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31 /* __ *\
32 ** ________ ___ / / ___ Scala API **
33 ** / __/ __// _ | / / / _ | (c) 2003-2011, LAMP/EPFL **
34 ** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
35 ** /____/\___/_/ |_/____/_/ | | **
36 ** |/ **
37 \* */
38
39 package scala.collection
40
41 import generic._
42 import mutable.{ Builder, ListBuffer }
43 import annotation.{tailrec, migration, bridge}
44 import annotation.unchecked.{ uncheckedVariance => uV }
45 import parallel.ParIterable
46
47 /** A template trait for traversable collections of type `Traversable[A]`.
48 *
49 * $traversableInfo
50 * @define mutability
51 * @define traversableInfo
52 * This is a base trait of all kinds of $mutability Scala collections. It
53 * implements the behavior common to all collections, in terms of a method
54 * `foreach` with signature:
55 * {{{
56 * def foreach[U](f: Elem => U): Unit
57 * }}}
58 * Collection classes mixing in this trait provide a concrete
59 * `foreach` method which traverses all the
60 * elements contained in the collection, applying a given function to each.
61 * They also need to provide a method `newBuilder`
62 * which creates a builder for collections of the same kind.
63 *
64 * A traversable class might or might not have two properties: strictness
65 * and orderedness. Neither is represented as a type.
66 *
67 * The instances of a strict collection class have all their elements
68 * computed before they can be used as values. By contrast, instances of
69 * a non-strict collection class may defer computation of some of their
70 * elements until after the instance is available as a value.
71 * A typical example of a non-strict collection class is a
72 * <a href="../immutable/Stream.html" target="ContentFrame">
73 * `scala.collection.immutable.Stream`</a>.
74 * A more general class of examples are `TraversableViews`.
75 *
76 * If a collection is an instance of an ordered collection class, traversing
77 * its elements with `foreach` will always visit elements in the
78 * same order, even for different runs of the program. If the class is not
79 * ordered, `foreach` can visit elements in different orders for
80 * different runs (but it will keep the same order in the same run).'
81 *
82 * A typical example of a collection class which is not ordered is a
83 * `HashMap` of objects. The traversal order for hash maps will
84 * depend on the hash codes of its elements, and these hash codes might
85 * differ from one run to the next. By contrast, a `LinkedHashMap`
86 * is ordered because it's `foreach` method visits elements in the
87 * order they were inserted into the `HashMap`.
88 *
89 * @author Martin Odersky
90 * @version 2.8
91 * @since 2.8
92 * @tparam A the element type of the collection
93 * @tparam Repr the type of the actual collection containing the elements.
94 *
95 * @define Coll Traversable
96 * @define coll traversable collection
97 */
98 trait TraversableLike[+A, +Repr] extends HasNewBuilder[A, Repr]
99 with FilterMonadic[A, Repr]
100 with TraversableOnce[A]
101 with GenTraversableLike[A, Repr]
102 with Parallelizable[A, ParIterable[A]]
103 {
104 self =>
105
106 import Traversable.breaks._
107
108 /** The type implementing this traversable */
109 protected type Self = Repr
110
111 /** The collection of type $coll underlying this `TraversableLike` object.
112 * By default this is implemented as the `TraversableLike` object itself,
113 * but this can be overridden.
114 */
115 def repr: Repr = this.asInstanceOf[Repr]
116
117 /** The underlying collection seen as an instance of `$Coll`.
118 * By default this is implemented as the current collection object itself,
119 * but this can be overridden.
120 */
121 protected[this] def thisCollection: Traversable[A] = this.asInstanceOf[Traversable[A]]
122
123 /** A conversion from collections of type `Repr` to `$Coll` objects.
124 * By default this is implemented as just a cast, but this can be overridden.
125 */
126 protected[this] def toCollection(repr: Repr): Traversable[A] = repr.asInstanceOf[Traversable[A]]
127
128 /** Creates a new builder for this collection type.
129 */
130 protected[this] def newBuilder: Builder[A, Repr]
131
132 protected[this] def parCombiner = ParIterable.newCombiner[A]
133
134 /** Applies a function `f` to all elements of this $coll.
135 *
136 * Note: this method underlies the implementation of most other bulk operations.
137 * It's important to implement this method in an efficient way.
