[feature request]fold2

[mars0i]

I wrote a compare function in terms of fold2, which seems generally useful. It would probably belong in owl_dense_matrix_generic.ml.

let fold2 f init m1 m2 =
  let rows, cols as dims = shape m1 in
  if dims <> shape m2 then failwith "fold2: matrices have different shapes"
  ;
  let last_col = cols - 1 in
  let apply_f acc i j = 
    f acc (get m1 i j) (get m2 i j)
  in
  let rec loop acc i j =
    if i < rows
    then loop (apply_f acc i j) (i + 1) j
    else if j < last_col         (* don't start on next col if at final col *)
         then loop acc 0 (j + 1) (* start over on next col *)
         else acc
  in loop init 0 0

It's not pretty, and the style probably shows my inexperience, but it's tail recursive. I didn't succeed in coming up with a more elegant purely functional definition. However, I've noticed that existing fold functions in owl use imperative methods internally. I assume that's more efficient, so perhaps you wouldn't want to a version of fold2 similar to this one. I'm not sure what the tradeoffs are between different iter function variants (iteri is defined in owl_dense_matrix_generic.ml, but iter there is defined by the iter in owl_dense_ndarray_generic.ml, which I haven't sorted out fully).

[mars0i]

Perhaps something like this would be useful, too. (I use it for a special-purpose compare function that I wrote.) It's like fold2, but will abort the loop if the accumulator comes to have a specified stop_val value. Other than the extra parameter, this differs from my fold2 by the addition of a single line of code in the loop function.

(** Fold f over matrices m1 and m2 starting with initial value init, 
    short-circuiting if stop_val is encountered:
    Folds f through all corresponding pairs of elements of matrices m1 
    and m2 by repeatedly applying f acc element_from_m1 element_from_m2,
    where acc is the result of previous applications.  init is the
    initial value for acc.  If f ever returns stop_val, this function
    returns immediately. *)
let short_circuit_fold2 stop_val f init m1 m2 =
  let rows, cols as dims = shape m1 in
  if dims <> shape m2 then failwith "matrices have different shapes"
  ;
  let last_col = cols - 1 in
  let apply_f acc i j = 
    f acc (get m1 i j) (get m2 i j)
  in
  let rec loop acc i j =
    if acc = stop_val then stop_val
    else if i < rows
    then loop (apply_f acc i j) (i + 1) j
    else if j < last_col         (* don't start on next col if at final col *)
         then loop acc 0 (j + 1) (* start over on next col *)
         else acc
  in loop init 0 0

[ryanrhymes]

Cool, Marshall, many thanks for the code, I will borrow it :) 👍