scala.collection.TraversableLike

trait TraversableLike[+A, +Repr] extends HasNewBuilder[A, Repr] with FilterMonadic[A, Repr] with TraversableOnce[A] with GenTraversableLike[A, Repr] with Parallelizable[A, ParIterable[A]]

A template trait for traversable collections of type Traversable[A] .

This is a base trait of all kinds of Scala collections. It implements the behavior common to all collections, in terms of a method foreach with signature:

def foreach[U](f: Elem => U): Unit

Collection classes mixing in this trait provide a concrete foreach method which traverses all the elements contained in the collection, applying a given function to each. They also need to provide a method newBuilder which creates a builder for collections of the same kind.

A traversable class might or might not have two properties: strictness and orderedness. Neither is represented as a type.

The instances of a strict collection class have all their elements computed before they can be used as values. By contrast, instances of a non-strict collection class may defer computation of some of their elements until after the instance is available as a value. A typical example of a non-strict collection class is a scala.collection.immutable.Stream. A more general class of examples are TraversableViews .

If a collection is an instance of an ordered collection class, traversing its elements with foreach will always visit elements in the same order, even for different runs of the program. If the class is not ordered, foreach can visit elements in different orders for different runs (but it will keep the same order in the same run).’

A typical example of a collection class which is not ordered is a HashMap of objects. The traversal order for hash maps will depend on the hash codes of its elements, and these hash codes might differ from one run to the next. By contrast, a LinkedHashMap is ordered because its foreach method visits elements in the order they were inserted into the HashMap .

• Self Type
  • TraversableLike [A, Repr]
• Source
  • TraversableLike.scala - Blame
  • TraversableLike.scala - History

Type Members

type Self = Repr

The type implementing this traversable

• Attributes
  • protected[this]

class WithFilter extends FilterMonadic[A, Repr]

A class supporting filtered operations. Instances of this class are returned by method withFilter .

• Source
  • TraversableLike.scala - Blame
  • TraversableLike.scala - History

Abstract Value Members From scala.collection.Parallelizable

abstract def seq: TraversableOnce[A]

• Definition Classes
  • Parallelizable

(defined at scala.collection.Parallelizable)

Concrete Value Members From scala.collection.Parallelizable

def par: ParIterable[A]

Returns a parallel implementation of this collection.

For most collection types, this method creates a new parallel collection by copying all the elements. For these collection, par takes linear time. Mutable collections in this category do not produce a mutable parallel collection that has the same underlying dataset, so changes in one collection will not be reflected in the other one.

Specific collections (e.g. ParArray or mutable.ParHashMap ) override this default behaviour by creating a parallel collection which shares the same underlying dataset. For these collections, par takes constant or sublinear time.

All parallel collections return a reference to themselves.

• returns
  • a parallel implementation of this collection
• Definition Classes
  • Parallelizable

(defined at scala.collection.Parallelizable)

Abstract Value Members From scala.collection.TraversableLike

abstract def newBuilder: Builder[A, Repr]

Creates a new builder for this collection type.

• Attributes
  • protected[this]
• Definition Classes
  • TraversableLike → HasNewBuilder

(defined at scala.collection.TraversableLike)

Concrete Value Members From scala.collection.TraversableLike

def ++:[B >: A, That](that: Traversable[B])(implicit bf: CanBuildFrom[Repr, B, That]): That

As with ++ , returns a new collection containing the elements from the left operand followed by the elements from the right operand.

It differs from ++ in that the right operand determines the type of the resulting collection rather than the left one. Mnemonic: the COLon is on the side of the new COLlection type.

Example:

scala> val x = List(1)
x: List[Int] = List(1)

scala> val y = LinkedList(2)
y: scala.collection.mutable.LinkedList[Int] = LinkedList(2)

scala> val z = x ++: y
z: scala.collection.mutable.LinkedList[Int] = LinkedList(1, 2)

This overload exists because: for the implementation of ++: we should reuse that of ++ because many collections override it with more efficient versions.

Since TraversableOnce has no ++ method, we have to implement that directly, but Traversable and down can use the overload.

• B
  • the element type of the returned collection.
• That
  • the class of the returned collection. Where possible, That is the same class as the current collection class Repr , but this depends on the element type B being admissible for that class, which means that an implicit instance of type CanBuildFrom[Repr, B, That] is found.
• that
  • the traversable to append.
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and and the new element type B .
• returns
  • a new collection of type That which contains all elements of this traversable collection followed by all elements of that .

(defined at scala.collection.TraversableLike)

def ++:[B >: A, That](that: TraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That

[use case]

As with ++ , returns a new collection containing the elements from the left operand followed by the elements from the right operand.

It differs from ++ in that the right operand determines the type of the resulting collection rather than the left one. Mnemonic: the COLon is on the side of the new COLlection type.

Example:

scala> val x = List(1)
x: List[Int] = List(1)

scala> val y = LinkedList(2)
y: scala.collection.mutable.LinkedList[Int] = LinkedList(2)

scala> val z = x ++: y
z: scala.collection.mutable.LinkedList[Int] = LinkedList(1, 2)

• B
  • the element type of the returned collection.
• that
  • the traversable to append.
• returns
  • a new traversable collection which contains all elements of this traversable collection followed by all elements of that .

(defined at scala.collection.TraversableLike)

def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That

[use case]

Returns a new traversable collection containing the elements from the left hand operand followed by the elements from the right hand operand. The element type of the traversable collection is the most specific superclass encompassing the element types of the two operands.

Example:

scala> val a = List(1)
a: List[Int] = List(1)

scala> val b = List(2)
b: List[Int] = List(2)

scala> val c = a ++ b
c: List[Int] = List(1, 2)

scala> val d = List('a')
d: List[Char] = List(a)

scala> val e = c ++ d
e: List[AnyVal] = List(1, 2, a)

• B
  • the element type of the returned collection.
• that
  • the traversable to append.
• returns
  • a new traversable collection which contains all elements of this traversable collection followed by all elements of that .
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[Repr, B, That]): That

[use case]

Builds a new collection by applying a partial function to all elements of this traversable collection on which the function is defined.

• B
  • the element type of the returned collection.
• pf
  • the partial function which filters and maps the traversable collection.
• returns
  • a new traversable collection resulting from applying the given partial function pf to each element on which it is defined and collecting the results. The order of the elements is preserved.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Unit

[use case]

Copies the elements of this traversable collection to an array. Fills the given array xs with at most len elements of this traversable collection, starting at position start . Copying will stop once either the end of the current traversable collection is reached, or the end of the target array is reached, or len elements have been copied.

Note: will not terminate for infinite-sized collections.

• xs
  • the array to fill.
• start
  • the starting index.
• len
  • the maximal number of elements to copy.
• Definition Classes
  • TraversableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableLike)

def drop(n: Int): Repr

Selects all elements except first n ones.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• n
  • the number of elements to drop from this traversable collection.
• returns
  • a traversable collection consisting of all elements of this traversable collection except the first n ones, or else the empty traversable collection, if this traversable collection has less than n elements.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def dropWhile(p: (A) ⇒ Boolean): Repr

Drops longest prefix of elements that satisfy a predicate.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• returns
  • the longest suffix of this traversable collection whose first element does not satisfy the predicate p .
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def exists(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for at least one element of this traversable collection.

