How to Get the True Count in Boolean Array in Kotlin

Question

I've read the kotlin documentation and tried BooleanArray.count(predicate: (Boolean) -> Boolean) , but it still returns the size of the array , not the number of true :

val checks = arrayOf(true, false, true, true, true)

val trueCount = checks.count { true }

What's wrong with my code?

Answer 1

You're passing in a predicate that would always return true here:

val trueCount = checks.count { true }

And that would indeed return the size of the array. I'd expect your code to be:

val trueCount = checks.count { it }

In other words, the count of items for which "with the item called it , the expression it returns true".

An alternative you might find easier to read that explicitly checks for equality:

val trueCount = checks.count { it == true }

Personally I tend to avoid comparisons against Boolean literals, but I can see how it's at least arguably clearer in this case.

Answer 2

Use count { it } instead of count { true }

The argument to count isn't a value to look for in the array; it's a predicate: a function that takes one argument and returns true or false . The predicate is called once for each item in the array, and the value ( true or false ) returned by that predicate determines whether the item is counted or not. So the result is the number of items in the array for which the predicate evaluated to true .

In this context, writing { true } is shorthand for { value -> true } , and is a constant function. It will always return true , regardless of what the value is. So "count the number of items for which { true } returns true " is exactly the same as "count the number of items" .

What you want instead is to count the number of items where the value is true . The predicate you should use is { value -> value } , which is a function that takes a single argument and returns the value of that argument. Since a single lambda parameter is implicitly named it when you don't give it a name, the shorthand for { value -> value } is just { it } .