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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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1 <html lang="en">
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3 <title>Column-major Format - FFTW 3.3.3</title>
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49 <a name="Column-major-Format"></a>
50 <a name="Column_002dmajor-Format"></a>
51 <p>
52 Next:&nbsp;<a rel="next" accesskey="n" href="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C">Fixed-size Arrays in C</a>,
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57
58 <h4 class="subsection">3.2.2 Column-major Format</h4>
59
60 <p><a name="index-column_002dmajor-118"></a>
61 Readers from the Fortran world are used to arrays stored in
62 <dfn>column-major</dfn> order (sometimes called &ldquo;Fortran order&rdquo;). This is
63 essentially the exact opposite of row-major order in that, here, the
64 <em>first</em> dimension's index varies most quickly.
65
66 <p>If you have an array stored in column-major order and wish to
67 transform it using FFTW, it is quite easy to do. When creating the
68 plan, simply pass the dimensions of the array to the planner in
69 <em>reverse order</em>. For example, if your array is a rank three
70 <code>N x M x L</code> matrix in column-major order, you should pass the
71 dimensions of the array as if it were an <code>L x M x N</code> matrix
72 (which it is, from the perspective of FFTW). This is done for you
73 <em>automatically</em> by the FFTW legacy-Fortran interface
74 (see <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran">Calling FFTW from Legacy Fortran</a>), but you must do it
75 manually with the modern Fortran interface (see <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>).
76 <a name="index-Fortran-interface-119"></a>
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