scala符号


Scala是数据挖掘算法领域最有力的编程语言之一，语言本身是面向函数，这也符合了数据挖掘算法的常用场景：在原始数据集上应用一系列的变换，语言本身也对集合操作提供了众多强大的函数，本文将以List类型为例子，介绍常见的集合变换操作。

一、常用操作符（操作符其实也是函数）

++ ++[B](that: GenTraversableOnce[B]): List[B] 从列表的尾部添加另外一个列表

++: ++:[B >: A, That](that: collection.Traversable[B])(implicit bf: CanBuildFrom[List[A], B, That]): That 在列表的头部添加一个列表

+: +:(elem: A): List[A] 在列表的头部添加一个元素

:+ :+(elem: A): List[A] 在列表的尾部添加一个元素

:: ::(x: A): List[A] 在列表的头部添加一个元素

::: :::(prefix: List[A]): List[A] 在列表的头部添加另外一个列表

:\ :[B](z: B)(op: (A, B) ⇒ B): B 与foldRight等价

<code class="hljs livecodeserver has-numbering">val left = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>)
val <span class="hljs-constant">right</span> = List(<span class="hljs-number">4</span>,<span class="hljs-number">5</span>,<span class="hljs-number">6</span>)
<span class="hljs-comment">
//以下操作等价</span>
left ++ <span class="hljs-constant">right</span>  <span class="hljs-comment"> // List(1,2,3,4,5,6)</span>
left ++: <span class="hljs-constant">right</span> <span class="hljs-comment"> // List(1,2,3,4,5,6)</span>
<span class="hljs-constant">right</span>.++:(left)   <span class="hljs-comment"> // List(1,2,3,4,5,6)</span>
<span class="hljs-constant">right</span>.:::(left) <span class="hljs-comment"> // List(1,2,3,4,5,6)</span>
<span class="hljs-comment">
//以下操作等价</span>
<span class="hljs-number">0</span> +: left   <span class="hljs-comment"> //List(0,1,2,3)</span>
left.+:(<span class="hljs-number">0</span>)  <span class="hljs-comment"> //List(0,1,2,3)</span>
<span class="hljs-comment">
//以下操作等价</span>
left :+ <span class="hljs-number">4</span>   <span class="hljs-comment"> //List(1,2,3,4)</span>
left.:+(<span class="hljs-number">4</span>)  <span class="hljs-comment"> //List(1,2,3,4)</span>
<span class="hljs-comment">
//以下操作等价</span>
<span class="hljs-number">0</span> :: left     <span class="hljs-comment"> //List(0,1,2,3)</span>
left.::(<span class="hljs-number">0</span>)    <span class="hljs-comment"> //List(0,1,2,3)</span></code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li></ul>

看到这里大家应该跟我一样有一点晕吧，怎么这么多奇怪的操作符，这里给大家一个提示，任何以冒号结果的操作符，都是右绑定的，即 0 :: List(1,2,3) = List(1,2,3).::(0) = List(0,1,2,3) 从这里可以看出操作::其实是右边List的操作符，而非左边Int类型的操作符

二、常用变换操作

1.map

map[B](f: (A) ⇒ B): List[B]

定义一个变换,把该变换应用到列表的每个元素中,原列表不变，返回一个新的列表数据

Example1 平方变换

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>)
<span class="hljs-keyword">val</span> square = (x: Int) => x*x
<span class="hljs-keyword">val</span> squareNums1 = nums.map(num => num*num)    <span class="hljs-comment">//List(1,4,9)</span>
<span class="hljs-keyword">val</span> squareNums2 = nums.map(math.pow(_,<span class="hljs-number">2</span>))    <span class="hljs-comment">//List(1,4,9)</span>
<span class="hljs-keyword">val</span> squareNums3 = nums.map(square)            <span class="hljs-comment">//List(1,4,9)</span></code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li></ul>