138 *
139 *
140 * @param f the function that is applied for its side-effect to every element.
141 * The result of function `f` is discarded.
142 *
143 * @tparam U the type parameter describing the result of function `f`.
144 * This result will always be ignored. Typically `U` is `Unit`,
145 * but this is not necessary.
146 *
147 * @usecase def foreach(f: A => Unit): Unit
148 */
149 def foreach[U](f: A => U): Unit
150
151 /** Tests whether this $coll is empty.
152 *
153 * @return `true` if the $coll contain no elements, `false` otherwise.
154 */
155 def isEmpty: Boolean = {
156 var result = true
157 breakable {
158 for (x <- this) {
159 result = false
160 break
161 }
162 }
163 result
164 }
165
166 /** Tests whether this $coll is known to have a finite size.
167 * All strict collections are known to have finite size. For a non-strict collection
168 * such as `Stream`, the predicate returns `true` if all elements have been computed.
169 * It returns `false` if the stream is not yet evaluated to the end.
170 *
171 * Note: many collection methods will not work on collections of infinite sizes.
172 *
173 * @return `true` if this collection is known to have finite size, `false` otherwise.
174 */
175 def hasDefiniteSize = true
176
177 def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
178 val b = bf(repr)
179 if (that.isInstanceOf[IndexedSeqLike[_, _]]) b.sizeHint(this, that.seq.size)
180 b ++= thisCollection
181 b ++= that.seq
182 b.result
183 }
184
185 @bridge
186 def ++[B >: A, That](that: TraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That =
187 ++(that: GenTraversableOnce[B])(bf)
188
189 /** Concatenates this $coll with the elements of a traversable collection.
190 * It differs from ++ in that the right operand determines the type of the
191 * resulting collection rather than the left one.
192 *
193 * @param that the traversable to append.
194 * @tparam B the element type of the returned collection.
195 * @tparam That $thatinfo
196 * @param bf $bfinfo
197 * @return a new collection of type `That` which contains all elements
198 * of this $coll followed by all elements of `that`.
199 *
200 * @usecase def ++:[B](that: TraversableOnce[B]): $Coll[B]
201 *
202 * @return a new $coll which contains all elements of this $coll
203 * followed by all elements of `that`.
204 */
205 def ++:[B >: A, That](that: TraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
206 val b = bf(repr)
207 if (that.isInstanceOf[IndexedSeqLike[_, _]]) b.sizeHint(this, that.size)
208 b ++= that
209 b ++= thisCollection
210 b.result
211 }
212
213 /** This overload exists because: for the implementation of ++: we should reuse
214 * that of ++ because many collections override it with more efficient versions.
215 * Since TraversableOnce has no '++' method, we have to implement that directly,
216 * but Traversable and down can use the overload.
217 */
218 def ++:[B >: A, That](that: Traversable[B])(implicit bf: CanBuildFrom[Repr, B, That]): That =
219 (that ++ seq)(breakOut)
220
221 def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
222 val b = bf(repr)
223 b.sizeHint(this)
224 for (x <- this) b += f(x)
225 b.result
226 }
227
228 def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
229 val b = bf(repr)
230 for (x <- this) b ++= f(x).seq
231 b.result
232 }
233
234 /** Selects all elements of this $coll which satisfy a predicate.
235 *
236 * @param p the predicate used to test elements.
237 * @return a new $coll consisting of all elements of this $coll that satisfy the given
238 * predicate `p`. The order of the elements is preserved.
239 */
240 def filter(p: A => Boolean): Repr = {
241 val b = newBuilder
242 for (x <- this)
243 if (p(x)) b += x
244 b.result
245 }
246
247 /** Selects all elements of this $coll which do not satisfy a predicate.
248 *
249 * @param p the predicate used to test elements.
250 * @return a new $coll consisting of all elements of this $coll that do not satisfy the given
251 * predicate `p`. The order of the elements is preserved.
252 */
253 def filterNot(p: A => Boolean): Repr = filter(!p(_))
254
255 def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
256 val b = bf(repr)
257 for (x <- this) if (pf.isDefinedAt(x)) b += pf(x)
258 b.result
259 }
260
261 /** Builds a new collection by applying an option-valued function to all
262 * elements of this $coll on which the function is defined.