Note: may not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• returns
  • false if this traversable collection is empty, otherwise true if the given predicate p holds for some of the elements of this traversable collection, otherwise false
• Definition Classes
  • TraversableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableLike)

def filter(p: (A) ⇒ Boolean): Repr

Selects all elements of this traversable collection which satisfy a predicate.

• p
  • the predicate used to test elements.
• returns
  • a new traversable collection consisting of all elements of this traversable collection that satisfy the given predicate p . The order of the elements is preserved.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def filterNot(p: (A) ⇒ Boolean): Repr

Selects all elements of this traversable collection which do not satisfy a predicate.

• p
  • the predicate used to test elements.
• returns
  • a new traversable collection consisting of all elements of this traversable collection that do not satisfy the given predicate p . The order of the elements is preserved.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def find(p: (A) ⇒ Boolean): Option[A]

Finds the first element of the traversable collection satisfying a predicate, if any.

Note: may not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• p
  • the predicate used to test elements.
• returns
  • an option value containing the first element in the traversable collection that satisfies p , or None if none exists.
• Definition Classes
  • TraversableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableLike)

def flatMap[B, That](f: (A) ⇒ GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That

[use case]

Builds a new collection by applying a function to all elements of this traversable collection and using the elements of the resulting collections.

For example:

def getWords(lines: Seq[String]): Seq[String] = lines flatMap (line => line split "\\W+")

The type of the resulting collection is guided by the static type of traversable collection. This might cause unexpected results sometimes. For example:

// lettersOf will return a Seq[Char] of likely repeated letters, instead of a Set
def lettersOf(words: Seq[String]) = words flatMap (word => word.toSet)

// lettersOf will return a Set[Char], not a Seq
def lettersOf(words: Seq[String]) = words.toSet flatMap (word => word.toSeq)

// xs will be an Iterable[Int]
val xs = Map("a" -> List(11,111), "b" -> List(22,222)).flatMap(_._2)

// ys will be a Map[Int, Int]
val ys = Map("a" -> List(1 -> 11,1 -> 111), "b" -> List(2 -> 22,2 -> 222)).flatMap(_._2)

• B
  • the element type of the returned collection.
• f
  • the function to apply to each element.
• returns
  • a new traversable collection resulting from applying the given collection-valued function f to each element of this traversable collection and concatenating the results.
• Definition Classes
  • TraversableLike → GenTraversableLike → FilterMonadic

(defined at scala.collection.TraversableLike)

def forall(p: (A) ⇒ Boolean): Boolean

Tests whether a predicate holds for all elements of this traversable collection.

Note: may not terminate for infinite-sized collections.

• p
  • the predicate used to test elements.
• returns
  • true if this traversable collection is empty or the given predicate p holds for all elements of this traversable collection, otherwise false .
• Definition Classes
  • TraversableLike → TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableLike)

abstract def foreach[U](f: (A) ⇒ U): Unit

[use case]

Applies a function f to all elements of this traversable collection.

Note: this method underlies the implementation of most other bulk operations. It’s important to implement this method in an efficient way.

• f
  • the function that is applied for its side-effect to every element. The result of function f is discarded.
• Definition Classes
  • TraversableLike → GenTraversableLike → TraversableOnce → GenTraversableOnce → FilterMonadic

(defined at scala.collection.TraversableLike)

def groupBy[K](f: (A) ⇒ K): immutable.Map[K, Repr]

Partitions this traversable collection into a map of traversable collections according to some discriminator function.

Note: this method is not re-implemented by views. This means when applied to a view it will always force the view and return a new traversable collection.

• K
  • the type of keys returned by the discriminator function.
• f
  • the discriminator function.
• returns
  • A map from keys to traversable collections such that the following invariant holds:

    (xs groupBy f)(k) = xs filter (x => f(x) == k)
    

That is, every key `k` is bound to a traversable collection of those
elements `x` for which `f(x)` equals `k` .

• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def map[B, That](f: (A) ⇒ B)(implicit bf: CanBuildFrom[Repr, B, That]): That

[use case]

Builds a new collection by applying a function to all elements of this traversable collection.

• B
  • the element type of the returned collection.
• f
  • the function to apply to each element.
• returns
  • a new traversable collection resulting from applying the given function f to each element of this traversable collection and collecting the results.
• Definition Classes
  • TraversableLike → GenTraversableLike → FilterMonadic

(defined at scala.collection.TraversableLike)

def parCombiner: Combiner[A, ParIterable[A]]

The default par implementation uses the combiner provided by this method to create a new parallel collection.

• returns
  • a combiner for the parallel collection of type ParRepr
• Attributes
  • protected[this]
• Definition Classes
  • TraversableLike → Parallelizable

(defined at scala.collection.TraversableLike)

def partition(p: (A) ⇒ Boolean): (Repr, Repr)

Partitions this traversable collection in two traversable collections according to a predicate.

• p
  • the predicate on which to partition.
• returns
  • a pair of traversable collections: the first traversable collection consists of all elements that satisfy the predicate p and the second traversable collection consists of all elements that don’t. The relative order of the elements in the resulting traversable collections is the same as in the original traversable collection.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def scanLeft[B, That](z: B)(op: (B, A) ⇒ B)(implicit bf: CanBuildFrom[Repr, B, That]): That

Produces a collection containing cumulative results of applying the operator going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• B
  • the type of the elements in the resulting collection
• That
  • the actual type of the resulting collection
• z
  • the initial value
• op
  • the binary operator applied to the intermediate result and the element
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and and the new element type B .
• returns
  • collection with intermediate results
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def scanRight[B, That](z: B)(op: (A, B) ⇒ B)(implicit bf: CanBuildFrom[Repr, B, That]): That

Produces a collection containing cumulative results of applying the operator going right to left. The head of the collection is the last cumulative result.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered.

Example:

List(1, 2, 3, 4).scanRight(0)(_ + _) == List(10, 9, 7, 4, 0)

• B
  • the type of the elements in the resulting collection
• That
  • the actual type of the resulting collection
• z
  • the initial value
• op
  • the binary operator applied to the intermediate result and the element
• bf
  • an implicit value of class CanBuildFrom which determines the result class That from the current representation type Repr and and the new element type B .
• returns
  • collection with intermediate results
• Definition Classes
  • TraversableLike → GenTraversableLike
• Annotations
  • @migration
• Migration
  • (Changed in version 2.9.0) The behavior of scanRight has changed. The previous behavior can be reproduced with scanRight.reverse.

(defined at scala.collection.TraversableLike)

def scan[B >: A, That](z: B)(op: (B, B) ⇒ B)(implicit cbf: CanBuildFrom[Repr, B, That]): That

Computes a prefix scan of the elements of the collection.

Note: The neutral element z may be applied more than once.