Example2 保存文本数据中的某几列

<code class="hljs livecodeserver has-numbering">val <span class="hljs-keyword">text</span> = List(<span class="hljs-string">"Homeway,25,Male"</span>,<span class="hljs-string">"XSDYM,23,Female"</span>)
val usersList = <span class="hljs-keyword">text</span>.map(_.<span class="hljs-built_in">split</span>(<span class="hljs-string">","</span>)(<span class="hljs-number">0</span>))
val usersWithAgeList = <span class="hljs-keyword">text</span>.map(<span class="hljs-built_in">line</span> => {
val fields = <span class="hljs-built_in">line</span>.<span class="hljs-built_in">split</span>(<span class="hljs-string">","</span>)
val user = fields(<span class="hljs-number">0</span>)
val age = fields(<span class="hljs-number">1</span>).toInt
(user,age)
})
</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li></ul>

2.flatMap, flatten

flatten: flatten[B]: List[B] 对列表的列表进行平坦化操作 flatMap: flatMap[B](f: (A) ⇒ GenTraversableOnce[B]): List[B] map之后对结果进行flatten

定义一个变换f, 把f应用列表的每个元素中，每个f返回一个列表，最终把所有列表连结起来。

<code class="hljs avrasm has-numbering">val text = List(<span class="hljs-string">"A,B,C"</span>,<span class="hljs-string">"D,E,F"</span>)
val textMapped = text<span class="hljs-preprocessor">.map</span>(_<span class="hljs-preprocessor">.split</span>(<span class="hljs-string">","</span>)<span class="hljs-preprocessor">.toList</span>) // List(List(<span class="hljs-string">"A"</span>,<span class="hljs-string">"B"</span>,<span class="hljs-string">"C"</span>),List(<span class="hljs-string">"D"</span>,<span class="hljs-string">"E"</span>,<span class="hljs-string">"F"</span>))
val textFlattened = textMapped<span class="hljs-preprocessor">.flatten</span>          // List(<span class="hljs-string">"A"</span>,<span class="hljs-string">"B"</span>,<span class="hljs-string">"C"</span>,<span class="hljs-string">"D"</span>,<span class="hljs-string">"E"</span>,<span class="hljs-string">"F"</span>)
val textFlatMapped = text<span class="hljs-preprocessor">.flatMap</span>(_<span class="hljs-preprocessor">.split</span>(<span class="hljs-string">","</span>)<span class="hljs-preprocessor">.toList</span>) // List(<span class="hljs-string">"A"</span>,<span class="hljs-string">"B"</span>,<span class="hljs-string">"C"</span>,<span class="hljs-string">"D"</span>,<span class="hljs-string">"E"</span>,<span class="hljs-string">"F"</span>)
</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li></ul>

3.reduce

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

定义一个变换f, f把两个列表的元素合成一个，遍历列表，最终把列表合并成单一元素

Example 列表求和

<code class="hljs fsharp has-numbering">
<span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>)
<span class="hljs-keyword">val</span> sum1 = nums.reduce((a,b) => a+b)   <span class="hljs-comment">//6</span>
<span class="hljs-keyword">val</span> sum2 = nums.reduce(_+_)            <span class="hljs-comment">//6</span>
<span class="hljs-keyword">val</span> sum3 = nums.sum                 <span class="hljs-comment">//6</span>
</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li></ul>

4.reduceLeft,reduceRight

reduceLeft: reduceLeft[B >: A](f: (B, A) ⇒ B): B

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

reduceLeft从列表的左边往右边应用reduce函数，reduceRight从列表的右边往左边应用reduce函数

Example

<code class="hljs fsharp has-numbering">
<span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">2.0</span>,<span class="hljs-number">2.0</span>,<span class="hljs-number">3.0</span>)
<span class="hljs-keyword">val</span> resultLeftReduce = nums.reduceLeft(math.pow)  <span class="hljs-comment">// = pow( pow(2.0,2.0) , 3.0) = 64.0</span>
<span class="hljs-keyword">val</span> resultRightReduce = nums.reduceRight(math.pow) <span class="hljs-comment">// = pow(2.0, pow(2.0,3.0)) = 256.0</span>
</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li></ul>

