92 lines
4.2 KiB
HTML
92 lines
4.2 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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<title>Column-major Format (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Multi_002ddimensional-Array-Format.html" rel="up" title="Multi-dimensional Array Format">
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<link href="Fixed_002dsize-Arrays-in-C.html" rel="next" title="Fixed-size Arrays in C">
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<link href="Row_002dmajor-Format.html" rel="prev" title="Row-major Format">
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<span id="Column_002dmajor-Format"></span><div class="header">
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<p>
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Next: <a href="Fixed_002dsize-Arrays-in-C.html" accesskey="n" rel="next">Fixed-size Arrays in C</a>, Previous: <a href="Row_002dmajor-Format.html" accesskey="p" rel="prev">Row-major Format</a>, Up: <a href="Multi_002ddimensional-Array-Format.html" accesskey="u" rel="up">Multi-dimensional Array Format</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Column_002dmajor-Format-1"></span><h4 class="subsection">3.2.2 Column-major Format</h4>
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<span id="index-column_002dmajor"></span>
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<p>Readers from the Fortran world are used to arrays stored in
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<em>column-major</em> order (sometimes called “Fortran order”). This is
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essentially the exact opposite of row-major order in that, here, the
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<em>first</em> dimension’s index varies most quickly.
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</p>
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<p>If you have an array stored in column-major order and wish to
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transform it using FFTW, it is quite easy to do. When creating the
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plan, simply pass the dimensions of the array to the planner in
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<em>reverse order</em>. For example, if your array is a rank three
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<code>N x M x L</code> matrix in column-major order, you should pass the
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dimensions of the array as if it were an <code>L x M x N</code> matrix
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(which it is, from the perspective of FFTW). This is done for you
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<em>automatically</em> by the FFTW legacy-Fortran interface
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(see <a href="Calling-FFTW-from-Legacy-Fortran.html">Calling FFTW from Legacy Fortran</a>), but you must do it
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manually with the modern Fortran interface (see <a href="Reversing-array-dimensions.html">Reversing array dimensions</a>).
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<span id="index-Fortran-interface"></span>
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</p>
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