• B
  • element type of the resulting collection
• That
  • type of the resulting collection
• z
  • neutral element for the operator op
• op
  • the associative operator for the scan
• cbf
  • combiner factory which provides a combiner
• returns
  • a new traversable collection containing the prefix scan of the elements in this traversable collection
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def slice(from: Int, until: Int): Repr

Selects an interval of elements. The returned collection is made up of all elements x which satisfy the invariant:

from <= indexOf(x) < until

Note: might return different results for different runs, unless the underlying collection type is ordered.

• returns
  • a traversable collection containing the elements greater than or equal to index from extending up to (but not including) index until of this traversable collection.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def span(p: (A) ⇒ Boolean): (Repr, Repr)

Splits this traversable collection into a prefix/suffix pair according to a predicate.

Note: c span p is equivalent to (but possibly more efficient than) (c takeWhile p, c dropWhile p) , provided the evaluation of the predicate p does not cause any side-effects.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• returns
  • a pair consisting of the longest prefix of this traversable collection whose elements all satisfy p , and the rest of this traversable collection.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def splitAt(n: Int): (Repr, Repr)

Splits this traversable collection into two at a given position. Note: c splitAt n is equivalent to (but possibly more efficient than) (c take n, c drop n) .

Note: might return different results for different runs, unless the underlying collection type is ordered.

• n
  • the position at which to split.
• returns
  • a pair of traversable collections consisting of the first n elements of this traversable collection, and the other elements.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def take(n: Int): Repr

Selects first n elements.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• n
  • the number of elements to take from this traversable collection.
• returns
  • a traversable collection consisting only of the first n elements of this traversable collection, or else the whole traversable collection, if it has less than n elements.
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def takeWhile(p: (A) ⇒ Boolean): Repr

Takes longest prefix of elements that satisfy a predicate.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• returns
  • the longest prefix of this traversable collection whose elements all satisfy the predicate p .
• Definition Classes
  • TraversableLike → GenTraversableLike

(defined at scala.collection.TraversableLike)

def toCollection(repr: Repr): Traversable[A]

A conversion from collections of type Repr to Traversable objects. By default this is implemented as just a cast, but this can be overridden.

• Attributes
  • protected[this]

(defined at scala.collection.TraversableLike)

def view(from: Int, until: Int): TraversableView[A, Repr]

Creates a non-strict view of a slice of this traversable collection.

Note: the difference between view and slice is that view produces a view of the current traversable collection, whereas slice produces a new traversable collection.

Note: view(from, to) is equivalent to view.slice(from, to)

Note: might return different results for different runs, unless the underlying collection type is ordered.

• from
  • the index of the first element of the view
• until
  • the index of the element following the view
• returns
  • a non-strict view of a slice of this traversable collection, starting at index from and extending up to (but not including) index until .

(defined at scala.collection.TraversableLike)

def view: TraversableView[A, Repr]

Creates a non-strict view of this traversable collection.

• returns
  • a non-strict view of this traversable collection.

(defined at scala.collection.TraversableLike)

def withFilter(p: (A) ⇒ Boolean): FilterMonadic[A, Repr]

Creates a non-strict filter of this traversable collection.

Note: the difference between c filter p and c withFilter p is that the former creates a new collection, whereas the latter only restricts the domain of subsequent map , flatMap , foreach , and withFilter operations.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• p
  • the predicate used to test elements.
• returns
  • an object of class WithFilter , which supports map , flatMap , foreach , and withFilter operations. All these operations apply to those elements of this traversable collection which satisfy the predicate p .
• Definition Classes
  • TraversableLike → FilterMonadic

(defined at scala.collection.TraversableLike)

Concrete Value Members From scala.collection.TraversableOnce

def /:[B](z: B)(op: (B, A) ⇒ B): B

Applies a binary operator to a start value and all elements of this traversable or iterator, going left to right.

Note: /: is alternate syntax for foldLeft ; z /: xs is the same as xs foldLeft z .

Examples:

Note that the folding function used to compute b is equivalent to that used to compute c.

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = (5 /: a)(_+_)
b: Int = 15

scala> val c = (5 /: a)((x,y) => x + y)
c: Int = 15

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

• B
  • the result type of the binary operator.
• z
  • the start value.
• op
  • the binary operator.
• returns
  • the result of inserting op between consecutive elements of this traversable or iterator, going left to right with the start value z on the left:

    op(...op(op(z, x_1), x_2), ..., x_n)
    

where `x1, ..., xn` are the elements of this traversable or iterator.

• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def :\[B](z: B)(op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator and a start value, going right to left.

Note: :\ is alternate syntax for foldRight ; xs :\ z is the same as xs foldRight z .

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

Examples:

Note that the folding function used to compute b is equivalent to that used to compute c.

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = (a :\ 5)(_+_)
b: Int = 15

scala> val c = (a :\ 5)((x,y) => x + y)
c: Int = 15

• B
  • the result type of the binary operator.
• z
  • the start value
• op
  • the binary operator
• returns
  • the result of inserting op between consecutive elements of this traversable or iterator, going right to left with the start value z on the right:

    op(x_1, op(x_2, ... op(x_n, z)...))
    

where `x1, ..., xn` are the elements of this traversable or iterator.

• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder): StringBuilder

Appends all elements of this traversable or iterator to a string builder. The written text consists of the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator without any separator string.

Example:

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = new StringBuilder()
b: StringBuilder =

scala> val h = a.addString(b)
h: StringBuilder = 1234

• b
  • the string builder to which elements are appended.
• returns
  • the string builder b to which elements were appended.
• Definition Classes
  • TraversableOnce

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder, sep: String): StringBuilder

Appends all elements of this traversable or iterator to a string builder using a separator string. The written text consists of the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator, separated by the string sep .

Example:

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = new StringBuilder()
b: StringBuilder =

scala> a.addString(b, ", ")
res0: StringBuilder = 1, 2, 3, 4

• b
  • the string builder to which elements are appended.
• sep
  • the separator string.
• returns
  • the string builder b to which elements were appended.
• Definition Classes
  • TraversableOnce

(defined at scala.collection.TraversableOnce)

def addString(b: StringBuilder, start: String, sep: String, end: String): StringBuilder

Appends all elements of this traversable or iterator to a string builder using start, end, and separator strings. The written text begins with the string start and ends with the string end . Inside, the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator are separated by the string sep .

Example:

scala> val a = List(1,2,3,4)
a: List[Int] = List(1, 2, 3, 4)

scala> val b = new StringBuilder()
b: StringBuilder =

scala> a.addString(b , "List(" , ", " , ")")
res5: StringBuilder = List(1, 2, 3, 4)

• b
  • the string builder to which elements are appended.
• start
  • the starting string.
• sep
  • the separator string.
• end
  • the ending string.
• returns
  • the string builder b to which elements were appended.
• Definition Classes
  • TraversableOnce

(defined at scala.collection.TraversableOnce)

def aggregate[B](z: ⇒ B)(seqop: (B, A) ⇒ B, combop: (B, B) ⇒ B): B

Aggregates the results of applying an operator to subsequent elements.

