Question

I have a project in C and I have to scan an array[10][15] diagonally as in the picture below.

I would like to help me to find how to scan the array this way…
Thank you very much.

P.S sorry for my english(I’m Greek)

Answer 1

Notice that along each diagonal, the difference* i - j is constant. So you have a nested double loop, where the outer one goes over the difference:

• First round: Difference -14, arr[0][14]

• Second round: Difference -13, arr[0][13], arr[1][14]

• …

• last round: Difference +9, arr[9][0].

In code:

for (int d = -14; d < 10; ++d)
{
    for (int i = 0; i < d + 15 && i < 10; ++i)
    {
      if (i < d) continue;

      // access arr[i][i - d];
      printf("[%d, %d]\n", i, i - d);
    }
}

Note the appearance of the numbers 10 and 15 in the code; the picture is readily generalized to arbitrary array bounds.

*) Or, as @Alexey points out, the sum i + j is constant, depending on where the origin is in your picture; in that case, modify the loop as follows:

for (int d = 0; d <= 9 + 14; ++d)
{
    for (int i = 0; i <= d && i < 10; ++i)
    {
        if (i + 15 <= d) continue;
        // use arr[i][d - i];
    }
}