if表达式的定义，if表达式是有值的，就是if或else中最后一行语句返回的值。
scala> val age = 30
age: Int = 30
scala> val isAdult = if(age > 18) 1 else 0
isAdult: Int = 1
if表达式的类型推断，if和else子句的值类型可能不同，Scala会自动进行推断，取两个类型的公共父类型。
scala> if(age>18) "adult" else 0
res3: Any = adult
# if和else的值分别是String和Int，则表达式的值是String和Int的公共父类型Any。
如果if后面没有跟else，则默认else的值是Unit，也用()表示，类似于Java中的void或null。
scala> if(age<12) "children"
res4: Any = ()
# 等价写法
scala> if(age<12) "children" else()
res6: Any = ()
将if语句放在多行中，默认情况下，REPL只能解释一行语句，但是if表达式通常需要放在多行，可以使用{}方式，或者使用:paste和ctrl+D的方式。
scala> if(age < 12) {
| "children"} else ()
res7: Any = ()
scala> :paste
// Entering paste mode (ctrl-D to finish)
if (age < 12)
"children"
else
"adult"
// Exiting paste mode, now interpreting.
res8: String = adult
语句终结符，默认情况下，Scala不需要语句终结符，默认将每一行作为一个语句。如果一行要放多条语句，则必须使用语句终结符分号。通常来说，对于多行语句，还是会使用花括号的方式。
scala> var a,b,c = 0
a: Int = 0
b: Int = 0
c: Int = 0
scala> if (a < 1) {b = b + 1; c = c + 1}
scala> b
res10: Int = 1
scala> c
res11: Int = 1
scala> :paste
// Entering paste mode (ctrl-D to finish)
if(a < 1) {
b = b + 1
c = c + 1
}
// Exiting paste mode, now interpreting.
scala> b
res13: Int = 2
scala> c
res14: Int = 2
块表达式，就是{}中的值，其中可以包含多条语句，最后一个语句的值就是块表达式的返回值。
scala> val d = if (a < 1) {b = b + 1; c = c + 1; b + c}
d: AnyVal = 6
print打印时不会加换行符，而println打印时会加一个换行符，printf可以用于进行格式化。readLine允许我们从控制台读取用户输入的数据。
scala> print("Hello World")
Hello World
scala> println("Hello World")
Hello World
scala>
scala> printf("Hello, my name is %s, I'm %d years old", "padluo", 30)
Hello, my name is padluo, I'm 30 years old
scala> readLine()
warning: there was one deprecation warning; re-run with -deprecation for details
res20: String = Hello World
scala> val name = readLine("Welcome to Game House. Please show your name:\n")
warning: there was one deprecation warning; re-run with -deprecation for details
Welcome to Game House. Please show your name:
name: String = padluo
scala> println("OK, welcome you, " + name)
OK, welcome you, padluo
scala> println("Then, please show your age:")
Then, please show your age:
scala> val age = readInt()
warning: there was one deprecation warning; re-run with -deprecation for details
age: Int = 30
scala> :paste
// Entering paste mode (ctrl-D to finish)
if(age > 18)
printf("Hi, %s, you are %d years old, so you are legal to come here!", name, age)
else {
printf("Sorry, boy, %s, you are only %d years old. you are illegal to come here.")
}
// Exiting paste mode, now interpreting.
Hi, padluo, you are 30 years old, so you are legal to come here!
while do循环，
scala> var n = 10
n: Int = 10
scala> :paste
// Entering paste mode (ctrl-D to finish)
while(n > 0) {
print(n + " ")
n -= 1
}
// Exiting paste mode, now interpreting.
10 9 8 7 6 5 4 3 2 1
Scala没有for循环，只能使用while替代for循环，或者使用简易版的for语句、或者使用until，也可以对字符串进行遍历，类似Java的增强for循环。
scala> val n = 10
n: Int = 10
scala> for(i <- 1 to n) print(i + " ")
1 2 3 4 5 6 7 8 9 10
scala> for (i <- 1 until n) print(i + " ")
1 2 3 4 5 6 7 8 9
scala> for(c <- "Hello World") print(c + " ")
H e l l o W o r l d
跳出循环语句，Scala没有类似于Java的break语句。可以使用boolean类型变量、return或者Breaks的break函数来替代使用。
scala> :paste
// Entering paste mode (ctrl-D to finish)
breakable {
var n = 10
for(c <- "Hello World") {
if(n == 5) break;
print(c)
n -= 1
}
}
// Exiting paste mode, now interpreting.
Hello
高级for循环，九九乘法表。
scala> :paste
// Entering paste mode (ctrl-D to finish)
for(i <- 1 to 9; j <- 1 to 9) {
if (j == 9) {
println(i * j)
}
else {
print(i * j + " ")
}
}
// Exiting paste mode, now interpreting.
1 2 3 4 5 6 7 8 9
2 4 6 8 10 12 14 16 18
3 6 9 12 15 18 21 24 27
4 8 12 16 20 24 28 32 36
5 10 15 20 25 30 35 40 45
6 12 18 24 30 36 42 48 54
7 14 21 28 35 42 49 56 63
8 16 24 32 40 48 56 64 72
9 18 27 36 45 54 63 72 81
scala>
if守卫，取偶数。
scala> for(i <- 1 to 20 if i % 2 == 0) print(i + " ")
2 4 6 8 10 12 14 16 18 20
for推导式，构造集合。
scala> for(i <- 1 to 10) yield i
res37: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
本文首发于steem，感谢阅读，转载请注明。
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