【scala初學】Trait Traversable

     Traversable是所有collections的父類

     Traversable是所有集合層級的最頂端的，他只有一個抽象方法， foreach:

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

     實現Traversable只需要定義這個方法，其他方法可以直接繼承得到。

foreach 方法以爲遍歷集合中的所有元素，並且應用傳入的方法，f , 對每一個元素。 函數（或者方法，操作）的類型是Elem => U, Elem 是集合元素的類型 而 U是一個任意的結果類型.  函數的f的調用是他的一個副作用，事實上函數的結果都會被 foreach方法丟棄。

　　函數分類：Traversable 的方法可以分類如下：

Addition：

         方法名： ++ 

      說明： 同時追加兩個traversable，或者追加一個迭代的所有元素到traversable

Map ： 

      方法名： map, flatMap, collect

        說明： 通過應用函數到集合的元素中，來產生一個新的集合

Conversions :

      方法名：toArray, toList, toIterable, toSeq, toIndexedSeq, toStream, toSet,toMap

   說明： 使得 Traversable 變的更加具體。所有這些轉換返回的接收器的參數不會改變，如果集合的運行時類型已經匹配了要求的集合類型。 如： 在一個列表中應用 toList，將會調用該列表的 toList方法。

Copying operations ：

      方法名 ： copyToBuffer,  copyToArray

   說明：作用如他們的名字

Size info：

      方法名：isEmpty, nonEmpty, size, and hasDefiniteSize: 

   說明：作用如其名,唯一需要說明的是hasDefiniteSize  ，判斷一個集合是否有窮還是無窮

Element retrieval： 

      方法名：head, last, headOption, lastOption, and find.

      說明： 頭尾方法的定義.  但是有些集合不能很好的定義頭尾方法。 如，hash存儲是key，value， 又如某些運行時會不斷改變元素順序的。 

Sub-collection retrieval operations ：

      方法名: tail, init, slice, take, drop, takeWhile,dropWhile, filter, filterNot, withFilter. 

   說明：返回子集合

Subdivision operations : 

      方法名: plitAt, span, partition, groupBy

   說明：分割集合成幾個子集合




Element tests: 

     方法名：exists, forall, count

     說明：測試斷言(書生：即方法做的事情)是否成立




Folds ：

     方法名：foldLeft, foldRight, /:, :\, reduceLeft, reduceRight

     說明：用二進制的方式操作連續的元素




Specific folds ： 

     方法名: sum, product, min, max    

     說明：用於特定類型，如數字




String ：

    方法名： mkString, addString, stringPrefix

  說明：轉換一個集合到字符串的方式

View：

     方法名： view

   說明：這個方法將單獨介紹

Class Traversable中的方法

WHAT IT IS	WHAT IT DOES
Abstract Method:
xs foreach f	Executes function f for every element of xs.
Addition:
xs ++ ys	A collection consisting of the elements of both xs and ys. ys is a TraversableOnce collection, i.e., either a Traversable or an Iterator.
Maps:
xs map f	The collection obtained from applying the function f to every element in xs.
xs flatMap f	The collection obtained from applying the collection-valued function f to every element in xs and concatenating the results.
xs collect f	The collection obtained from applying the partial function f to every element in xs for which it is defined and collecting the results.
Conversions:
xs.toArray	Converts the collection to an array.
xs.toList	Converts the collection to a list.
xs.toIterable	Converts the collection to an iterable.
xs.toSeq	Converts the collection to a sequence.
xs.toIndexedSeq	Converts the collection to an indexed sequence.
xs.toStream	Converts the collection to a lazily computed stream.
xs.toSet	Converts the collection to a set.
xs.toMap	Converts the collection of key/value pairs to a map. If the collection does not have pairs as elements, calling this operation results in a static type error.
Copying:
xs copyToBuffer buf	Copies all elements of the collection to buffer buf.
xs copyToArray(arr, s, n)	Copies at most n elements of the collection to array arr starting at index s. The last two arguments are optional.
Size info:
xs.isEmpty	Tests whether the collection is empty.
xs.nonEmpty	Tests whether the collection contains elements.
xs.size	The number of elements in the collection.
xs.hasDefiniteSize	True if xs is known to have finite size.
Element Retrieval:
xs.head	The first element of the collection (or, some element, if no order is defined).
xs.headOption	The first element of xs in an option value, or None if xs is empty.
xs.last	The last element of the collection (or, some element, if no order is defined).
xs.lastOption	The last element of xs in an option value, or None if xs is empty.
xs find p	An option containing the first element in xs that satisfies p, or None if no element qualifies.
Subcollections:
xs.tail	The rest of the collection except xs.head.
xs.init	The rest of the collection except xs.last.
xs slice (from, to)	A collection consisting of elements in some index range of xs (from from up to, and excluding to).
xs take n	A collection consisting of the first n elements of xs (or, some arbitrary n elements, if no order is defined).
xs drop n	The rest of the collection except xs take n.
xs takeWhile p	The longest prefix of elements in the collection that all satisfy p.
xs dropWhile p	The collection without the longest prefix of elements that all satisfy p.
xs filter p	The collection consisting of those elements of xs that satisfy the predicate p.
xs withFilter p	A non-strict filter of this collection. Subsequent calls to map, flatMap, foreach, and withFilter will only apply to those elements of xs for which the condition p is true.
xs filterNot p	The collection consisting of those elements of xs that do not satisfy the predicate p.
Subdivisions:
xs splitAt n	Split xs at a position, giving the pair of collections (xs take n, xs drop n).
xs span p	Split xs according to a predicate, giving the pair of collections (xs takeWhile p, xs.dropWhile p).
xs partition p	Split xs into a pair of two collections; one with elements that satisfy the predicate p, the other with elements that do not, giving the pair of collections (xs filter p, xs.filterNot p)
xs groupBy f	Partition xs into a map of collections according to a discriminator function f.
Element Conditions:
xs forall p	A boolean indicating whether the predicate p holds for all elements of xs.
xs exists p	A boolean indicating whether the predicate p holds for some element in xs.
xs count p	The number of elements in xs that satisfy the predicate p.
Folds:
(z /: xs)(op)	Apply binary operation op between successive elements of xs, going left to right and starting with z.
(xs :\ z)(op)	Apply binary operation op between successive elements of xs, going right to left and starting with z.
xs.foldLeft(z)(op)	Same as (z /: xs)(op).
xs.foldRight(z)(op)	Same as (xs :\ z)(op).
xs reduceLeft op	Apply binary operation op between successive elements of non-empty collection xs, going left to right.
xs reduceRight op	Apply binary operation op between successive elements of non-empty collection xs, going right to left.
Specific Folds:
xs.sum	The sum of the numeric element values of collection xs.
xs.product	The product of the numeric element values of collection xs.
xs.min	The minimum of the ordered element values of collection xs.
xs.max	The maximum of the ordered element values of collection xs.
Strings:
xs addString (b, start, sep, end)	Adds a string to StringBuilder b that shows all elements of xs between separators sep enclosed in strings start and end. start, sep, end are all optional.
xs mkString (start, sep, end)	Converts the collection to a string that shows all elements of xs between separators sep enclosed in strings start and end. start, sep, end are all optional.
xs.stringPrefix	The collection name at the beginning of the string returned from xs.toString.
Views:
xs.view	Produces a view over xs.
xs view (from, to)	Produces a view that represents the elements in some index range of xs.