This is a more general form of fold and reduce . It is similar to foldLeft in that it doesn’t require the result to be a supertype of the element type. In addition, it allows parallel collections to be processed in chunks, and then combines the intermediate results.

aggregate splits the traversable or iterator into partitions and processes each partition by sequentially applying seqop , starting with z (like foldLeft ). Those intermediate results are then combined by using combop (like fold ). The implementation of this operation may operate on an arbitrary number of collection partitions (even 1), so combop may be invoked an arbitrary number of times (even 0).

As an example, consider summing up the integer values of a list of chars. The initial value for the sum is 0. First, seqop transforms each input character to an Int and adds it to the sum (of the partition). Then, combop just needs to sum up the intermediate results of the partitions:

List('a', 'b', 'c').aggregate(0)({ (sum, ch) => sum + ch.toInt }, { (p1, p2) => p1 + p2 })

• B
  • the type of accumulated results
• z
  • the initial value for the accumulated result of the partition - this will typically be the neutral element for the seqop operator (e.g. Nil for list concatenation or 0 for summation) and may be evaluated more than once
• seqop
  • an operator used to accumulate results within a partition
• combop
  • an associative operator used to combine results from different partitions
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def collectFirst[B](pf: PartialFunction[A, B]): Option[B]

Finds the first element of the traversable or iterator for which the given partial function is defined, and applies the partial function to it.

Note: may not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered.

• pf
  • the partial function
• returns
  • an option value containing pf applied to the first value for which it is defined, or None if none exists.
• Definition Classes
  • TraversableOnce

Example:

Seq("a", 1, 5L).collectFirst({ case x: Int => x*10 }) = Some(10)

(defined at scala.collection.TraversableOnce)

def copyToArray[B >: A](xs: Array[B]): Unit

[use case]

Copies the elements of this traversable collection to an array. Fills the given array xs with values of this traversable collection. Copying will stop once either the end of the current traversable collection is reached, or the end of the target array is reached.

Note: will not terminate for infinite-sized collections.

• xs
  • the array to fill.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def copyToArray[B >: A](xs: Array[B], start: Int): Unit

[use case]

Copies the elements of this traversable collection to an array. Fills the given array xs with values of this traversable collection, beginning at index start . Copying will stop once either the end of the current traversable collection is reached, or the end of the target array is reached.

Note: will not terminate for infinite-sized collections.

• xs
  • the array to fill.
• start
  • the starting index.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def copyToBuffer[B >: A](dest: Buffer[B]): Unit

Copies all elements of this traversable or iterator to a buffer.

Note: will not terminate for infinite-sized collections.

• dest
  • The buffer to which elements are copied.
• Definition Classes
  • TraversableOnce

(defined at scala.collection.TraversableOnce)

def count(p: (A) ⇒ Boolean): Int

Counts the number of elements in the traversable or iterator which satisfy a predicate.

• p
  • the predicate used to test elements.
• returns
  • the number of elements satisfying the predicate p .
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def foldLeft[B](z: B)(op: (B, A) ⇒ B): B

Applies a binary operator to a start value and all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

• B
  • the result type of the binary operator.
• z
  • the start value.
• op
  • the binary operator.
• returns
  • the result of inserting op between consecutive elements of this traversable or iterator, going left to right with the start value z on the left:

    op(...op(z, x_1), x_2, ..., x_n)
    

where `x1, ..., xn` are the elements of this traversable or iterator.
Returns `z` if this traversable or iterator is empty.

• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def foldRight[B](z: B)(op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator and a start value, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

• B
  • the result type of the binary operator.
• z
  • the start value.
• op
  • the binary operator.
• returns
  • the result of inserting op between consecutive elements of this traversable or iterator, going right to left with the start value z on the right:

    op(x_1, op(x_2, ... op(x_n, z)...))
    

where `x1, ..., xn` are the elements of this traversable or iterator.
Returns `z` if this traversable or iterator is empty.

• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def fold[A1 >: A](z: A1)(op: (A1, A1) ⇒ A1): A1

Folds the elements of this traversable or iterator using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

Note: will not terminate for infinite-sized collections.

• A1
  • a type parameter for the binary operator, a supertype of A .
• z
  • a neutral element for the fold operation; may be added to the result an arbitrary number of times, and must not change the result (e.g., Nil for list concatenation, 0 for addition, or 1 for multiplication).
• op
  • a binary operator that must be associative.
• returns
  • the result of applying the fold operator op between all the elements and z , or z if this traversable or iterator is empty.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def maxBy[B](f: (A) ⇒ B)(implicit cmp: Ordering[B]): A

[use case]

Finds the first element which yields the largest value measured by function f.

• B
  • The result type of the function f.
• f
  • The measuring function.
• returns
  • the first element of this traversable collection with the largest value measured by function f.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def minBy[B](f: (A) ⇒ B)(implicit cmp: Ordering[B]): A

[use case]

Finds the first element which yields the smallest value measured by function f.

• B
  • The result type of the function f.
• f
  • The measuring function.
• returns
  • the first element of this traversable collection with the smallest value measured by function f.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def mkString(sep: String): String

Displays all elements of this traversable or iterator in a string using a separator string.

• sep
  • the separator string.
• returns
  • a string representation of this traversable or iterator. In the resulting string the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator are separated by the string sep .
• Definition Classes
  • TraversableOnce → GenTraversableOnce

Example:

List(1, 2, 3).mkString("|") = "1|2|3"

(defined at scala.collection.TraversableOnce)

def mkString(start: String, sep: String, end: String): String

Displays all elements of this traversable or iterator in a string using start, end, and separator strings.

• start
  • the starting string.
• sep
  • the separator string.
• end
  • the ending string.
• returns
  • a string representation of this traversable or iterator. The resulting string begins with the string start and ends with the string end . Inside, the string representations (w.r.t. the method toString ) of all elements of this traversable or iterator are separated by the string sep .
• Definition Classes
  • TraversableOnce → GenTraversableOnce

Example:

List(1, 2, 3).mkString("(", "; ", ")") = "(1; 2; 3)"

(defined at scala.collection.TraversableOnce)

def reduceLeftOption[B >: A](op: (B, A) ⇒ B): Option[B]

Optionally applies a binary operator to all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

• B
  • the result type of the binary operator.
• op
  • the binary operator.
• returns
  • an option value containing the result of reduceLeft(op) if this traversable or iterator is nonempty, None otherwise.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceLeft[B >: A](op: (B, A) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator, going left to right.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

• B
  • the result type of the binary operator.
• op
  • the binary operator.
• returns
  • the result of inserting op between consecutive elements of this traversable or iterator, going left to right:

    op( op( ... op(x_1, x_2) ..., x_{n-1}), x_n)
    

where `x1, ..., xn` are the elements of this traversable or iterator.

• Definition Classes
  • TraversableOnce
• Exceptions thrown
  • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

def reduceOption[A1 >: A](op: (A1, A1) ⇒ A1): Option[A1]

Reduces the elements of this traversable or iterator, if any, using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

• A1
  • A type parameter for the binary operator, a supertype of A .
• op
  • A binary operator that must be associative.
• returns
  • An option value containing result of applying reduce operator op between all the elements if the collection is nonempty, and None otherwise.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceRightOption[B >: A](op: (A, B) ⇒ B): Option[B]

Optionally applies a binary operator to all elements of this traversable or iterator, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

• B
  • the result type of the binary operator.
• op
  • the binary operator.
• returns
  • an option value containing the result of reduceRight(op) if this traversable or iterator is nonempty, None otherwise.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def reduceRight[B >: A](op: (A, B) ⇒ B): B

Applies a binary operator to all elements of this traversable or iterator, going right to left.

