I've got an input into my system, current a current state, and a previous_states. My first direction of thought would be to use a when statement, but Kotlin doesn't support using multiple variables in there:

when (input, state, previous_states) {  
  (KeyPower, PowerOff, matched) -> ...
  (KeyPower, PowerOn,  matched) -> ...
  (KeyMenu,  PowerOff, listOf(MainView, _)) -> ...
}

Is just a syntax error. Is there a workaround for this / an entirely different approach?

CodePudding user response：

Not checking all parameters

It is possible using when, but note that you need to add necessary 'else' branch:

fun getDescription(color: String, shape: String, size: String) : String {
    return when {
        (color == "red" && shape == "square" && size == "small") -> "You have a small red square"
        (color == "red" && shape == "square" && size == "large") -> "You have a large red square"
        (color == "blue" && shape == "triangle") -> "You have a blue triangle"
        else -> "undefined"
    }
}

You can also throw an exception on the else branch

else -> throw IllegalStateException("Invalid input")

Always checking all given parameters

If you always want to check the same number of parameters, you can also use the to operator:

fun getDescription(color: String, shape: String, size: String): String {
    return when (color to shape to size) {
        ("red" to "square" to "small") -> "You have a small red square"
        ("red" to "square" to "large") -> "You have a large red square"
        ("blue" to "triangle" to "small") -> "You have a small blue triangle"
        else -> "undefined"
    }
}

"red" to "square" to "large" is of type Pair<Pair<String, String>, String> and therefore incompatible to be compared with e.g. "blue" to "triangle" which is of type Pair<String, String>

CodePudding user response：

You can use a when in this case, but you will have to wrap the three independent vars/vals in a Triple.

Here's an example that uses Strings, because I don't know the types of your vars/vals:

fun main() {
    val input = "KeyPower"
    val state = "PowerOn"
    val previous_states = "matched"
    
    when (Triple(input, state, previous_states)) {
          Triple("KeyPower", "PowerOff", "matched") -> println("Power, Off, matched")
          Triple("KeyPower", "PowerOn",  "matched") -> println("Power, On, matched")
          Triple("KeyMenu",  "PowerOff", "matched") -> println("Menu, Off, matched")
          else -> println("no match")
    }
}

Output:

Power, On, matched

The when statement requires a single object, it seems…