Finding position of a value in an array

[mipri]

On Sunday, 29 December 2019 at 08:26:58 UTC, Daren Scot Wilson 
wrote:
> Reading documentation... Array, Algorithms, ... maybe I've been 
> up too late... how does one obtain the index of, say, 55 in an 
> array like this
>
>     int[] a = [77,66,55,44];
>
> I want to do something like:
>
>     int i = a.find_value_returning_its_index(55);
>     assert(i==2)
>
> I'm sure it's obvious but I'm not seeing it right now.

int i = a.countUntil!(v => v == 55);
assert(i == 2);

It might not be an obvious application of countUntil, but it's
listed at https://dlang.org/phobos/std_algorithm_searching.html