Kotlin学习笔记——基础语法篇之控制语句

if…else…

用法

Kotlin中if...else...基本用法与C/C++，java中相同

例子

button.setOnClickListener {
    if (flag) {
        text.text = "I love you, at the first sight of you."
    } else {
        text.text = "I love three things in this world.Sun, moon and you. " +
                "Sun for morning, moon for night, and you forever."
    }
}

而Kotlin的if else可以有返回值

例子

button.setOnClickListener {
    text.text = if (flag) {
        "I love you, at the first sight of you."
    } else {
        "I love three things in this world.Sun, moon and you. " +
                "Sun for morning, moon for night, and you forever."
    }
}

Kotlin中没有java，C/C++的三目运算符，但是可以用if…else…取代

例子

button.setOnClickListener {
    text.text = if (flag) (16).toString() else (153.6).toString()
    /*
    像极了三目元算符：(假装这里是C/C++或java)
    text.text = flag ? (16).toString() : (153.6).toString();
    */
}

when…else…

用法

Kotlin中的when...else...多路分支相当于C/C++，java中的switch...case...，但是用法稍有不同

1. Kotlin的when...else...和Kotlin的if...else...一样，允许有返回值
2. Kotlin的when...else...各个分支中，可以不是常量，变量也可以
3. Kotlin的when...else...不用写break，每个分支结束后自动退出when...else...语句块

例子

button.setOnClickListener {
    text.text = when(type) {
        1,2,3 -> "I love you, at the first sight of you."//多个值走同一个分支，用逗号隔开
        in 4..10 -> "I love three things in this world.Sun, moon and you. " +
                "Sun for morning, moon for night, and you forever."//表示在4到10之间
        !in 1..10 -> "We don't talk anymore."//表示不在1到10之间
        else -> "error"
    }
}

循环

for循环

Kotlin居然取消了常见的for循环，tmd

遍历循环

1. for-in循环

类似C++/java中的for_each形式的循环，可以对字符串、数组、Array<>, 队列、映射、集合进行遍历

例子

btn.setOnClickListener {
    var str:String = "0123456789"
    for (item in str) {//item自动类型推断
        Toast.makeText(this, "${item}", Toast.Toast.LENGTH_SHORT).show()
    }
}

例子(下标数组遍历)

btn.setOnClickListener {
    var str:String = "0123456789"
    for (i in str.indices) {//indices是下标数组
        Toast.makeText(this, "${str[i]}", Toast.Toast.LENGTH_SHORT).show()
    }
}

条件循环

格式

for(i in 11 until 66) {}
//左闭右开区间，[11,66)
for (i in 23..89 step 4) {}
//每次循环，i += 4，如果条件允许，可以到89
for (i in 50 downTo 7) {}
//从50 递减到 7

while循环

用法同java/C/C++

do-while循环

用法同java/C/C++

跳出多重循环

和java类似，如果想一次性跳出多个循环，可以在循环外面加"标签"

例子

var i:Int = 0
var j:Int = 0
@outside while (i <= 10) {
    j = 10;
    while (i * j != 50) {
        j--
        if (j == 0) {
            break@outside
        }
    }
    i++
}