263 *
264 * @param f the option-valued function which filters and maps the $coll.
265 * @tparam B the element type of the returned collection.
266 * @tparam That $thatinfo
267 * @param bf $bfinfo
268 * @return a new collection of type `That` resulting from applying the option-valued function
269 * `f` to each element and collecting all defined results.
270 * The order of the elements is preserved.
271 *
272 * @usecase def filterMap[B](f: A => Option[B]): $Coll[B]
273 *
274 * @param pf the partial function which filters and maps the $coll.
275 * @return a new $coll resulting from applying the given option-valued function
276 * `f` to each element and collecting all defined results.
277 * The order of the elements is preserved.
278 def filterMap[B, That](f: A => Option[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
279 val b = bf(repr)
280 for (x <- this)
281 f(x) match {
282 case Some(y) => b += y
283 case _ =>
284 }
285 b.result
286 }
287 */
288
289 /** Partitions this $coll in two ${coll}s according to a predicate.
290 *
291 * @param p the predicate on which to partition.
292 * @return a pair of ${coll}s: the first $coll consists of all elements that
293 * satisfy the predicate `p` and the second $coll consists of all elements
294 * that don't. The relative order of the elements in the resulting ${coll}s
295 * is the same as in the original $coll.
296 */
297 def partition(p: A => Boolean): (Repr, Repr) = {
298 val l, r = newBuilder
299 for (x <- this) (if (p(x)) l else r) += x
300 (l.result, r.result)
301 }
302
303 def groupBy[K](f: A => K): immutable.Map[K, Repr] = {
304 val m = mutable.Map.empty[K, Builder[A, Repr]]
305 for (elem <- this) {
306 val key = f(elem)
307 val bldr = m.getOrElseUpdate(key, newBuilder)
308 bldr += elem
309 }
310 val b = immutable.Map.newBuilder[K, Repr]
311 for ((k, v) <- m)
312 b += ((k, v.result))
313
314 b.result
315 }
316
317 /** Tests whether a predicate holds for all elements of this $coll.
318 *
319 * $mayNotTerminateInf
320 *
321 * @param p the predicate used to test elements.
322 * @return `true` if the given predicate `p` holds for all elements
323 * of this $coll, otherwise `false`.
324 */
325 def forall(p: A => Boolean): Boolean = {
326 var result = true
327 breakable {
328 for (x <- this)
329 if (!p(x)) { result = false; break }
330 }
331 result
332 }
333
334 /** Tests whether a predicate holds for some of the elements of this $coll.
335 *
336 * $mayNotTerminateInf
337 *
338 * @param p the predicate used to test elements.
339 * @return `true` if the given predicate `p` holds for some of the
340 * elements of this $coll, otherwise `false`.
341 */
342 def exists(p: A => Boolean): Boolean = {
343 var result = false
344 breakable {
345 for (x <- this)
346 if (p(x)) { result = true; break }
347 }
348 result
349 }
350
351 /** Finds the first element of the $coll satisfying a predicate, if any.
352 *
353 * $mayNotTerminateInf
354 * $orderDependent
355 *
356 * @param p the predicate used to test elements.
357 * @return an option value containing the first element in the $coll
358 * that satisfies `p`, or `None` if none exists.
359 */
360 def find(p: A => Boolean): Option[A] = {
361 var result: Option[A] = None
362 breakable {
363 for (x <- this)
364 if (p(x)) { result = Some(x); break }
365 }
366 result
367 }
368
369 def scan[B >: A, That](z: B)(op: (B, B) => B)(implicit cbf: CanBuildFrom[Repr, B, That]): That = scanLeft(z)(op)
370
371 def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
372 val b = bf(repr)
373 b.sizeHint(this, 1)
374 var acc = z
375 b += acc
376 for (x <- this) { acc = op(acc, x); b += acc }
377 b.result
378 }
379
380 @migration(2, 9,
381 "This scanRight definition has changed in 2.9.\n" +
382 "The previous behavior can be reproduced with scanRight.reverse."
383 )
384 def scanRight[B, That](z: B)(op: (A, B) => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
385 var scanned = List(z)
386 var acc = z
387 for (x <- reversed) {
388 acc = op(x, acc)
389 scanned ::= acc
390 }
391 val b = bf(repr)
392 for (elem <- scanned) b += elem
393 b.result
394 }
395
396 /** Selects the first element of this $coll.
