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3.2.2 Column-major Format

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Readers from the Fortran world are used to arrays stored in Chris@82: column-major order (sometimes called “Fortran order”). This is Chris@82: essentially the exact opposite of row-major order in that, here, the Chris@82: first dimension’s index varies most quickly. Chris@82:

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If you have an array stored in column-major order and wish to Chris@82: transform it using FFTW, it is quite easy to do. When creating the Chris@82: plan, simply pass the dimensions of the array to the planner in Chris@82: reverse order. For example, if your array is a rank three Chris@82: N x M x L matrix in column-major order, you should pass the Chris@82: dimensions of the array as if it were an L x M x N matrix Chris@82: (which it is, from the perspective of FFTW). This is done for you Chris@82: automatically by the FFTW legacy-Fortran interface Chris@82: (see Calling FFTW from Legacy Fortran), but you must do it Chris@82: manually with the modern Fortran interface (see Reversing array dimensions). Chris@82: Chris@82:

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