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if Expressions

You can use if, in combination with else if and else where required, to choose between different execution paths in a Kotlin program. The syntax is basically the same as in C:

if (number < 0) {
    println("Number too low")
}
else if (number > 100) {
    println("Number too high")
}
else {
    println("Number OK")
}

if (number < 0) {
    printf("Number too low\n");
}
else if (number > 100) {
    printf("Number too high\n");
}
else {
    printf("Number OK\n");
}

if number < 0:
    print("Number too low")
elif number > 100:
    print("Number too high")
else:
    print("Number OK")

However, note that the Kotlin compiler is stricter than a C compiler. It will require the test performed by if or else if to be a proper boolean expression.

You can construct more complex boolean expressions in Kotlin using && and ||, just as in C:

if (number < 0 || number > 100) {
    println("Number is out of range")
}

One important difference between Kotlin and C or Python is that, in Kotlin, selection structures are expressions, not statements. This means that a selection structure can be regarded as having a value, which can be assigned to a variable. The value will be determined by the chosen execution path, so each path should yield a result of the appropriate type.

Thus you could rewrite the example above like this:

val message = if (number < 0) {
    "Number too low"
}
else if (number > 100) {
    "Number too high"
}
else {
    "Number OK"
}

println(message)

Each branch yields a string, so the compiler will infer the type of message to be String.

Caution

In the original version of this example, we could have omitted the else branch if we didn’t want to print a message when number is in range.

We cannot do that here; this new version requires an else branch.

The reason is that we are assigning the result of the if expression to a variable. The expression must therefore always yield a result. The else branch guarantees this by providing a value that can be used in cases where none of the tests in the other branches evaluate to true.

You don’t need to write an else branch if you are not using the result of the expression in some way.

Whilst C and Python don’t allow you to duplicate an example like the one above, they do support a simpler form of conditional expression. Here’s an example of that simpler form, written for all three languages so you can compare the syntax:

val sign = if (number < 0) '-' else '+'

char sign = (number < 0) ? '-' : '+';

sign = '-' if number < 0 else '+'

This is often described as a ternary expression, because it has three parts: a boolean expression; some code to evaluate if that expression has the value true; and some code to evaluate if that expression has the value false.

In the examples above, Python arguably has most natural and readable syntax for ternary expressions, with C being the least readable in this regard.