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Row major means that all your data depend on the width.

When parsing the data (an array), you start at index 0, when index = width - 1 then you have parsed through all the first row. So when index is in [0, width - 1] : row = 0 and column = index.

Now if you are in the second row then index will be in [width, 2*width - 1]: row = 1 and column = index - width

For third row index is between [2*width, 3*width-1] : row = 2 and column = index - 2*width

We see a pattern here and we can deduce the formula for row and column. The column is always the rest of index minus a multiple of the width, hence a modulo so column = index % width. Now this multiple corresponds to the row so row = (index - index % width)/width