Scala Library: scala.collection.TraversableOnce
scala.collection.TraversableOnce
A template trait for collections which can be traversed either once only or one or more times.
This trait exists primarily to eliminate code duplication between Iterator
and
Traversable
, and thus implements some of the common methods that can be
implemented solely in terms of foreach without access to a Builder
. It also
includes a number of abstract methods whose implementations are provided by
Iterator
, Traversable
, etc. It contains implementations common to
Iterators
and Traversables
, such as folds, conversions, and other
operations which traverse some or all of the elements and return a derived
value. Directly subclassing TraversableOnce
is not recommended - instead,
consider declaring an Iterator
with a next
and hasNext
method, creating an
Iterator
with one of the methods on the Iterator
object, or declaring a
subclass of Traversable
.
- Self Type
- TraversableOnce [A]
- Source
- Version
- 2.8
- Since
- 2.8
Abstract Value Members From scala.collection.TraversableOnce
abstract def exists(p: (A) ⇒ Boolean): Boolean
Tests whether a predicate holds for at least one element of this traversable or iterator.
Note: may not terminate for infinite-sized collections.
- p
- the predicate used to test elements.
- returns
true
if the given predicatep
is satisfied by at least one element of this traversable or iterator, otherwisefalse
- Definition Classes
- TraversableOnce → GenTraversableOnce
(defined at scala.collection.TraversableOnce)
abstract def find(p: (A) ⇒ Boolean): Option[A]
Finds the first element of the traversable or iterator 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 or iterator
that satisfies
p
, orNone
if none exists.
- an option value containing the first element in the traversable or iterator
that satisfies
- Definition Classes
- TraversableOnce → GenTraversableOnce
(defined at scala.collection.TraversableOnce)
abstract def forall(p: (A) ⇒ Boolean): Boolean
Tests whether a predicate holds for all elements of this traversable or iterator.
Note: may not terminate for infinite-sized collections.
- p
- the predicate used to test elements.
- returns
true
if this traversable or iterator is empty or the given predicatep
holds for all elements of this traversable or iterator, otherwisefalse
.
- Definition Classes
- TraversableOnce → GenTraversableOnce
(defined at scala.collection.TraversableOnce)
abstract def seq: TraversableOnce[A]
A version of this collection with all of the operations implemented sequentially (i.e., in a single-threaded manner).
This method returns a reference to this collection. In parallel collections, it is redefined to return a sequential implementation of this collection. In both cases, it has O(1) complexity.
- returns
- a sequential view of the collection.
- Definition Classes
- TraversableOnce → GenTraversableOnce
(defined at scala.collection.TraversableOnce)
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.
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 valuez
on the left:
- the result of inserting
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.
- 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 valuez
on the right:
- the result of inserting
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:
- b
- the string builder to which elements are appended.
- returns
- the string builder
b
to which elements were appended.
- the string builder
(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:
- b
- the string builder to which elements are appended.
- sep
- the separator string.
- returns
- the string builder
b
to which elements were appended.
- the string builder
(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:
- 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.
- the string builder
(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:
- 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 or0
for summation) and may be evaluated more than once
- the initial value for the accumulated result of the partition - this will
typically be the neutral element for the
- 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.
- an option value containing pf applied to the first value for which it is
defined, or
Example:
(defined at scala.collection.TraversableOnce)
def copyToArray[B >: A](xs: Array[B]): Unit
[use case]
Copies the elements of this traversable or iterator to an array. Fills the given
array xs
with values of this traversable or iterator. Copying will stop once
either the end of the current traversable or iterator 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 or iterator to an array. Fills the given
array xs
with values of this traversable or iterator, beginning at index
start
. Copying will stop once either the end of the current traversable or
iterator 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)
abstract def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Unit
[use case]
Copies the elements of this traversable or iterator to an array. Fills the given
array xs
with at most len
elements of this traversable or iterator, starting
at position start
. Copying will stop once either the end of the current
traversable or iterator 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
- 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.
(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
.
- the number of elements satisfying the predicate
- 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 valuez
on the left:
- the result of inserting
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 valuez
on the right:
- the result of inserting
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
.
- a type parameter for the binary operator, a supertype of
- 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).
- 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.,
- op
- a binary operator that must be associative.
- returns
- the result of applying the fold operator
op
between all the elements andz
, orz
if this traversable or iterator is empty.
- the result of applying the fold operator
- Definition Classes
- TraversableOnce → GenTraversableOnce
(defined at scala.collection.TraversableOnce)
abstract def foreach[U](f: (A) ⇒ U): Unit
[use case]
Applies a function f
to all elements of this traversable or iterator.
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.
- the function that is applied for its side-effect to every element. The
result of function
- 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 or iterator 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 or iterator 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 stringsep
.
- a string representation of this traversable or iterator. In the resulting
string the string representations (w.r.t. the method
- Definition Classes
- TraversableOnce → GenTraversableOnce
Example:
(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 stringend
. Inside, the string representations (w.r.t. the methodtoString
) of all elements of this traversable or iterator are separated by the stringsep
.
- a string representation of this traversable or iterator. The resulting
string begins with the string
- Definition Classes
- TraversableOnce → GenTraversableOnce
Example:
(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.
- an option value containing the result of
- 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:
- the result of inserting
where `x1, ..., xn` are the elements of this traversable or iterator.
- 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
.
- A type parameter for the binary operator, a supertype of
- 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, andNone
otherwise.
- An option value containing result of applying reduce operator
- 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.
- an option value containing the result of
- 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:
- the result of inserting
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
.
- A type parameter for the binary operator, a supertype of
- 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.
- The result of applying reduce operator
- 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 or iterator 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 or iterator.
- a map of type
- 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.TraversableOnce.MonadOps
def filter(p: (A) ⇒ Boolean): TraversableOnce[A]
- Implicit information
- This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
- Definition Classes
- MonadOps
(added by implicit convertion: scala.collection.TraversableOnce.MonadOps)
def flatMap[B](f: (A) ⇒ GenTraversableOnce[B]): TraversableOnce[B]
- Implicit information
- This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
- Definition Classes
- MonadOps
(added by implicit convertion: scala.collection.TraversableOnce.MonadOps)
def map[B](f: (A) ⇒ B): TraversableOnce[B]
- Implicit information
- This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
- Definition Classes
- MonadOps
(added by implicit convertion: scala.collection.TraversableOnce.MonadOps)
def withFilter(p: (A) ⇒ Boolean): Iterator[A]
- Implicit information
- This member is added by an implicit conversion from TraversableOnce [A] to MonadOps [A] performed by method MonadOps in scala.collection.TraversableOnce.
- Definition Classes
- MonadOps (added by implicit convertion: scala.collection.TraversableOnce.MonadOps)
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