Kuba Ober a écrit :
  

I need functionality of map, but done in such a way that the output array
is given as the argument, not as a return value. The closest I could get was

let inplace_map f a b = Array.blit (map f a) 0 b 0 (Array.length a)

Arrays a and b are of the same size.
This seems very inelegant.
    

I guess you mean Array.map. Indeed, this is not optimal because
Array.map allocates a new array, whose contents is immediately copied
into b.

  

One could use an adapter function and iteri, but that adds very noticeable 
overhead, and doesn't seem too elegant either.

let inplace_map f a b = Array.iteri (fun i src -> b.(i) <- f src; ()) a
    

You may find this inelegant too, but it is clearly more efficient than
your previous version using map. Note that "; ()" can be omitted, since
the assignment <- already has type unit.

Equivalently, you could use a for loop instead of Array.iteri.