5.fold,foldLeft,foldRight

fold: fold[A1 >: A](z: A1)(op: (A1, A1) ⇒ A1): A1 带有初始值的reduce,从一个初始值开始，从左向右将两个元素合并成一个，最终把列表合并成单一元素。

foldLeft: foldLeft[B](z: B)(f: (B, A) ⇒ B): B 带有初始值的reduceLeft

foldRight: foldRight[B](z: B)(op: (A, B) ⇒ B): B 带有初始值的reduceRight

<code class="hljs fsharp has-numbering">
<span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">4</span>)
<span class="hljs-keyword">val</span> sum = nums.fold(<span class="hljs-number">1</span>)(_+_)  <span class="hljs-comment">// = 1+2+3+4 = 9</span>

<span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">2.0</span>,<span class="hljs-number">3.0</span>)
<span class="hljs-keyword">val</span> result1 = nums.foldLeft(<span class="hljs-number">4.0</span>)(math.pow) <span class="hljs-comment">// = pow(pow(4.0,2.0),3.0) = 4096</span>
<span class="hljs-keyword">val</span> result2 = nums.foldRight(<span class="hljs-number">1.0</span>)(math.pow) <span class="hljs-comment">// = pow(1.0,pow(2.0,3.0)) = 8.0</span>
</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li></ul>

6.sortBy,sortWith,sorted

sortBy: sortBy[B](f: (A) ⇒ B)(implicit ord: math.Ordering[B]): List[A] 按照应用函数f之后产生的元素进行排序

sorted： sorted[B >: A](implicit ord: math.Ordering[B]): List[A] 按照元素自身进行排序

sortWith： sortWith(lt: (A, A) ⇒ Boolean): List[A] 使用自定义的比较函数进行排序

<code class="hljs coffeescript has-numbering">val nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">3</span>,<span class="hljs-number">2</span>,<span class="hljs-number">4</span>)
val sorted = nums.sorted  <span class="hljs-regexp">//</span>List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">4</span>)

val users = List((<span class="hljs-string">"HomeWay"</span>,<span class="hljs-number">25</span>),(<span class="hljs-string">"XSDYM"</span>,<span class="hljs-number">23</span>))
val sortedByAge = users.sortBy{<span class="hljs-reserved">case</span><span class="hljs-function"><span class="hljs-params">(user,age)</span> =></span> age}  <span class="hljs-regexp">//</span>List((<span class="hljs-string">"XSDYM"</span>,<span class="hljs-number">23</span>),(<span class="hljs-string">"HomeWay"</span>,<span class="hljs-number">25</span>))
val sortedWith = users.sortWith{<span class="hljs-reserved">case</span><span class="hljs-function"><span class="hljs-params">(user1,user2)</span> =></span> user1._2 < user2._2} <span class="hljs-regexp">//</span>List((<span class="hljs-string">"XSDYM"</span>,<span class="hljs-number">23</span>),(<span class="hljs-string">"HomeWay"</span>,<span class="hljs-number">25</span>))
</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li></ul>

7.filter, filterNot

filter: filter(p: (A) ⇒ Boolean): List[A]

filterNot: filterNot(p: (A) ⇒ Boolean): List[A]

filter 保留列表中符合条件p的列表元素 ， filterNot，保留列表中不符合条件p的列表元素

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">4</span>)
<span class="hljs-keyword">val</span> odd = nums.filter( _ % <span class="hljs-number">2</span> != <span class="hljs-number">0</span>) <span class="hljs-comment">// List(1,3)</span>
<span class="hljs-keyword">val</span> even = nums.filterNot( _ % <span class="hljs-number">2</span> != <span class="hljs-number">0</span>) <span class="hljs-comment">// List(2,4)</span>
</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li></ul>

