Filter family

Another common task is filtering a collection. You know the drill. You iterate over it and put only values that fit your criteria in a new collection. For example, if given a range of numbers between 1-10, we would like to return only odd ones. Of course, we've already learned this lesson from the previous example, and wouldn't simply create a function called filterOdd(), as later we would be required to also implement filterEven(), filterPrime(), and so on. We'll receive a lambda as the second argument right away:

fun filter(numbers: List<Int>, check: (Int)->Boolean): MutableList<Int> {
    val result = mutableListOf<Int>()

    for (n in numbers) {
        if (check(n)) {
            result.add(n)
        }
    }

    return result
}

Invoking it will print only odd numbers. How odd:

println(filter((1..10).toList()) {
    it % 2 != 0
}) ...