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Next: <a href="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C" accesskey="n" rel="next">Fixed-size Arrays in C</a>, Previous: <a href="Row_002dmajor-Format.html#Row_002dmajor-Format" accesskey="p" rel="prev">Row-major Format</a>, Up: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" accesskey="u" rel="up">Multi-dimensional Array Format</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<h4 class="subsection">3.2.2 Column-major Format</h4>
<a name="index-column_002dmajor"></a>

<p>Readers from the Fortran world are used to arrays stored in
<em>column-major</em> order (sometimes called &ldquo;Fortran order&rdquo;).  This is
essentially the exact opposite of row-major order in that, here, the
<em>first</em> dimension&rsquo;s index varies most quickly.
</p>
<p>If you have an array stored in column-major order and wish to
transform it using FFTW, it is quite easy to do.  When creating the
plan, simply pass the dimensions of the array to the planner in
<em>reverse order</em>.  For example, if your array is a rank three
<code>N x M x L</code> matrix in column-major order, you should pass the
dimensions of the array as if it were an <code>L x M x N</code> matrix
(which it is, from the perspective of FFTW).  This is done for you
<em>automatically</em> by the FFTW legacy-Fortran interface
(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
manually with the modern Fortran interface (see <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>).
<a name="index-Fortran-interface"></a>
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Mon, 02 Mar 2020 14:03:47 +0000
parents	bd3cc4d1df30
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