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How can I use functional code to achieve the same effect as the following procedural code

Do[
    Do[
        f[i,j],
        {j,1,m}
      ],
    {i,1,n}
   ]
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    $\begingroup$ There's the unnested Do[f[i, j], {i, n}, {j, m}], which is almost as fast as Array and Outer (faster if m*n is very large) and quite a bit faster than Apple or Scan. And Do uses very little memory. -- Now, my understanding is that in functional programming, one generally aims for functions to have no side effects, in which case, Null seems the most efficient to accomplish the above. :) In other words, looking for functional code to accomplish a non-functional task seems odd. $\endgroup$
    – Michael E2
    Commented Apr 16, 2022 at 22:49
  • $\begingroup$ Scan might be useful in this setting. $\endgroup$
    – Daniel Lichtblau
    Commented Apr 17, 2022 at 0:53

1 Answer 1

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9
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Defining a function with some side-effect for demonstration purposes.

f[i_, j_] := Echo[{i, j}]

Here are some more WL-esque options:

Array[f, {2, 3}];

Outer[f, Range[2], Range[3], 1];

f @@@ Tuples[{Range[2], Range[3]}];

Note the ; above to suppress the output. If your function indeed only has side-effects and you don't want the output (supposedly that's why you chose Do instead of Table), you can also use Scan:

Scan[Echo, Tuples[{Range[2], Range[3]}]]
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