8.count

count(p: (A) ⇒ Boolean): Int

计算列表中所有满足条件p的元素的个数，等价于 filter(p).length

val nums = List(-1,-2,0,1,2) val plusCnt1 = nums.count( > 0) val plusCnt2 = nums.filter( > 0).length 

9. diff, union, intersect

diff:diff(that: collection.Seq[A]): List[A] 保存列表中那些不在另外一个列表中的元素，即从集合中减去与另外一个集合的交集

union : union(that: collection.Seq[A]): List[A] 与另外一个列表进行连结

intersect: intersect(that: collection.Seq[A]): List[A] 与另外一个集合的交集

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums1 = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>)
<span class="hljs-keyword">val</span> nums2 = List(<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">4</span>)
<span class="hljs-keyword">val</span> diff1 = nums1 diff nums2   <span class="hljs-comment">// List(1)</span>
<span class="hljs-keyword">val</span> diff2 = nums2.diff(num1)   <span class="hljs-comment">// List(4)</span>
<span class="hljs-keyword">val</span> union1 = nums1 union nums2  <span class="hljs-comment">// List(1,2,3,2,3,4)</span>
<span class="hljs-keyword">val</span> union2 = nums2 ++ nums1        <span class="hljs-comment">// List(2,3,4,1,2,3)</span>
<span class="hljs-keyword">val</span> intersection = nums1 intersect nums2  <span class="hljs-comment">//List(2,3)</span></code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li></ul>

10.distinct

distinct: List[A] 保留列表中非重复的元素，相同的元素只会被保留一次

<code class="hljs mathematica has-numbering">val list = <span class="hljs-keyword">List</span>(<span class="hljs-string">"A"</span>,<span class="hljs-string">"B"</span>,<span class="hljs-string">"C"</span>,<span class="hljs-string">"A"</span>,<span class="hljs-string">"B"</span>) val distincted = list.distinct // <span class="hljs-keyword">List</span>(<span class="hljs-string">"A"</span>,<span class="hljs-string">"B"</span>,<span class="hljs-string">"C"</span>)</code><ul class="pre-numbering"><li>1</li></ul>

11.groupBy, grouped

groupBy : groupBy[K](f: (A) ⇒ K): Map[K, List[A]] 将列表进行分组，分组的依据是应用f在元素上后产生的新元素 

grouped: grouped(size: Int): Iterator[List[A]] 按列表按照固定的大小进行分组

<code class="hljs mathematica has-numbering">val data = <span class="hljs-keyword">List</span>((<span class="hljs-string">"HomeWay"</span>,<span class="hljs-string">"Male"</span>),(<span class="hljs-string">"XSDYM"</span>,<span class="hljs-string">"Femail"</span>),(<span class="hljs-string">"Mr.Wang"</span>,<span class="hljs-string">"Male"</span>))
val group1 = data.groupBy(_._2) // = <span class="hljs-keyword">Map</span>(<span class="hljs-string">"Male"</span> -> <span class="hljs-keyword">List</span>((<span class="hljs-string">"HomeWay"</span>,<span class="hljs-string">"Male"</span>),(<span class="hljs-string">"Mr.Wang"</span>,<span class="hljs-string">"Male"</span>)),<span class="hljs-string">"Female"</span> -> <span class="hljs-keyword">List</span>((<span class="hljs-string">"XSDYM"</span>,<span class="hljs-string">"Femail"</span>)))
val group2 = data.groupBy<span class="hljs-list">{case (name,sex) => sex}</span> // = <span class="hljs-keyword">Map</span>(<span class="hljs-string">"Male"</span> -> <span class="hljs-keyword">List</span>((<span class="hljs-string">"HomeWay"</span>,<span class="hljs-string">"Male"</span>),(<span class="hljs-string">"Mr.Wang"</span>,<span class="hljs-string">"Male"</span>)),<span class="hljs-string">"Female"</span> -> <span class="hljs-keyword">List</span>((<span class="hljs-string">"XSDYM"</span>,<span class="hljs-string">"Femail"</span>)))
val fixSizeGroup = data.grouped(<span class="hljs-number">2</span>).toList // = <span class="hljs-keyword">Map</span>(<span class="hljs-string">"Male"</span> -> <span class="hljs-keyword">List</span>((<span class="hljs-string">"HomeWay"</span>,<span class="hljs-string">"Male"</span>),(<span class="hljs-string">"XSDYM"</span>,<span class="hljs-string">"Femail"</span>)),<span class="hljs-string">"Female"</span> -> <span class="hljs-keyword">List</span>((<span class="hljs-string">"Mr.Wang"</span>,<span class="hljs-string">"Male"</span>)))

