Introduction to Kotlin (Part 2): if, when, and looping

In part one of this Introduction to Kotlin series, we talked about what Kotlin is and about some of the basics of the language(variables, types, and functions). If you are completely new to Kotlin, that article is probably a good read before starting this one.

Making decisions (if and when)

Like most other programming languages, Kotlin includes support for conditional logic with if/else. The basic structure is the same as in Java.

if(booleanExpression) {
    println("I'm true!")
}
else {
    println("I'm false!")
}

Unlike in Java, if/else blocks are expressions that can return a value. When used as an expression the last line of each block is returned as the value.

val result = if(booleanExpression) {
    "I'm true!"
}
else {
    "I'm false!"
}
println(result)

If the if and else blocks contain a single expression, then this code can be simplified further.

val result = if(booleanExpression) "I'm true!" else "I'm false!"
println(result)

This pattern is similar to the ternary (? :) operator in Java, which Kotlin does not have.

In addition to if Kotlin also has another conditional called when. The when expression is a powerful alternative, and is especially useful when doing many comparisons together.

val cost = 12
val result = when {
  cost < 5 -> "It is cheap!"
  cost > 50 -> "It is expensive!"
  else -> "It's in between."
}
println(result)

When multiple conditional branches are required the when expression results in a much more concise code. The when expression requires that all possible branches must be covered. So, in this case, we must have an else branch. When using enumerations an else can be omitted when all values are covered by an existing branch.

If all conditional branches are going to be comparisons with a single value we can pass that as an argument to the when expression.

val count = 5
result = when (count) {
  1 -> "Only one!"
  2,3 -> "Two or three!"
  in 4..6 -> "Four to six!"
  else -> "Seven or more!!"
}
println(result)

For this type of when expression we can either specify a single value, a comma-separated list of values, or a range using the in keyword. Once again we still must have an else statement to catch any non-matching values.

In this form, the when expression looks very similar to the switch statement in Java, only with more power and simpler syntax.

Looping

The for loop in Kotlin allows for iterating over any object that provides an iterator, or over ranges. The in keyword The type on the variable defined in the loop can, in most cases, be omitted.

for(item: Int in someCollection) {
    println(item)
}

Kotlin does not utilize the C style for loop for iterating over a range of numbers. Instead, iteration is specified using Range Expressions. Ranges are created using the .. operator (which is associated with the rangeTo() function).

for(i in 1..5) {
    print(i)
}
// Prints: 1, 2, 3, 4, and 5

By default iterating over a range will increment the value by one each time. In some cases, you may want to increment by other values. This can be achieved by specifying step and a number. In the example below, the number will be increased by two each time until the ending value is reached.

for(i in 1..10 step 2) {
    print(i)
}
// Prints: 1, 3, 5, 7, and 9

If you want the range to go in reverse order you can use downTo instead of ... This will result in the starting number being decremented by one (or another value when step is specified) until the ending value is reached.

for(i in 5 downTo 1) {
    println(i)
}
// Prints: 5, 4, 3, 2, and 1

Kotlin also support while and do while loops. These loops work just as you would expect them to in Java.

while(booleanExpression) {
    // ...
}

do {
    // ...
}
while(booleanExpression)

What next?

In part three of this series, we will talk about Kotlin classes. Also, the Kotlin Reference documentation covers all of these items discussed in this post in much more detail.