Note: will not terminate for infinite-sized collections.

Note: might return different results for different runs, unless the underlying collection type is ordered or the operator is associative and commutative.

• B
  • the result type of the binary operator.
• op
  • the binary operator.
• returns
  • the result of inserting op between consecutive elements of this traversable or iterator, going right to left:

    op(x_1, op(x_2, ..., op(x_{n-1}, x_n)...))
    

where `x1, ..., xn` are the elements of this traversable or iterator.

• Definition Classes
  • TraversableOnce → GenTraversableOnce
• Exceptions thrown
  • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

def reduce[A1 >: A](op: (A1, A1) ⇒ A1): A1

Reduces the elements of this traversable or iterator using the specified associative binary operator.

The order in which operations are performed on elements is unspecified and may be nondeterministic.

• A1
  • A type parameter for the binary operator, a supertype of A .
• op
  • A binary operator that must be associative.
• returns
  • The result of applying reduce operator op between all the elements if the traversable or iterator is nonempty.
• Definition Classes
  • TraversableOnce → GenTraversableOnce
• Exceptions thrown
  • UnsupportedOperationException if this traversable or iterator is empty.

(defined at scala.collection.TraversableOnce)

def toBuffer[B >: A]: Buffer[B]

Uses the contents of this traversable or iterator to create a new mutable buffer.

Note: will not terminate for infinite-sized collections.

• returns
  • a buffer containing all elements of this traversable or iterator.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toIndexedSeq: immutable.IndexedSeq[A]

Converts this traversable or iterator to an indexed sequence.

Note: will not terminate for infinite-sized collections.

• returns
  • an indexed sequence containing all elements of this traversable or iterator.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toMap[T, U](implicit ev: <:<[A, (T, U)]): immutable.Map[T, U]

[use case]

Converts this traversable collection to a map. This method is unavailable unless the elements are members of Tuple2, each ((T, U)) becoming a key-value pair in the map. Duplicate keys will be overwritten by later keys: if this is an unordered collection, which key is in the resulting map is undefined.

Note: will not terminate for infinite-sized collections.

• returns
  • a map of type immutable.Map[T, U] containing all key/value pairs of type (T, U) of this traversable collection.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

def toSet[B >: A]: immutable.Set[B]

Converts this traversable or iterator to a set.

Note: will not terminate for infinite-sized collections.

• returns
  • a set containing all elements of this traversable or iterator.
• Definition Classes
  • TraversableOnce → GenTraversableOnce

(defined at scala.collection.TraversableOnce)

Concrete Value Members From Implicit scala.collection.parallel.CollectionsHaveToParArray ——————————————————————————–

def toParArray: ParArray[T]

• Implicit information
  • This member is added by an implicit conversion from TraversableLike [A, Repr] to CollectionsHaveToParArray [TraversableLike [A, Repr], T] performed by method CollectionsHaveToParArray in scala.collection.parallel. This conversion will take place only if an implicit value of type ( TraversableLike [A, Repr]) ⇒ GenTraversableOnce [T] is in scope.
• Definition Classes
  • CollectionsHaveToParArray (added by implicit convertion: scala.collection.parallel.CollectionsHaveToParArray)

Full Source:

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala
package collection

import generic._
import mutable.{ Builder }
import scala.annotation.migration
import scala.annotation.unchecked.{ uncheckedVariance => uV }
import parallel.ParIterable
import scala.language.higherKinds

/** A template trait for traversable collections of type `Traversable[A]`.
 *
 *  $traversableInfo
 *  @define mutability
 *  @define traversableInfo
 *  This is a base trait of all kinds of $mutability Scala collections. It
 *  implements the behavior common to all collections, in terms of a method
 *  `foreach` with signature:
 * {{{
 *     def foreach[U](f: Elem => U): Unit
 * }}}
 *  Collection classes mixing in this trait provide a concrete
 *  `foreach` method which traverses all the
 *  elements contained in the collection, applying a given function to each.
 *  They also need to provide a method `newBuilder`
 *  which creates a builder for collections of the same kind.
 *
 *  A traversable class might or might not have two properties: strictness
 *  and orderedness. Neither is represented as a type.
 *
 *  The instances of a strict collection class have all their elements
 *  computed before they can be used as values. By contrast, instances of
 *  a non-strict collection class may defer computation of some of their
 *  elements until after the instance is available as a value.
 *  A typical example of a non-strict collection class is a
 *  [[scala.collection.immutable.Stream]].
 *  A more general class of examples are `TraversableViews`.
 *
 *  If a collection is an instance of an ordered collection class, traversing
 *  its elements with `foreach` will always visit elements in the
 *  same order, even for different runs of the program. If the class is not
 *  ordered, `foreach` can visit elements in different orders for
 *  different runs (but it will keep the same order in the same run).'
 *
 *  A typical example of a collection class which is not ordered is a
 *  `HashMap` of objects. The traversal order for hash maps will
 *  depend on the hash codes of its elements, and these hash codes might
 *  differ from one run to the next. By contrast, a `LinkedHashMap`
 *  is ordered because its `foreach` method visits elements in the
 *  order they were inserted into the `HashMap`.
 *
 *  @author Martin Odersky
 *  @version 2.8
 *  @since   2.8
 *  @tparam A    the element type of the collection
 *  @tparam Repr the type of the actual collection containing the elements.
 *
 *  @define Coll Traversable
 *  @define coll traversable collection
 */
trait TraversableLike[+A, +Repr] extends Any
                                    with HasNewBuilder[A, Repr]
                                    with FilterMonadic[A, Repr]
                                    with TraversableOnce[A]
                                    with GenTraversableLike[A, Repr]
                                    with Parallelizable[A, ParIterable[A]]
{
  self =>

  import Traversable.breaks._

  /** The type implementing this traversable */
  protected[this] type Self = Repr

  /** The collection of type $coll underlying this `TraversableLike` object.
   *  By default this is implemented as the `TraversableLike` object itself,
   *  but this can be overridden.
   */
  def repr: Repr = this.asInstanceOf[Repr]

  final def isTraversableAgain: Boolean = true

  /** The underlying collection seen as an instance of `$Coll`.
   *  By default this is implemented as the current collection object itself,
   *  but this can be overridden.
   */
  protected[this] def thisCollection: Traversable[A] = this.asInstanceOf[Traversable[A]]

  /** A conversion from collections of type `Repr` to `$Coll` objects.
   *  By default this is implemented as just a cast, but this can be overridden.
   */
  protected[this] def toCollection(repr: Repr): Traversable[A] = repr.asInstanceOf[Traversable[A]]

  /** Creates a new builder for this collection type.
   */
  protected[this] def newBuilder: Builder[A, Repr]

  protected[this] def parCombiner = ParIterable.newCombiner[A]