</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li></ul>

12.scan

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

由一个初始值开始，从左向右，进行积累的op操作，这个比较难解释，具体的看例子吧。

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>)
<span class="hljs-keyword">val</span> result = nums.scan(<span class="hljs-number">10</span>)(_+_)   <span class="hljs-comment">// List(10,10+1,10+1+2,10+1+2+3) = List(10,11,12,13)</span></code><ul class="pre-numbering"><li>1</li><li>2</li></ul>

13.scanLeft,scanRight

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

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

scanLeft: 从左向右进行scan函数的操作，scanRight：从右向左进行scan函数的操作

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1.0</span>,<span class="hljs-number">2.0</span>,<span class="hljs-number">3.0</span>)
<span class="hljs-keyword">val</span> result = nums.scanLeft(<span class="hljs-number">2.0</span>)(math.pow)   <span class="hljs-comment">// List(2.0,pow(2.0,1.0), pow(pow(2.0,1.0),2.0),pow(pow(pow(2.0,1.0),2.0),3.0) = List(2.0,2.0,4.0,64.0)</span>
<span class="hljs-keyword">val</span> result = nums.scanRight(<span class="hljs-number">2.0</span>)(math.pow)  <span class="hljs-comment">// List(2.0,pow(3.0,2.0), pow(2.0,pow(3.0,2.0)), pow(1.0,pow(2.0,pow(3.0,2.0))) = List(1.0,512.0,9.0,2.0)</span></code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li></ul>

14.take,takeRight,takeWhile

take : takeRight(n: Int): List[A] 提取列表的前n个元素 takeRight: takeRight(n: Int): List[A] 提取列表的最后n个元素 takeWhile: takeWhile(p: (A) ⇒ Boolean): List[A] 从左向右提取列表的元素，直到条件p不成立

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">4</span>,<span class="hljs-number">4</span>,<span class="hljs-number">4</span>,<span class="hljs-number">4</span>)
<span class="hljs-keyword">val</span> left = nums.take(<span class="hljs-number">4</span>)   <span class="hljs-comment">// List(1,1,1,1)</span>
<span class="hljs-keyword">val</span> right = nums.takeRight(<span class="hljs-number">4</span>) <span class="hljs-comment">// List(4,4,4,4)</span>
<span class="hljs-keyword">val</span> headNums = nums.takeWhile( _ == nums.head)  <span class="hljs-comment">// List(1,1,1,1)</span></code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li></ul>

15.drop,dropRight,dropWhile

drop: drop(n: Int): List[A] 丢弃前n个元素，返回剩下的元素 dropRight: dropRight(n: Int): List[A] 丢弃最后n个元素，返回剩下的元素 dropWhile: dropWhile(p: (A) ⇒ Boolean): List[A] 从左向右丢弃元素，直到条件p不成立

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">4</span>,<span class="hljs-number">4</span>,<span class="hljs-number">4</span>,<span class="hljs-number">4</span>)
<span class="hljs-keyword">val</span> left = nums.drop(<span class="hljs-number">4</span>)   <span class="hljs-comment">// List(4,4,4,4)</span>
<span class="hljs-keyword">val</span> right = nums.dropRight(<span class="hljs-number">4</span>) <span class="hljs-comment">// List(1,1,1,1)</span>
<span class="hljs-keyword">val</span> tailNums = nums.dropWhile( _ == nums.head)  <span class="hljs-comment">// List(4,4,4,4)</span></code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li></ul>