397 * $orderDependent
398 * @return the first element of this $coll.
399 * @throws `NoSuchElementException` if the $coll is empty.
400 */
401 def head: A = {
402 var result: () => A = () => throw new NoSuchElementException
403 breakable {
404 for (x <- this) {
405 result = () => x
406 break
407 }
408 }
409 result()
410 }
411
412 /** Optionally selects the first element.
413 * $orderDependent
414 * @return the first element of this $coll if it is nonempty, `None` if it is empty.
415 */
416 def headOption: Option[A] = if (isEmpty) None else Some(head)
417
418 /** Selects all elements except the first.
419 * $orderDependent
420 * @return a $coll consisting of all elements of this $coll
421 * except the first one.
422 * @throws `UnsupportedOperationException` if the $coll is empty.
423 */
424 override def tail: Repr = {
425 if (isEmpty) throw new UnsupportedOperationException("empty.tail")
426 drop(1)
427 }
428
429 /** Selects the last element.
430 * $orderDependent
431 * @return The last element of this $coll.
432 * @throws NoSuchElementException If the $coll is empty.
433 */
434 def last: A = {
435 var lst = head
436 for (x <- this)
437 lst = x
438 lst
439 }
440
441 /** Optionally selects the last element.
442 * $orderDependent
443 * @return the last element of this $coll$ if it is nonempty, `None` if it is empty.
444 */
445 def lastOption: Option[A] = if (isEmpty) None else Some(last)
446
447 /** Selects all elements except the last.
448 * $orderDependent
449 * @return a $coll consisting of all elements of this $coll
450 * except the last one.
451 * @throws `UnsupportedOperationException` if the $coll is empty.
452 */
453 def init: Repr = {
454 if (isEmpty) throw new UnsupportedOperationException("empty.init")
455 var lst = head
456 var follow = false
457 val b = newBuilder
458 b.sizeHint(this, -1)
459 for (x <- this.seq) {
460 if (follow) b += lst
461 else follow = true
462 lst = x
463 }
464 b.result
465 }
466
467 def take(n: Int): Repr = slice(0, n)
468
469 def drop(n: Int): Repr =
470 if (n <= 0) {
471 val b = newBuilder
472 b.sizeHint(this)
473 b ++= thisCollection result
474 }
475 else sliceWithKnownDelta(n, Int.MaxValue, -n)
476
477 def slice(from: Int, until: Int): Repr = sliceWithKnownBound(math.max(from, 0), until)
478
479 // Precondition: from >= 0, until > 0, builder already configured for building.
480 private[this] def sliceInternal(from: Int, until: Int, b: Builder[A, Repr]): Repr = {
481 var i = 0
482 breakable {
483 for (x <- this.seq) {
484 if (i >= from) b += x
485 i += 1
486 if (i >= until) break
487 }
488 }
489 b.result
490 }
491 // Precondition: from >= 0
492 private[scala] def sliceWithKnownDelta(from: Int, until: Int, delta: Int): Repr = {
493 val b = newBuilder
494 if (until <= from) b.result
495 else {
496 b.sizeHint(this, delta)
497 sliceInternal(from, until, b)
498 }
499 }
500 // Precondition: from >= 0
501 private[scala] def sliceWithKnownBound(from: Int, until: Int): Repr = {
502 val b = newBuilder
503 if (until <= from) b.result
504 else {
505 b.sizeHintBounded(until - from, this)
506 sliceInternal(from, until, b)
507 }
508 }
509
510 def takeWhile(p: A => Boolean): Repr = {
511 val b = newBuilder
512 breakable {
513 for (x <- this) {
514 if (!p(x)) break
515 b += x
516 }
517 }
518 b.result
519 }
520
521 def dropWhile(p: A => Boolean): Repr = {
522 val b = newBuilder
523 var go = false
524 for (x <- this) {
525 if (!p(x)) go = true
526 if (go) b += x
527 }
528 b.result
529 }
530
531 def span(p: A => Boolean): (Repr, Repr) = {
532 val l, r = newBuilder
533 var toLeft = true
534 for (x <- this) {
535 toLeft = toLeft && p(x)
536 (if (toLeft) l else r) += x
537 }
538 (l.result, r.result)
539 }
540
541 def splitAt(n: Int): (Repr, Repr) = {
542 val l, r = newBuilder
543 l.sizeHintBounded(n, this)
544 if (n >= 0) r.sizeHint(this, -n)
545 var i = 0
546 for (x <- this) {
547 (if (i < n) l else r) += x
548 i += 1
549 }
550 (l.result, r.result)
551 }
552
553 /** Iterates over the tails of this $coll. The first value will be this
554 * $coll and the final one will be an empty $coll, with the intervening
555 * values the results of successive applications of `tail`.