  /** Applies a function `f` to all elements of this $coll.
   *
   *  @param  f   the function that is applied for its side-effect to every element.
   *              The result of function `f` is discarded.
   *
   *  @tparam  U  the type parameter describing the result of function `f`.
   *              This result will always be ignored. Typically `U` is `Unit`,
   *              but this is not necessary.
   *
   *  @usecase def foreach(f: A => Unit): Unit
   *    @inheritdoc
   *
   *    Note: this method underlies the implementation of most other bulk operations.
   *    It's important to implement this method in an efficient way.
   *
   */
  def foreach[U](f: A => U): Unit

  /** Tests whether this $coll is empty.
   *
   *  @return    `true` if the $coll contain no elements, `false` otherwise.
   */
  def isEmpty: Boolean = {
    var result = true
    breakable {
      for (x <- this) {
        result = false
        break
      }
    }
    result
  }

  /** Tests whether this $coll is known to have a finite size.
   *  All strict collections are known to have finite size. For a non-strict
   *  collection such as `Stream`, the predicate returns `'''true'''` if all
   *  elements have been computed. It returns `'''false'''` if the stream is
   *  not yet evaluated to the end.
   *
   *  Note: many collection methods will not work on collections of infinite sizes.
   *
   *  @return  `'''true'''` if this collection is known to have finite size,
   *           `'''false'''` otherwise.
   */
  def hasDefiniteSize = true

  def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    val b = bf(repr)
    if (that.isInstanceOf[IndexedSeqLike[_, _]]) b.sizeHint(this, that.seq.size)
    b ++= thisCollection
    b ++= that.seq
    b.result
  }

  /** As with `++`, returns a new collection containing the elements from the left operand followed by the
   *  elements from the right operand.
   *
   *  It differs from `++` in that the right operand determines the type of
   *  the resulting collection rather than the left one.
   *  Mnemonic: the COLon is on the side of the new COLlection type.
   *
   *  @param that   the traversable to append.
   *  @tparam B     the element type of the returned collection.
   *  @tparam That  $thatinfo
   *  @param bf     $bfinfo
   *  @return       a new collection of type `That` which contains all elements
   *                of this $coll followed by all elements of `that`.
   *
   *  @usecase def ++:[B](that: TraversableOnce[B]): $Coll[B]
   *    @inheritdoc
   *
   *    Example:
   *    {{{
   *      scala> val x = List(1)
   *      x: List[Int] = List(1)
   *
   *      scala> val y = LinkedList(2)
   *      y: scala.collection.mutable.LinkedList[Int] = LinkedList(2)
   *
   *      scala> val z = x ++: y
   *      z: scala.collection.mutable.LinkedList[Int] = LinkedList(1, 2)
   *    }}}
   *
   *    @return       a new $coll which contains all elements of this $coll
   *                  followed by all elements of `that`.
   */
  def ++:[B >: A, That](that: TraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    val b = bf(repr)
    if (that.isInstanceOf[IndexedSeqLike[_, _]]) b.sizeHint(this, that.size)
    b ++= that
    b ++= thisCollection
    b.result
  }

  /** As with `++`, returns a new collection containing the elements from the
   *  left operand followed by the elements from the right operand.
   *
   *  It differs from `++` in that the right operand determines the type of
   *  the resulting collection rather than the left one.
   *  Mnemonic: the COLon is on the side of the new COLlection type.
   *
   *  Example:
   *  {{{
   *     scala> val x = List(1)
   *     x: List[Int] = List(1)
   *
   *     scala> val y = LinkedList(2)
   *     y: scala.collection.mutable.LinkedList[Int] = LinkedList(2)
   *
   *     scala> val z = x ++: y
   *     z: scala.collection.mutable.LinkedList[Int] = LinkedList(1, 2)
   *  }}}
   *
   * This overload exists because: for the implementation of `++:` we should
   *  reuse that of `++` because many collections override it with more
   *  efficient versions.
   *
   *  Since `TraversableOnce` has no `++` method, we have to implement that
   *  directly, but `Traversable` and down can use the overload.
   *
   *  @param that   the traversable to append.
   *  @tparam B     the element type of the returned collection.
   *  @tparam That  $thatinfo
   *  @param bf     $bfinfo
   *  @return       a new collection of type `That` which contains all elements
   *                of this $coll followed by all elements of `that`.
   */
  def ++:[B >: A, That](that: Traversable[B])(implicit bf: CanBuildFrom[Repr, B, That]): That =
    (that ++ seq)(breakOut)

  def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    def builder = { // extracted to keep method size under 35 bytes, so that it can be JIT-inlined
      val b = bf(repr)
      b.sizeHint(this)
      b
    }
    val b = builder
    for (x <- this) b += f(x)
    b.result
  }

  def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    def builder = bf(repr) // extracted to keep method size under 35 bytes, so that it can be JIT-inlined
    val b = builder
    for (x <- this) b ++= f(x).seq
    b.result
  }

  private[scala] def filterImpl(p: A => Boolean, isFlipped: Boolean): Repr = {
    val b = newBuilder
    for (x <- this)
      if (p(x) != isFlipped) b += x

    b.result
  }

  /** Selects all elements of this $coll which satisfy a predicate.
   *
   *  @param p     the predicate used to test elements.
   *  @return      a new $coll consisting of all elements of this $coll that satisfy the given
   *               predicate `p`. The order of the elements is preserved.
   */
  def filter(p: A => Boolean): Repr = filterImpl(p, isFlipped = false)

  /** Selects all elements of this $coll which do not satisfy a predicate.
   *
   *  @param p     the predicate used to test elements.
   *  @return      a new $coll consisting of all elements of this $coll that do not satisfy the given
   *               predicate `p`. The order of the elements is preserved.
   */
  def filterNot(p: A => Boolean): Repr = filterImpl(p, isFlipped = true)

  def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    val b = bf(repr)
    foreach(pf.runWith(b += _))
    b.result
  }

  /** Builds a new collection by applying an option-valued function to all
   *  elements of this $coll on which the function is defined.
   *
   *  @param f      the option-valued function which filters and maps the $coll.
   *  @tparam B     the element type of the returned collection.
   *  @tparam That  $thatinfo
   *  @param bf     $bfinfo
   *  @return       a new collection of type `That` resulting from applying the option-valued function
   *                `f` to each element and collecting all defined results.
   *                The order of the elements is preserved.
   *
   *  @usecase def filterMap[B](f: A => Option[B]): $Coll[B]
   *    @inheritdoc
   *
   *    @param pf     the partial function which filters and maps the $coll.
   *    @return       a new $coll resulting from applying the given option-valued function
   *                  `f` to each element and collecting all defined results.
   *                  The order of the elements is preserved.
  def filterMap[B, That](f: A => Option[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    val b = bf(repr)
    for (x <- this)
      f(x) match {
        case Some(y) => b += y
        case _ =>
      }
    b.result
  }
   */

