Scala学习笔记01_条件控制与循环

in #cn7 years ago

if表达式

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

语句终结符与块表达式

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

输入和输出

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

综合案例:游戏厅门禁

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!

循环

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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@padluo, 伦家就觉得你写得不错嘛~~~ img