16.span, splitAt, partition

span : span(p: (A) ⇒ Boolean): (List[A], List[A]) 从左向右应用条件p进行判断，直到条件p不成立，此时将列表分为两个列表

splitAt: splitAt(n: Int): (List[A], List[A]) 将列表分为前n个，与，剩下的部分

partition: partition(p: (A) ⇒ Boolean): (List[A], List[A]) 将列表分为两部分，第一部分为满足条件p的元素，第二部分为不满足条件p的元素

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">2</span>,<span class="hljs-number">1</span>)
<span class="hljs-keyword">val</span> (prefix,suffix) = nums.span( _ == <span class="hljs-number">1</span>) <span class="hljs-comment">// prefix = List(1,1,1), suffix = List(2,3,2,1)</span>
<span class="hljs-keyword">val</span> (prefix,suffix) = nums.splitAt(<span class="hljs-number">3</span>)  <span class="hljs-comment">// prefix = List(1,1,1), suffix = List(2,3,2,1)</span>
<span class="hljs-keyword">val</span> (prefix,suffix) = nums.partition( _ == <span class="hljs-number">1</span>) <span class="hljs-comment">// prefix = List(1,1,1,1), suffix = List(2,3,2)</span></code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li></ul>

17.padTo

padTo(len: Int, elem: A): List[A]

将列表扩展到指定长度，长度不够的时候，使用elem进行填充，否则不做任何操作。

<code class="hljs fsharp has-numbering"> <span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">1</span>)
<span class="hljs-keyword">val</span> padded = nums.padTo(<span class="hljs-number">6</span>,<span class="hljs-number">2</span>)   <span class="hljs-comment">// List(1,1,1,2,2,2)</span></code><ul class="pre-numbering"><li>1</li><li>2</li></ul>

18.combinations,permutations

combinations: combinations(n: Int): Iterator[List[A]] 取列表中的n个元素进行组合，返回不重复的组合列表，结果一个迭代器

permutations: permutations: Iterator[List[A]] 对列表中的元素进行排列，返回不重得的排列列表，结果是一个迭代器

<code class="hljs mathematica has-numbering">val nums = <span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">3</span>)
val combinations = nums.combinations(<span class="hljs-number">2</span>).toList //<span class="hljs-keyword">List</span>(<span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">3</span>))
val permutations = nums.permutations.toList        // <span class="hljs-keyword">List</span>(<span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">3</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">3</span>,<span class="hljs-number">1</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">3</span>,<span class="hljs-number">1</span>,<span class="hljs-number">1</span>))</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li></ul>

19.zip, zipAll, zipWithIndex, unzip,unzip3

zip: zip[B](that: GenIterable[B]): List[(A, B)] 与另外一个列表进行拉链操作，将对应位置的元素组成一个pair，返回的列表长度为两个列表中短的那个

zipAll: zipAll[B](that: collection.Iterable[B], thisElem: A, thatElem: B): List[(A, B)] 与另外一个列表进行拉链操作，将对应位置的元素组成一个pair，若列表长度不一致，自身列表比较短的话使用thisElem进行填充，对方列表较短的话使用thatElem进行填充

zipWithIndex：zipWithIndex: List[(A, Int)] 将列表元素与其索引进行拉链操作，组成一个pair

unzip: unzip[A1, A2](implicit asPair: (A) ⇒ (A1, A2)): (List[A1], List[A2]) 解开拉链操作

unzip3: unzip3[A1, A2, A3](implicit asTriple: (A) ⇒ (A1, A2, A3)): (List[A1], List[A2], List[A3]) 3个元素的解拉链操作

