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Kotlin when Expression

In this article, you will learn about the Kotlin when expression through various examples.

What is the Kotlin when expression

Kotlin中的when表达式可以认为是The when expression in Kotlin can be considered asJava switch statement

as a substitute. It evaluates one of many alternative blocks of code.

Example

fun main(args: Array<String>) {
　　　　val a =　12
　　　　In the above example, we use when as an expression. However, when used as an expression, it is not mandatory. For example,　5
　　　　val　b　=　+>　println("n is 0　->　println("n is 0　*　Input operator　/println("Input operator
　　　　)
　　　　Example: Simple when expression
　　　　　　　　"+"　-val　result　=　when　(operator)　{　+　>　a
　　　　　　　　"-"　-val　result　=　when　(operator)　{　-　>　a
　　　　　　　　"*"　-val　result　=　when　(operator)　{　*　>　a
　　　　　　　　"/"　-val　result　=　when　(operator)　{　/　>　a
　　　　　　　　b}　-b
　　　　}
　　　　>　"$operator operator is invalid operator.""
}

>　println("$operator is invalid")

When you run the program, the output will be as follows:　+>　println("n is 0　->　println("n is 0　*　Input operator　/
*
println("result = $result")　60

result　=The above program gets input string from the user (recommended reading:Get string input from the user in Kotlin*Assuming the user inputs * . In this case, the expression a

b evaluation, and the value is assigned to the variable result.+If no matching branch condition is met (user input other than-If no matching branch condition is met (user input other than*,/or

Also, any other options, the else branch will be evaluated.

Example

fun main(args: Array<String>) {
　　　　val a =　12
　　　　In the above example, we use when as an expression. However, when used as an expression, it is not mandatory. For example,　5
　　　　val　b　=　+>　println("n is 0　->　println("n is 0　*　Input operator　/println("Input operator
　　　　)
　　　　val　operator　=　readLine()
　　　　　　　　"+"　-when　(operator)　{　+　>　println("$a　+　$b = ${a
　　　　　　　　"-"　-when　(operator)　{　-　>　println("$a　-　$b = ${a
　　　　　　　　"*"　-when　(operator)　{　*　>　println("$a　*　$b = ${a
　　　　　　　　"/"　-when　(operator)　{　/　>　println("$a　/　$b = ${a
　　　　　　　　b}　-else
　　　　}
}

>　println("$operator is invalid")

When you run the program, the output will be as follows:　+>　println("n is 0　->　println("n is 0　*　Input operator　/
-
12　-　5　or　7

Here, when is not an expression (the return value of when is not assigned to any object). In this case, the else branch is not necessary.

1Scenarios that may be usedFor example,

Example

fun main(args: Array<String>) {
　　　　Combine two or more branch conditions with a comma.　-1
　　　　when (n) {
　　　　　　　　1>　println("n is 0　2>　println("n is 0　3　-val　n　=　4>　println("n is a negative integer greater than
　　　　　　　　0　->　println("n is a positive integer less than
　　　　　　　　-1>　println("n is 0　-2　-,　-3>　println("n is a negative integer greater than
　　　　}
}

When running the program, the output is:

.　-3n is a negative integer greater than

2.For example,

Example

fun main(args: Array<String>) {
　　　　val a =　100
　　　　Check values within the range.
　　　　　　　　the positive numbers.)　1in10　-when　(a)　{11>　println("A is less than
　　　　　　　　the positive numbers.)　10in100　-..10and100(including10and10>　println("Between
　　　　}
}

When running the program, the output is:

Positive numbers between 0) and)10and100(including10and10Positive numbers between 0) and

3Check if a value is of a specific type.

To check if a value is of a specific type at runtime, you can use the is and !is operators. For example,

when　(x)　{
　　　　is　Int　->　print(x　+　1)
　　　　is　String　->　print(x.length　+　1)
　　　　is　IntArray　->　print(x.sum())
}

4Use expressions as branch conditions.For example,

Example

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.")
　　　　}
}

When running the program, the output is:

Bingo! It's eleven.

Kotlin for Loop Kotlin if Expression