scala的多種集合的使用(3)之遍歷集合的方法

遍歷集合的方法

1.用foreach循環遍歷一個集合

foreach接收一個函數作為參數。定義的函數應該接收一個元素作為輸入參數，然后不要返回任何的東西。輸入的參數的類型應該匹配集合中的類型。隨着foreach的執行，它每次都會把一個元素傳給你的函數，直到集合中最后一個元素。

foreach常用的就是輸出信息：

scala> val x = Vector(1,2,3)
x: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3)

scala> x.foreach((i:Int) => println(i))
1
2
3

對於表達式，還有其他寫法：

x.foreach(i => println(i))

x.foreach(println(_))

x.foreach(println)　

對於最后一種表達式，情況是一個只有一條語句組成並且接受一個參數的函數可以簡寫成這種形式。

2.用for循環遍歷一個集合

 可以用for循環遍歷任意一種Traversable的類型。

scala> val fruits = Traversable("apple","orange","banana")
fruits: Traversable[String] = List(apple, orange, banana)

scala> for(i <- fruits) println(i)
apple
orange
banana

如果算法很長，可以在for循環的代碼塊里執行。

下面展示了for循環里使用計數器的幾種方式：

scala> val fruits = Array("apple","banana","orange")
fruits: Array[String] = Array(apple, banana, orange)

scala> for(i <- 0 until fruits.size) println(s"$i is ${fruits(i)}")
0 is apple
1 is banana
2 is orange

scala> val fruits = Array("apple","banana","orange")
fruits: Array[String] = Array(apple, banana, orange)

scala> for((elem,count) <- fruits.zipWithIndex){
     | println(s"$count is $elem")
     | }
0 is apple
1 is banana
2 is orange

scala> val fruits = Array("apple","banana","orange")
fruits: Array[String] = Array(apple, banana, orange)

scala> for((elem,count) <- fruits.zip(Stream from 1)){
     | println(s"$count is $elem")
     | }
1 is apple
2 is banana
3 is orange

3.用zipWithIndex創建循環計數器

 (1)以foreach的形式遍歷集合

scala> val days = Array("Sunday","Monday","Tusday","Wednsday","Thursday","Friday","Saturday")
days: Array[String] = Array(Sunday, Monday, Tusday, Wednsday, Thursday, Friday,Saturday)

scala> days.zipWithIndex.foreach{
	 | case(day,count) => println(s"${count+1} is $day")
	 | }
1 is Sunday
2 is Monday
3 is Tusday
4 is Wednsday
5 is Thursday
6 is Friday
7 is Saturday

　另外的一種表示方法：

scala> val days = Array("Sunday","Monday","Tusday","Wednsday","Thursday","Friday","Saturday")
days: Array[String] = Array(Sunday, Monday, Tusday, Wednsday, Thursday, Friday,Saturday)

scala> days.zipWithIndex.foreach{ d=>
	 | println(s"${d._2+1} is ${d._1}")
	 | }
1 is Sunday
2 is Monday
3 is Tusday
4 is Wednsday
5 is Thursday
6 is Friday
7 is Saturday

(2) 以for的形式循環遍歷

scala> val days = Array("Sunday","Monday","Tusday","Wednsday","Thursday","Friday","Saturday")
days: Array[String] = Array(Sunday, Monday, Tusday, Wednsday, Thursday, Friday,Saturday)

scala> for ((day,count) <- days.zipWithIndex){
	 | println(s"${count+1} is $day")
	 | }
1 is Sunday
2 is Monday
3 is Tusday
4 is Wednsday
5 is Thursday
6 is Friday
7 is Saturday

4.用zip創建循環計數器

 在zip中使用Stream是一種生存計數器的方法。

scala> val days = Array("Sunday","Monday","Tusday","Wednsday","Thursday","Friday","Saturday")
days: Array[String] = Array(Sunday, Monday, Tusday, Wednsday, Thursday, Friday,Saturday)

scala> for ((day,count) <- days.zip(Stream from 1)){
	 | println(s"$count is $day")
	 | }
1 is Sunday
2 is Monday
3 is Tusday
4 is Wednsday
5 is Thursday
6 is Friday
7 is Saturday

5.range循環計數器

 如果只是需要重復做某事多次，可以用range。

scala> val fruits = Array("apple","banana","orange")
fruits: Array[String] = Array(apple, banana, orange)

scala> for(i <- 0 until fruits.size) {
	 | println(s"$i is ${fruits(i)}")}
0 is apple
1 is banana
2 is orange

6.用reduce方法遍歷集合

使用reduceLeft和reduceRight方法來遍歷序列中的元素，把相鄰的元素傳給你的函數成成一個新的結果，之后和序列的下一個元素比較在生成新的結果。

reduceLeft方法是從左到右遍歷一個序列，在算法中首先會對前兩個元素進行比較，然后返回一個結果。該結果會與第三個元素進行比較，比較之后在產生一個新結果，接着在於第四個元素比較，以此類推。

scala> val a = Array(12,3,4,5,67)
a: Array[Int] = Array(12, 3, 4, 5, 67)

scala> a.reduceLeft(_ + _)
res4: Int = 91

scala> a.reduceLeft(_ min _)
res5: Int = 3

scala> a.reduceLeft(_ max _)
res6: Int = 67

這里面的兩個下划線，它們代表傳給函數的兩個參數。

reduceRight與reduceLeft同理的。

7.用fold方法遍歷集合

foldLeft方法如同reduceLeft，它會設置一個種子值用於第一個元素。foldLeft接收兩個參數列表。第一個列表有一個字段，種子值。第二個列表是要運行的代碼塊。

scala> val a = Array(12,3,4,5,67)
a: Array[Int] = Array(12, 3, 4, 5, 67)

scala> a.reduceLeft(_ + _)
res8: Int = 91

scala> a.foldLeft(20)(_ + _)
res9: Int = 111

我們來看一下foldLeft和foldRight的執行過程：　　

下面的/:是foldLeft的簡化版。

scala> ((1 to 4).foldLeft(5))((i,sum) => i-sum)
res25: Int = -5

scala> (1 to 4).foldLeft(5)(_-_)
res8: Int = -5

scala> (5/:(1 to 4))(_-_)
res9: Int = -5

foldLeft的執行過程：5-1-2-3-4=-5

下面的:\是foldRight的簡化版。

scala> ((1 to 4).foldRight(5))((i,sum) => i-sum)
res22: Int = 3

scala> (1 to 4).foldRight(5)(_-_)
res14: Int = 3

scala> ((1 to 4):\5)(_-_)
res15: Int = 3

foldRight的執行過程：1-(2-(3-(4-(5-0))))=3