if ... else
传统用法
if（testExpression）{
//如果testExpression为true，则运行代码
}else{
//如果testExpression为false，则运行代码
}
与Java（和其他许多编程语言）不同，if可以在Kotlin中使用表达式并且返回一个值，例如：
fun main() {
val number = -10
val result = if (number > 0) {
"Positive number"
} else {
"Negative number"
}
println(result)
}
结果：
Negative number
如果if分支块包含多个表达式，则最后一个表达式将作为块的值返回，例如：
fun main() {
val a = -9
val b = -11
val max = if (a > b) {
println("$a is larger than $b.")
println("max variable holds value of a.")
a
} else {
println("$b is larger than $a.")
println("max variable holds value of b.")
b
}
println("max = $max")
}
结果：
-9 is larger than -11.
max variable holds value of a.
max = -9
when表达式
fun main() {
val a = 12
val b = 5
println("Enter operator either +, -, * or /")
val operator = readLine()
val result = when (operator) {
"+" -> a + b
"-" -> a - b
"*" -> a * b
"/" -> a / b
else -> "$operator operator is invalid operator."
}
println("result = $result")
}
结果：
Enter operator either +, -, * or /
*
result = 60
when有点像switch..case
使用逗号组合两个或多个分支条件的表达式，例如：
fun main() {
val n = -1
when (n) {
1, 2, 3 -> println("n is a positive integer less than 4.")
0 -> println("n is zero")
-1, -2 -> println("n is a negative integer greater than 3.")
}
}
结果：n is a negative integer greater than 3.
检查范围内的值，例如：
fun main(args: Array<String>) {
val a = 100
when (a) {
in 1..10 -> println("A positive number less than 11.")
in 10..100 -> println("A positive number between 10 and 100 (inclusive)")
}
}
检查值是否属于特定类型（is、！is）,例如：
when (x) {
is Int -> print(x + 1)
is String -> print(x.length + 1)
is IntArray -> print(x.sum())
}
使用表达式作为分支条件，例如：
fun main(args: Array<String>) {
val a = 11
val n = "11"
when (n) {
"cat" -> println("Cat? Really?")
12.toString() -> println("Close but not close enough.")
a.toString() -> println("Bingo! It's eleven.")
}
}
结果：Bingo! It's eleven.
Break
first@ for (i in 1..4) {
second@ for (j in 1..2) {
println("i = $i; j = $j")
if (i == 2)
break@second
}
}
break终止标有 @second的循环.
continue 跳过
for (i in 1..5) {
println("$i Always printed.")
if (i > 1 && i < 5) {
continue
}
println("$i Not always printed.")
}
输出：
1始终打印。
1并不总是打印。
2始终打印。
3始终打印。
4始终打印。
5始终打印。
5并不总是打印。
For循环语法：
for（item in collection）{
//循环体
}
例如：
print("for (i in 1..5) print(i) = ")
for (i in 1..5) print(i)
println()
print("for (i in 5..1) print(i) = ")
for (i in 5..1) print(i) // 无任何输出
println()
print("for (i in 5 downTo 1) print(i) = ")
for (i in 5 downTo 1) print(i)//从5倒序输出到1
println()
print("for (i in 1..4 step 2) print(i) = ")
for (i in 1..5 step 2) print(i)//间隔为2的输出
println()
print("for (i in 4 downTo 1 step 2) print(i) = ")
for (i in 5 downTo 1 step 2) print(i)//间隔为2的倒序输出
println()
print("for ((i in 1 until 5) print(i) = ")
for (i in 1 until 5) print(i)//输出1..4 ，去掉末尾的遍历
结果：
for (i in 1..5) print(i) = 12345
for (i in 5..1) print(i) =
for (i in 5 downTo 1) print(i) = 54321
for (i in 1..4 step 2) print(i) = 135
for (i in 4 downTo 1 step 2) print(i) = 531
for ( (i in 1 until 5) print(i) = 1234