  /** Partitions this $coll in two ${coll}s according to a predicate.
   *
   *  @param p the predicate on which to partition.
   *  @return  a pair of ${coll}s: the first $coll consists of all elements that
   *           satisfy the predicate `p` and the second $coll consists of all elements
   *           that don't. The relative order of the elements in the resulting ${coll}s
   *           is the same as in the original $coll.
   */
  def partition(p: A => Boolean): (Repr, Repr) = {
    val l, r = newBuilder
    for (x <- this) (if (p(x)) l else r) += x
    (l.result, r.result)
  }

  def groupBy[K](f: A => K): immutable.Map[K, Repr] = {
    val m = mutable.Map.empty[K, Builder[A, Repr]]
    for (elem <- this) {
      val key = f(elem)
      val bldr = m.getOrElseUpdate(key, newBuilder)
      bldr += elem
    }
    val b = immutable.Map.newBuilder[K, Repr]
    for ((k, v) <- m)
      b += ((k, v.result))

    b.result
  }

  def forall(p: A => Boolean): Boolean = {
    var result = true
    breakable {
      for (x <- this)
        if (!p(x)) { result = false; break }
    }
    result
  }

  /** Tests whether a predicate holds for at least one element of this $coll.
   *
   *  $mayNotTerminateInf
   *
   *  @param   p     the predicate used to test elements.
   *  @return        `false` if this $coll is empty, otherwise `true` if the given predicate `p`
    *                holds for some of the elements of this $coll, otherwise `false`
   */
  def exists(p: A => Boolean): Boolean = {
    var result = false
    breakable {
      for (x <- this)
        if (p(x)) { result = true; break }
    }
    result
  }

  def find(p: A => Boolean): Option[A] = {
    var result: Option[A] = None
    breakable {
      for (x <- this)
        if (p(x)) { result = Some(x); break }
    }
    result
  }

  def scan[B >: A, That](z: B)(op: (B, B) => B)(implicit cbf: CanBuildFrom[Repr, B, That]): That = scanLeft(z)(op)

  def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    val b = bf(repr)
    b.sizeHint(this, 1)
    var acc = z
    b += acc
    for (x <- this) { acc = op(acc, x); b += acc }
    b.result
  }

  @migration("The behavior of `scanRight` has changed. The previous behavior can be reproduced with scanRight.reverse.", "2.9.0")
  def scanRight[B, That](z: B)(op: (A, B) => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
    var scanned = List(z)
    var acc = z
    for (x <- reversed) {
      acc = op(x, acc)
      scanned ::= acc
    }
    val b = bf(repr)
    for (elem <- scanned) b += elem
    b.result
  }

  /** Selects the first element of this $coll.
   *  $orderDependent
   *  @return  the first element of this $coll.
   *  @throws NoSuchElementException if the $coll is empty.
   */
  def head: A = {
    var result: () => A = () => throw new NoSuchElementException
    breakable {
      for (x <- this) {
        result = () => x
        break
      }
    }
    result()
  }

  /** Optionally selects the first element.
   *  $orderDependent
   *  @return  the first element of this $coll if it is nonempty,
   *           `None` if it is empty.
   */
  def headOption: Option[A] = if (isEmpty) None else Some(head)

  /** Selects all elements except the first.
   *  $orderDependent
   *  @return  a $coll consisting of all elements of this $coll
   *           except the first one.
   *  @throws `UnsupportedOperationException` if the $coll is empty.
   */
  override def tail: Repr = {
    if (isEmpty) throw new UnsupportedOperationException("empty.tail")
    drop(1)
  }

  /** Selects the last element.
    * $orderDependent
    * @return The last element of this $coll.
    * @throws NoSuchElementException If the $coll is empty.
    */
  def last: A = {
    var lst = head
    for (x <- this)
      lst = x
    lst
  }

  /** Optionally selects the last element.
   *  $orderDependent
   *  @return  the last element of this $coll$ if it is nonempty,
   *           `None` if it is empty.
   */
  def lastOption: Option[A] = if (isEmpty) None else Some(last)

  /** Selects all elements except the last.
   *  $orderDependent
   *  @return  a $coll consisting of all elements of this $coll
   *           except the last one.
   *  @throws UnsupportedOperationException if the $coll is empty.
   */
  def init: Repr = {
    if (isEmpty) throw new UnsupportedOperationException("empty.init")
    var lst = head
    var follow = false
    val b = newBuilder
    b.sizeHint(this, -1)
    for (x <- this) {
      if (follow) b += lst
      else follow = true
      lst = x
    }
    b.result
  }

  def take(n: Int): Repr = slice(0, n)

  def drop(n: Int): Repr =
    if (n <= 0) {
      val b = newBuilder
      b.sizeHint(this)
      (b ++= thisCollection).result
    }
    else sliceWithKnownDelta(n, Int.MaxValue, -n)

  def slice(from: Int, until: Int): Repr =
    sliceWithKnownBound(scala.math.max(from, 0), until)

  // Precondition: from >= 0, until > 0, builder already configured for building.
  private[this] def sliceInternal(from: Int, until: Int, b: Builder[A, Repr]): Repr = {
    var i = 0
    breakable {
      for (x <- this) {
        if (i >= from) b += x
        i += 1
        if (i >= until) break
      }
    }
    b.result
  }
  // Precondition: from >= 0
  private[scala] def sliceWithKnownDelta(from: Int, until: Int, delta: Int): Repr = {
    val b = newBuilder
    if (until <= from) b.result
    else {
      b.sizeHint(this, delta)
      sliceInternal(from, until, b)
    }
  }
  // Precondition: from >= 0
  private[scala] def sliceWithKnownBound(from: Int, until: Int): Repr = {
    val b = newBuilder
    if (until <= from) b.result
    else {
      b.sizeHintBounded(until - from, this)
      sliceInternal(from, until, b)
    }
  }

  def takeWhile(p: A => Boolean): Repr = {
    val b = newBuilder
    breakable {
      for (x <- this) {
        if (!p(x)) break
        b += x
      }
    }
    b.result
  }

  def dropWhile(p: A => Boolean): Repr = {
    val b = newBuilder
    var go = false
    for (x <- this) {
      if (!go && !p(x)) go = true
      if (go) b += x
    }
    b.result
  }

  def span(p: A => Boolean): (Repr, Repr) = {
    val l, r = newBuilder
    var toLeft = true
    for (x <- this) {
      toLeft = toLeft && p(x)
      (if (toLeft) l else r) += x
    }
    (l.result, r.result)
  }

  def splitAt(n: Int): (Repr, Repr) = {
    val l, r = newBuilder
    l.sizeHintBounded(n, this)
    if (n >= 0) r.sizeHint(this, -n)
    var i = 0
    for (x <- this) {
      (if (i < n) l else r) += x
      i += 1
    }
    (l.result, r.result)
  }

  /** Iterates over the tails of this $coll. The first value will be this
   *  $coll and the final one will be an empty $coll, with the intervening
   *  values the results of successive applications of `tail`.
   *
   *  @return   an iterator over all the tails of this $coll
   *  @example  `List(1,2,3).tails = Iterator(List(1,2,3), List(2,3), List(3), Nil)`
   */
  def tails: Iterator[Repr] = iterateUntilEmpty(_.tail)

