Scala if Statements and for Loop
Scala if Statements and for Loop
Scala if Statements
scala 中的 if 和其它编程语言中不同的一点是 if 表达式的返回值。比如下面这样，
val configFilePath = if (configFile.exists()) {
configFile.getAbsolutePath
} else {
configFile.createNewFile()
configFile.getAbsolutePath
}
configFilePath: String = /Applications/IntelliJ IDEA 15.app/Contents/bin/somefile.txt
Scala for Loops
一个基本的for循环语句，
val dogBreeds = List("Doberman", "Yorkshire Terrier", "Dachshund", "Scottish Terrier", "Great Dane", "Portuguese Water Dog")
for (bread <- dogBreeds)
println(bread)
Generator Expressions
表达式
bread <- dogBreeds
叫做generator expression，这样命名是因为它从一个集合里产生一个个体值。left arrow operator (<-) 是用来遍历一个集合的。比如下面这样，
for (i <- 1 to 10) println(i)
Scala for Loops:Filtering Values
在for循环中，可以过滤数据，如下，
val dogBreeds = List("Doberman", "Yorkshire Terrier", "Dachshund", "Scottish Terrier", "Great Dane", "Portuguese Water Dog")
for (bread <- dogBreeds
if bread.contains("Terrier")
) println(bread)
还可以使用多个过滤条件，
for (bread <- dogBreeds
if bread.contains("Terrier")
if !bread.startsWith("Yorkshire")
) println(bread)
这两种方式是等价的，
for (bread <- dogBreeds
if bread.contains("Terrier") && !bread.startsWith("Yorkshire")
) println(bread)
Yielding
上面那段话的意义就是，for 循环中的 yield 会把当前的元素记下来，保存在集合中，循环结束后将返回该集合。Scala 中 for 循环是有返回值的。如果被循环的是 Map，返回的就是 Map，被循环的是 List，返回的就是 List，以此类推。
scala> val filteredBreeds =
| for (breed <- dogBreeds
| if breed.contains("Terrier") && !breed.startsWith("Yorkshire")
| ) yield breed
filteredBreeds: List[String] = List(Scottish Terrier)
scala>
scala> filteredBreeds.toString()
res3: String = List(Scottish Terrier)
下面遍历Map，
scala> val map = Map("key1" -> "value1", "key2" -> "value2", "key3" -> "value3")
map: scala.collection.immutable.Map[String,String] = Map(key1 -> value1, key2 -> value2, key3 -> value3)
scala> val temp_map = for (m <- map
| if m._1.equals("key1")
| ) yield m
temp_map: scala.collection.immutable.Map[String,String] = Map(key1 -> value1)
scala> temp_map.toString()
res4: String = Map(key1 -> value1)
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