Which is better, a single for loop or double for loop to iterate through 2-D array? C++

Question

Let's say that I have a 9-by-9 2-D array. Is there a difference between looping through with a single loop or multiple loops?

for (int i = 0; i < 81; i++)
  currentEdit[i / 9][i % 9] = 0;

VS.

for (int i = 0; i < 9; i++)
   for (int j = 0; j < 9; j++)
      currentEdit[i][j] = 0;

Well, the first uses divisions, the second doesn't. Assuming the compiler doesn't just make them the same on its own, the second would be better since it doesn't have divisions, and is easier to read. — ChrisMM
– ChrisMM, Commented Apr 9, 2020 at 23:28
The second set of loops will generate better assembly.. but I honestly don't see it making much of a difference unless this is extremely critical.. — Brandon
– Brandon, Commented Apr 9, 2020 at 23:49
You should not assume the memory layout, and the code should be transparent as to what it's actually doing. 2nd version. — user207421
– user207421, Commented Apr 9, 2020 at 23:51

Rodrigo · Accepted Answer · 2020-04-09 23:41:41Z

2

The right choice is multiple loops. Keep in mind that it will perform much less operations since it does not have to divide or calculate the module to access the array position.

This is the right choice:

for (int i = 0; i < 9; i++)
   for (int j = 0; j < 9; j++)
      currentEdit[i][j] = 0;

answered Apr 9, 2020 at 23:41

Rodrigo

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