556 *
557 * @return an iterator over all the tails of this $coll
558 * @example `List(1,2,3).tails = Iterator(List(1,2,3), List(2,3), List(3), Nil)`
559 */
560 def tails: Iterator[Repr] = iterateUntilEmpty(_.tail)
561
562 /** Iterates over the inits of this $coll. The first value will be this
563 * $coll and the final one will be an empty $coll, with the intervening
564 * values the results of successive applications of `init`.
565 *
566 * @return an iterator over all the inits of this $coll
567 * @example `List(1,2,3).inits = Iterator(List(1,2,3), List(1,2), List(1), Nil)`
568 */
569 def inits: Iterator[Repr] = iterateUntilEmpty(_.init)
570
571 /** Copies elements of this $coll to an array.
572 * Fills the given array `xs` with at most `len` elements of
573 * this $coll, starting at position `start`.
574 * Copying will stop once either the end of the current $coll is reached,
575 * or the end of the array is reached, or `len` elements have been copied.
576 *
577 * $willNotTerminateInf
578 *
579 * @param xs the array to fill.
580 * @param start the starting index.
581 * @param len the maximal number of elements to copy.
582 * @tparam B the type of the elements of the array.
583 *
584 *
585 * @usecase def copyToArray(xs: Array[A], start: Int, len: Int): Unit
586 */
587 def copyToArray[B >: A](xs: Array[B], start: Int, len: Int) {
588 var i = start
589 val end = (start + len) min xs.length
590 breakable {
591 for (x <- this) {
592 if (i >= end) break
593 xs(i) = x
594 i += 1
595 }
596 }
597 }
598
599 def toTraversable: Traversable[A] = thisCollection
600 def toIterator: Iterator[A] = toStream.iterator
601 def toStream: Stream[A] = toBuffer.toStream
602
603 /** Converts this $coll to a string.
604 *
605 * @return a string representation of this collection. By default this
606 * string consists of the `stringPrefix` of this $coll,
607 * followed by all elements separated by commas and enclosed in parentheses.
608 */
609 override def toString = mkString(stringPrefix + "(", ", ", ")")
610
611 /** Defines the prefix of this object's `toString` representation.
612 *
613 * @return a string representation which starts the result of `toString`
614 * applied to this $coll. By default the string prefix is the
615 * simple name of the collection class $coll.
616 */
617 def stringPrefix : String = {
618 var string = repr.asInstanceOf[AnyRef].getClass.getName
619 val idx1 = string.lastIndexOf('.' : Int)
620 if (idx1 != -1) string = string.substring(idx1 + 1)
621 val idx2 = string.indexOf('$')
622 if (idx2 != -1) string = string.substring(0, idx2)
623 string
624 }
625
626 /** Creates a non-strict view of this $coll.
627 *
628 * @return a non-strict view of this $coll.
629 */
630 def view = new TraversableView[A, Repr] {
631 protected lazy val underlying = self.repr
632 override def foreach[U](f: A => U) = self foreach f
633 }
634
635 /** Creates a non-strict view of a slice of this $coll.
636 *
637 * Note: the difference between `view` and `slice` is that `view` produces
638 * a view of the current $coll, whereas `slice` produces a new $coll.
639 *
640 * Note: `view(from, to)` is equivalent to `view.slice(from, to)`
641 * $orderDependent
642 *
643 * @param from the index of the first element of the view
644 * @param until the index of the element following the view
645 * @return a non-strict view of a slice of this $coll, starting at index `from`
646 * and extending up to (but not including) index `until`.
