Another expression that controls the flow of execution in a program is the when expression.

A when expression is made up of multiple branches that contain values, or arguments, to check for. If the value of the variable being evaluated matches one of the branches’ arguments the instructions contained in that branch will be executed.

For example:

var lightColor = "red"

when (lightColor) {
  "green" -> println("Go.")
  "yellow" -> println("Slow down.")
  "red" -> println("Stop.")
  else -> println("Not a valid traffic light color.")
}
// Prints: Stop.

• The keyword when is followed by a variable placed within parentheses.
• The set up for each branch starts with a value to check for, an arrow(->), and then an instruction to execute.
• The else at the end of a when expression is an optional addition that can be used to catch any unexpected or additional values the variable being evaluated could contain.

In the example above, the value of lightColor was "red", so the instruction println("Stop.") was executed.

Programmers often use when expressions in lieu of if/else, else-if conditionals because of their significantly shorter syntax. For example, we can create a when expression that evaluates multiple expressions:

var num = 19
when {
  num < 0 -> println("$num is negative.")
  num == 0 -> println("$num is zero.")
  num > 0 -> println("$num is positive.")
  else -> println("Not a valid number.")
}
// Prints: 19 is positive.

Note how in the example above, we did not place a value inside parentheses after the when keyword and instead placed full expressions inside the branches’ arguments.

To explore other ways we can implement a when expression in our programs, check out the Kotlin Language Guide.