.where for performing masked operations

[CGenie]

This is a follow-up of #48

(I think boolean arrays are not implemented yet, so this leaves us at integer array with 0 as false, 1 as true)

In numpy one can do:

np.where(condition, x, y)

https://numpy.org/doc/2.2/reference/generated/numpy.where.html

x and y are "array-like" i.e. can be an array or scalar. The result is an array with x where condition is true and y otherwise.

I guess this can be implemented via mapIndexed but it would be nice to have in the stdlib.

[CGenie]

My Kotlin is poor but something along this:

@JvmName("whereNDArray")
inline fun <T, D: Dimension> NDArray<Int, D>.where(x: NDArray<T, D>, default: T): NDArray<T, D> {
    val ret = x.copy()
    val indexIter = ret.multiIndices.iterator()
    indexIter.forEach {
        if (this[it] == 0) {
            ret[it] = default;
        }
    }

    return ret
}

and then

var a = mk.ndarray(mk[1, 2, 3])
var a_cond = a.map { x -> if (x > 2) 0 else 1 }
println(a_cond.where(a, -1))

[CGenie]

BTW, this throws a runtime exception:

println(a.map { x -> x > 2 })

Exception in thread "main" java.lang.IllegalStateException: One of the primitive types was expected, got Boolean