647 */
648 def view(from: Int, until: Int): TraversableView[A, Repr] = view.slice(from, until)
649
650 /** Creates a non-strict filter of this $coll.
651 *
652 * Note: the difference between `c filter p` and `c withFilter p` is that
653 * the former creates a new collection, whereas the latter only
654 * restricts the domain of subsequent `map`, `flatMap`, `foreach`,
655 * and `withFilter` operations.
656 * $orderDependent
657 *
658 * @param p the predicate used to test elements.
659 * @return an object of class `WithFilter`, which supports
660 * `map`, `flatMap`, `foreach`, and `withFilter` operations.
661 * All these operations apply to those elements of this $coll which
662 * satisfy the predicate `p`.
663 */
664 def withFilter(p: A => Boolean): FilterMonadic[A, Repr] = new WithFilter(p)
665
666 /** A class supporting filtered operations. Instances of this class are
667 * returned by method `withFilter`.
668 */
669 class WithFilter(p: A => Boolean) extends FilterMonadic[A, Repr] {
670
671 /** Builds a new collection by applying a function to all elements of the
672 * outer $coll containing this `WithFilter` instance that satisfy predicate `p`.
673 *
674 * @param f the function to apply to each element.
675 * @tparam B the element type of the returned collection.
676 * @tparam That $thatinfo
677 * @param bf $bfinfo
678 * @return a new collection of type `That` resulting from applying
679 * the given function `f` to each element of the outer $coll
680 * that satisfies predicate `p` and collecting the results.
681 *
682 * @usecase def map[B](f: A => B): $Coll[B]
683 *
684 * @return a new $coll resulting from applying the given function
685 * `f` to each element of the outer $coll that satisfies
686 * predicate `p` and collecting the results.
687 */
688 def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
689 val b = bf(repr)
690 for (x <- self)
691 if (p(x)) b += f(x)
692 b.result
693 }
694
695 /** Builds a new collection by applying a function to all elements of the
696 * outer $coll containing this `WithFilter` instance that satisfy
697 * predicate `p` and concatenating the results.
698 *
699 * @param f the function to apply to each element.
700 * @tparam B the element type of the returned collection.
701 * @tparam That $thatinfo
702 * @param bf $bfinfo
703 * @return a new collection of type `That` resulting from applying
704 * the given collection-valued function `f` to each element
705 * of the outer $coll that satisfies predicate `p` and
706 * concatenating the results.
707 *
708 * @usecase def flatMap[B](f: A => TraversableOnce[B]): $Coll[B]
709 *
710 * @return a new $coll resulting from applying the given collection-valued function
711 * `f` to each element of the outer $coll that satisfies predicate `p` and concatenating the results.
712 */
713 def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
714 val b = bf(repr)
715 for (x <- self)
716 if (p(x)) b ++= f(x).seq
717 b.result
718 }
719
720 /** Applies a function `f` to all elements of the outer $coll containing
721 * this `WithFilter` instance that satisfy predicate `p`.
722 *
723 * @param f the function that is applied for its side-effect to every element.
724 * The result of function `f` is discarded.
725 *
726 * @tparam U the type parameter describing the result of function `f`.
727 * This result will always be ignored. Typically `U` is `Unit`,
728 * but this is not necessary.
729 *
730 * @usecase def foreach(f: A => Unit): Unit
731 */
732 def foreach[U](f: A => U): Unit =
733 for (x <- self)
734 if (p(x)) f(x)
735
736 /** Further refines the filter for this $coll.
737 *
738 * @param q the predicate used to test elements.
739 * @return an object of class `WithFilter`, which supports
740 * `map`, `flatMap`, `foreach`, and `withFilter` operations.
741 * All these operations apply to those elements of this $coll which
742 * satisfy the predicate `q` in addition to the predicate `p`.
743 */
744 def withFilter(q: A => Boolean): WithFilter =
745 new WithFilter(x => p(x) && q(x))
746 }
747
748 // A helper for tails and inits.
749 private def iterateUntilEmpty(f: Traversable[A @uV] => Traversable[A @uV]): Iterator[Repr] = {
750 val it = Iterator.iterate(thisCollection)(f) takeWhile (x => !x.isEmpty)
751 it ++ Iterator(Nil) map (newBuilder ++= _ result)
752 }
753 }
754
755
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