Finding the nearest/closest value in a collection but not exceeding it

Question

I have a collection and want to return the nearest value to some fixed value but not exceed the fixed value. For example, if my collection looks like this:

val numbers = mutableListOf(23, 12, 64, 47, 36, 55)

and I have a target fixed value of 35, the value returned from the collection would be 23. Here are some other examples:

Target -> Returned
29 -> 23
5 -> null (there is no value less than 12, so null is returned)
70 -> 64

Is there some Collection function that I can use that would provide the expected result? Note: The list of numbers is not sorted. In the real app, these are not numbers but objects that do contain an integer property, but I could just as well sort the collection first on that value if that would help in the solution.

Answer 1

You can use fold function to hold the closest value in an accumulator. For instance,

val numbers = mutableListOf(23, 12, 64, 47, 36, 55)

val target = 35

val answer = numbers.fold(null){acc: Int?, num ->
    if(num <= target && (acc == null || num > acc)) num
    else acc
}

In case you want to break the loop if the target matches one of the values in the list you can have the following

val numbers = mutableListOf(23, 12, 64, 47, 36, 55)
val target = 35

fun MutableList<Int>.findClosest(input: Int) = fold(null) { acc: Int?, num ->
    val closest = if (num <= input && (acc == null || num > acc)) num else acc
    if (closest == input) return@findClosest closest else return@fold closest
}

val answer = numbers.findClosest(target)

The return keyword in the inner function will return from findClosest function as soon as the target matches a particular value