Populating and array in a matrix style - arrays

Question

I want to populate an array in a matrix style, on columns. So for example, if I had an array input:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Then the matrix would be (for numRows = 3, numCols=5):
1 4 7 10 13
2 5 8 11 14
3 6 9 12 15
This means the real array would be:
1 4 7 10 13 2 5 8 11 14 3 6 9 12 15
How would I do that?
Here is my take:
int v[15] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
int matrix[15];
int row = 0, col = 0;
for (int i = 0; i < 15; ++i) {
matrix[col * 5 + row] = v[i];
printf("row = %d col = %d\n", row, col);
row++;
if (row == 3) {
row = 0;
col++;
}
}
```

Answer 1

You were very close in your last code example. The short answer is that rather than this:
matrix[col * 5 + row] = v[i];
You want:
matrix[row * 5 + col] = v[i];
In other words, 'row' and 'col' need to be the other way around.
Here's your example with that change made, and the results printed at the end:
int main(int argc, const char * argv[]) {
int v[15] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
int matrix[15];
int row = 0, col = 0;
for (int i = 0; i < 15; ++i) {
matrix[row * 5 + col] = v[i];
printf("row = %d col = %d\n", row, col);
row++;
if (row == 3) {
row = 0;
col++;
}
}
for (int i = 0; i < 15; ++i) {
printf("%d ", matrix[i]);
}
printf("\n");
}
The output at the end for me is:
1 4 7 10 13 2 5 8 11 14 3 6 9 12 15
Which appears to match the output you were after in your post.
There are multiple conversions that can be made here: from 2-d row/column indices to 1-d indices in row- or column-major order, and from 1-d indices in row- or column-major order to 2-d row/column indices. The math is fairly simple for all the conversions (converting from 1-d to 2-d indices involves integer division and modulus). Once you figure out the conversions you need, I suggest creating a little library of functions for performing the conversions so that you have the functionality available when you need it.
Lastly, a couple general suggestions. Your original code wrote outside array bounds, which wasn't checked for, at least in what you posted. For exploratory work like this, I'd suggest using bounds checking so that you can detect errors more easily. (For example, in this case you could use std::array and the at() function.) This will help you catch a certain class of mistakes early.
I'd also suggest using named constants, even in a small example like this, e.g. 'numRows' and 'numCols' instead of 3 and 5, and 'numElements' as the product of those instead of 15. This will also help avoid errors, and will also make the code easier to read and understand (both for you and others).