<code class="hljs perl has-numbering">val alphabet = List(<span class="hljs-string">"A"</span>,B<span class="hljs-string">","</span>C<span class="hljs-string">")
val nums = List(1,2)
val zipped = alphabet zip nums   // List(("</span>A<span class="hljs-string">",1),("</span>B<span class="hljs-string">",2))
val zippedAll = alphabet.zipAll(nums,"</span><span class="hljs-variable">*"</span>,-<span class="hljs-number">1</span>)   // List((<span class="hljs-string">"A"</span>,<span class="hljs-number">1</span>),(<span class="hljs-string">"B"</span>,<span class="hljs-number">2</span>),(<span class="hljs-string">"C"</span>,-<span class="hljs-number">1</span>))
val zippedIndex = alphabet.zipWithIndex  // List((<span class="hljs-string">"A"</span>,<span class="hljs-number">0</span>),(<span class="hljs-string">"B"</span>,<span class="hljs-number">1</span>),(<span class="hljs-string">"C"</span>,<span class="hljs-number">3</span>))
val (list1,list2) = zipped.unzip        // list1 = List(<span class="hljs-string">"A"</span>,<span class="hljs-string">"B"</span>), list2 = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>)
val (l1,l2,l3) = List((<span class="hljs-number">1</span>, <span class="hljs-string">"one"</span>, <span class="hljs-string">'1'</span>),(<span class="hljs-number">2</span>, <span class="hljs-string">"two"</span>, <span class="hljs-string">'2'</span>),(<span class="hljs-number">3</span>, <span class="hljs-string">"three"</span>, <span class="hljs-string">'3'</span>)).unzip3   // l1=List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>),l2=List(<span class="hljs-string">"one"</span>,<span class="hljs-string">"two"</span>,<span class="hljs-string">"three"</span>),l3=List(<span class="hljs-string">'1'</span>,<span class="hljs-string">'2'</span>,<span class="hljs-string">'3'</span>)</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li></ul>

20.slice

slice(from: Int, until: Int): List[A] 提取列表中从位置from到位置until(不含该位置)的元素列表

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">4</span>,<span class="hljs-number">5</span>)
<span class="hljs-keyword">val</span> sliced = nums.slice(<span class="hljs-number">2</span>,<span class="hljs-number">4</span>)  <span class="hljs-comment">//List(3,4)</span></code><ul class="pre-numbering"><li>1</li><li>2</li></ul>

21.sliding

sliding(size: Int, step: Int): Iterator[List[A]] 将列表按照固定大小size进行分组，步进为step，step默认为1,返回结果为迭代器

<code class="hljs mathematica has-numbering">val nums = <span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">3</span>,<span class="hljs-number">4</span>,<span class="hljs-number">4</span>)
val groupStep2 = nums.sliding(<span class="hljs-number">2</span>,<span class="hljs-number">2</span>).toList  //<span class="hljs-keyword">List</span>(<span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">2</span>,<span class="hljs-number">2</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">3</span>,<span class="hljs-number">3</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">4</span>,<span class="hljs-number">4</span>))
val groupStep1 = nums.sliding(<span class="hljs-number">2</span>).toList //<span class="hljs-keyword">List</span>(<span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">1</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">2</span>,<span class="hljs-number">2</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">2</span>,<span class="hljs-number">3</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">3</span>,<span class="hljs-number">3</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">3</span>,<span class="hljs-number">4</span>),<span class="hljs-keyword">List</span>(<span class="hljs-number">4</span>,<span class="hljs-number">4</span>))</code><ul class="pre-numbering"><li>1</li><li>2</li><li>3</li></ul>

22.updated

updated(index: Int, elem: A): List[A] 对列表中的某个元素进行更新操作

<code class="hljs fsharp has-numbering"><span class="hljs-keyword">val</span> nums = List(<span class="hljs-number">1</span>,<span class="hljs-number">2</span>,<span class="hljs-number">3</span>,<span class="hljs-number">3</span>)
<span class="hljs-keyword">val</span> fixed = nums.updated(<span class="hljs-number">3</span>,<span class="hljs-number">4</span>)  <span class="hljs-comment">// List(1,2,3,4)</span></code><ul class="pre-numbering"><li>1</li><li>2</li></ul>