c++ use 1D Array with 2D Data

Question

I do not use any matrix library, but instead plain std::vector for my matrix data.

To fill it with 2D data I use this code:

data[iy + dataPointsY * ix] = value;

I would like to know is this is correct or if it must be the other way (ix first). To my understanding fftw needs 'Row-major Format'. Since I use it the formula should be according to row-major format.

The key point is, they way you store the data should be exactly the way you retrieve. — phoxis
– phoxis, Commented Nov 4, 2016 at 15:26
sure, but I know that other libs rely on row.major format. Since I use it I should order the data according to it. — Matthias Pospiech
– Matthias Pospiech, Commented Nov 4, 2016 at 15:27
If you are trying to encode the 2d indexing in a linear format which some other library expects, then you have to follow exactly the format specified by that library. Like MPI functions would require all the data elements of a matrix should be adjacent in memory. — phoxis
– phoxis, Commented Nov 4, 2016 at 15:29
Also, if you traverse the matrix column wise, (that is top to bottom for one single column), then you should consider storing it in a column-major format, as in that case the elements would be adjacent, allowing large datasets being traversed in lesser page faults. — phoxis
– phoxis, Commented Nov 4, 2016 at 15:30
The question is not whether this format is "correct". The question is what format does the fftw library expect. — Chris Drew
– Chris Drew, Commented Nov 4, 2016 at 15:31

Kh40tiK · Accepted Answer · 2016-11-04 17:32:56Z

3

Assuming you want row major format for fftw, what you want is:

data[ix + iy*dataPointsY]

The point of row-major is, when the combined index increased by 1, the corresponding row index would be same (assuming not overflowing to the next row).

double m[4][4];
mp = (double*)m;
mp[1+2*3] == m[2][1]; //true
mp[2+2*3] == m[2][2]; //true
mp[2+2*3] == m[3][1]; //false

In general, there's no "right" way to store a matrix. Row major format is also called "C-style" matrix, while column major is called "fortran-style" matrix. The naming is due to different multidimensional array indexing scheme between the two language.

edited Nov 4, 2016 at 17:32

answered Nov 4, 2016 at 17:27

Kh40tiK

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