Finding position of a value in an array

[Daren Scot Wilson]

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.