scala中yield关键字讲解

1.yield作用

   当我们使用for或者if，或者for与if联合使用，会过滤出符合你条件的集合，但是当你并不需要打印过滤出来的集合时，

你需要对过滤后的集合进行处理，使用yield关键字可以在for表达式中生成新的集合。

2.yield例子

    2.1有yield关键字

val fruits = List("banana fruit","apple fruit","peach","grapes","pear")

val filterFruit = for {
    fruit <- fruits
    if fruit.contains("fruit") && !fruit.startsWith("banana")
}yield fruit

结果为：

fruits: List[String] = List(banana fruit, apple fruit, peach, grapes, pear)
filterFruit: List[String] = List(apple fruit)

我们看到没有打印出过滤后的集合。

    2.2没有yield关键字

val fruits = List("banana fruit","apple fruit","peach","grapes","pear")

val filterFruit = for {
    fruit <- fruits
    if fruit.contains("fruit") && !fruit.startsWith("banana")
} fruit

结果为：

fruits: List[String] = List(banana fruit, apple fruit, peach, grapes, pear)
filterFruit: Unit = ()

   2.3带println关键字

val fruits = List("banana fruit","apple fruit","peach","grapes","pear")

val filterFruit = for {
    fruit <- fruits
    if fruit.contains("fruit") && !fruit.startsWith("banana")
} println(fruit)

结果为：

fruits: List[String] = List(banana fruit, apple fruit, peach, grapes, pear)

apple fruit

filterFruit: Unit = ()

3.完整代码及结果

   3.1带yield关键字

   3.2无yield关键字

   3.3带prinltn关键字