Moving through each element of a (potentially non square) 2d array, diagonally - arrays

Question

I was playing around with 2d arrays in c, and I am wondering how to traverse a 2d array, fully and diagonally.
Horizontally, in the matrix of dimensions width,height
you can just move through each index i, and inspect elements at index j
Something like:
const int width = 10;
const int height = 10;
const int mat[width][height] = {0};
for (i = 0, i<width, i++){
for (j = 0; j<height; j++){
mat[i][0] = j;
}
}
I just added in something random so the loop did something..., however, the key is that I was traversing in the correct direction
vertically would be similar, with some flipped parameters
however diagonally...I am a bit lost; I cannot think of a way to traverse in a diagonal way. Conceptually I may want to hit the 4x3 matrix in the following order:
1 2 4 7
3 5 8 10
6 9 11 12
Or with indices i,j :
0,0 ->
1,0 -> 0,1 ->
2,0 -> 1,1 -> 0,2 ->
2,1 -> 1,2 -> 0,3 ->
2,2 -> 1,3 ->
2,3
Is there a straightforward way to hit these elements(not necessarily in that order per say, but I think it would be useful to increment diagonally)
Also, is it possible to check the diagonals in the opposite direction ?

Answer 1

For diagonal traverse, use only one for loop, as both i and j are identical:
for (i = 0, i < min(width, hight), i++){
// something to do with element[i][i]
}

Answer 2

This is a duplicate of Traverse Matrix in diagonal strips where is it suggested something like :
int main(void)
{
int m[3][4] = { 1, 2, 4, 7,
3, 5, 8,10,
6, 9,11,12 };
const int h = 3; /* height*/
const int w = 4; /* width */
for (int diag = 0; diag < (w + h - 1); ++diag)
{
printf("diag %d:",diag);
int i_start = diag < w ? 0 : diag - w + 1;
int i_end = diag < h ? diag + 1 : h;
for (int i = i_start; i < i_end; ++i)
{
printf("%d ",m[i][diag - i]);
}
printf("\n");
}
return 0;
}
Output:
diag 0:1
diag 1:2 3
diag 2:4 5 6
diag 3:7 8 9
diag 4:10 11
diag 5:12