  /** Iterates over the inits of this $coll. The first value will be this
   *  $coll and the final one will be an empty $coll, with the intervening
   *  values the results of successive applications of `init`.
   *
   *  @return  an iterator over all the inits of this $coll
   *  @example  `List(1,2,3).inits = Iterator(List(1,2,3), List(1,2), List(1), Nil)`
   */
  def inits: Iterator[Repr] = iterateUntilEmpty(_.init)

  def copyToArray[B >: A](xs: Array[B], start: Int, len: Int) {
    var i = start
    val end = (start + len) min xs.length
    breakable {
      for (x <- this) {
        if (i >= end) break
        xs(i) = x
        i += 1
      }
    }
  }

  @deprecatedOverriding("Enforce contract of toTraversable that if it is Traversable it returns itself.", "2.11.0")
  def toTraversable: Traversable[A] = thisCollection

  def toIterator: Iterator[A] = toStream.iterator
  def toStream: Stream[A] = toBuffer.toStream
  // Override to provide size hint.
  override def to[Col[_]](implicit cbf: CanBuildFrom[Nothing, A, Col[A @uV]]): Col[A @uV] = {
    val b = cbf()
    b.sizeHint(this)
    b ++= thisCollection
    b.result
  }

  /** Converts this $coll to a string.
   *
   *  @return   a string representation of this collection. By default this
   *            string consists of the `stringPrefix` of this $coll, followed
   *            by all elements separated by commas and enclosed in parentheses.
   */
  override def toString = mkString(stringPrefix + "(", ", ", ")")

  /** Defines the prefix of this object's `toString` representation.
   *
   *  @return  a string representation which starts the result of `toString`
   *           applied to this $coll. By default the string prefix is the
   *           simple name of the collection class $coll.
   */
  def stringPrefix : String = {
    var string = repr.getClass.getName
    val idx1 = string.lastIndexOf('.' : Int)
    if (idx1 != -1) string = string.substring(idx1 + 1)
    val idx2 = string.indexOf('$')
    if (idx2 != -1) string = string.substring(0, idx2)
    string
  }

  /** Creates a non-strict view of this $coll.
   *
   *  @return a non-strict view of this $coll.
   */
  def view = new TraversableView[A, Repr] {
    protected lazy val underlying = self.repr
    override def foreach[U](f: A => U) = self foreach f
  }

  /** Creates a non-strict view of a slice of this $coll.
   *
   *  Note: the difference between `view` and `slice` is that `view` produces
   *        a view of the current $coll, whereas `slice` produces a new $coll.
   *
   *  Note: `view(from, to)` is equivalent to `view.slice(from, to)`
   *  $orderDependent
   *
   *  @param from   the index of the first element of the view
   *  @param until  the index of the element following the view
   *  @return a non-strict view of a slice of this $coll, starting at index `from`
   *  and extending up to (but not including) index `until`.
   */
  def view(from: Int, until: Int): TraversableView[A, Repr] = view.slice(from, until)

  /** Creates a non-strict filter of this $coll.
   *
   *  Note: the difference between `c filter p` and `c withFilter p` is that
   *        the former creates a new collection, whereas the latter only
   *        restricts the domain of subsequent `map`, `flatMap`, `foreach`,
   *        and `withFilter` operations.
   *  $orderDependent
   *
   *  @param p   the predicate used to test elements.
   *  @return    an object of class `WithFilter`, which supports
   *             `map`, `flatMap`, `foreach`, and `withFilter` operations.
   *             All these operations apply to those elements of this $coll
   *             which satisfy the predicate `p`.
   */
  def withFilter(p: A => Boolean): FilterMonadic[A, Repr] = new WithFilter(p)

  /** A class supporting filtered operations. Instances of this class are
   *  returned by method `withFilter`.
   */
  class WithFilter(p: A => Boolean) extends FilterMonadic[A, Repr] {

    /** Builds a new collection by applying a function to all elements of the
     *  outer $coll containing this `WithFilter` instance that satisfy predicate `p`.
     *
     *  @param f      the function to apply to each element.
     *  @tparam B     the element type of the returned collection.
     *  @tparam That  $thatinfo
     *  @param bf     $bfinfo
     *  @return       a new collection of type `That` resulting from applying
     *                the given function `f` to each element of the outer $coll
     *                that satisfies predicate `p` and collecting the results.
     *
     *  @usecase def map[B](f: A => B): $Coll[B]
     *    @inheritdoc
     *
     *    @return       a new $coll resulting from applying the given function
     *                  `f` to each element of the outer $coll that satisfies
     *                  predicate `p` and collecting the results.
     */
    def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
      val b = bf(repr)
      for (x <- self)
        if (p(x)) b += f(x)
      b.result
    }

    /** Builds a new collection by applying a function to all elements of the
     *  outer $coll containing this `WithFilter` instance that satisfy
     *  predicate `p` and concatenating the results.
     *
     *  @param f      the function to apply to each element.
     *  @tparam B     the element type of the returned collection.
     *  @tparam That  $thatinfo
     *  @param bf     $bfinfo
     *  @return       a new collection of type `That` resulting from applying
     *                the given collection-valued function `f` to each element
     *                of the outer $coll that satisfies predicate `p` and
     *                concatenating the results.
     *
     *  @usecase def flatMap[B](f: A => TraversableOnce[B]): $Coll[B]
     *    @inheritdoc
     *
     *    The type of the resulting collection will be guided by the static type
     *    of the outer $coll.
     *
     *    @return       a new $coll resulting from applying the given
     *                  collection-valued function `f` to each element of the
     *                  outer $coll that satisfies predicate `p` and concatenating
     *                  the results.
     */
    def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
      val b = bf(repr)
      for (x <- self)
        if (p(x)) b ++= f(x).seq
      b.result
    }

    /** Applies a function `f` to all elements of the outer $coll containing
     *  this `WithFilter` instance that satisfy predicate `p`.
     *
     *  @param  f   the function that is applied for its side-effect to every element.
     *              The result of function `f` is discarded.
     *
     *  @tparam  U  the type parameter describing the result of function `f`.
     *              This result will always be ignored. Typically `U` is `Unit`,
     *              but this is not necessary.
     *
     *  @usecase def foreach(f: A => Unit): Unit
     *    @inheritdoc
     */
    def foreach[U](f: A => U): Unit =
      for (x <- self)
        if (p(x)) f(x)

    /** Further refines the filter for this $coll.
     *
     *  @param q   the predicate used to test elements.
     *  @return    an object of class `WithFilter`, which supports
     *             `map`, `flatMap`, `foreach`, and `withFilter` operations.
     *             All these operations apply to those elements of this $coll which
     *             satisfy the predicate `q` in addition to the predicate `p`.
     */
    def withFilter(q: A => Boolean): WithFilter =
      new WithFilter(x => p(x) && q(x))
  }

  // A helper for tails and inits.
  private def iterateUntilEmpty(f: Traversable[A @uV] => Traversable[A @uV]): Iterator[Repr] = {
    val it = Iterator.iterate(thisCollection)(f) takeWhile (x => !x.isEmpty)
    it ++ Iterator(Nil) map (x => (newBuilder ++= x).result)
  }
}