Chris@42: <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Chris@42: <html>
Chris@42: <!-- This manual is for FFTW
Chris@42: (version 3.3.5, 30 July 2016).
Chris@42:
Chris@42: Copyright (C) 2003 Matteo Frigo.
Chris@42:
Chris@42: Copyright (C) 2003 Massachusetts Institute of Technology.
Chris@42:
Chris@42: Permission is granted to make and distribute verbatim copies of this
Chris@42: manual provided the copyright notice and this permission notice are
Chris@42: preserved on all copies.
Chris@42:
Chris@42: Permission is granted to copy and distribute modified versions of this
Chris@42: manual under the conditions for verbatim copying, provided that the
Chris@42: entire resulting derived work is distributed under the terms of a
Chris@42: permission notice identical to this one.
Chris@42:
Chris@42: Permission is granted to copy and distribute translations of this manual
Chris@42: into another language, under the above conditions for modified versions,
Chris@42: except that this permission notice may be stated in a translation
Chris@42: approved by the Free Software Foundation. -->
Chris@42: <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
Chris@42: <head>
Chris@42: <title>FFTW 3.3.5: Dynamic Arrays in C</title>
Chris@42:
Chris@42: <meta name="description" content="FFTW 3.3.5: Dynamic Arrays in C">
Chris@42: <meta name="keywords" content="FFTW 3.3.5: Dynamic Arrays in C">
Chris@42: <meta name="resource-type" content="document">
Chris@42: <meta name="distribution" content="global">
Chris@42: <meta name="Generator" content="makeinfo">
Chris@42: <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Chris@42: <link href="index.html#Top" rel="start" title="Top">
Chris@42: <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
Chris@42: <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@42: <link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format">
Chris@42: <link href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way" rel="next" title="Dynamic Arrays in C-The Wrong Way">
Chris@42: <link href="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C" rel="prev" title="Fixed-size Arrays in C">
Chris@42: <style type="text/css">
Chris@42: <!--
Chris@42: a.summary-letter {text-decoration: none}
Chris@42: blockquote.smallquotation {font-size: smaller}
Chris@42: div.display {margin-left: 3.2em}
Chris@42: div.example {margin-left: 3.2em}
Chris@42: div.indentedblock {margin-left: 3.2em}
Chris@42: div.lisp {margin-left: 3.2em}
Chris@42: div.smalldisplay {margin-left: 3.2em}
Chris@42: div.smallexample {margin-left: 3.2em}
Chris@42: div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
Chris@42: div.smalllisp {margin-left: 3.2em}
Chris@42: kbd {font-style:oblique}
Chris@42: pre.display {font-family: inherit}
Chris@42: pre.format {font-family: inherit}
Chris@42: pre.menu-comment {font-family: serif}
Chris@42: pre.menu-preformatted {font-family: serif}
Chris@42: pre.smalldisplay {font-family: inherit; font-size: smaller}
Chris@42: pre.smallexample {font-size: smaller}
Chris@42: pre.smallformat {font-family: inherit; font-size: smaller}
Chris@42: pre.smalllisp {font-size: smaller}
Chris@42: span.nocodebreak {white-space:nowrap}
Chris@42: span.nolinebreak {white-space:nowrap}
Chris@42: span.roman {font-family:serif; font-weight:normal}
Chris@42: span.sansserif {font-family:sans-serif; font-weight:normal}
Chris@42: ul.no-bullet {list-style: none}
Chris@42: -->
Chris@42: </style>
Chris@42:
Chris@42:
Chris@42: </head>
Chris@42:
Chris@42: <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42: <a name="Dynamic-Arrays-in-C"></a>
Chris@42: <div class="header">
Chris@42: <p>
Chris@42: Next: <a href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way" accesskey="n" rel="next">Dynamic Arrays in C-The Wrong Way</a>, Previous: <a href="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C" accesskey="p" rel="prev">Fixed-size Arrays in C</a>, Up: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" 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#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@42: </div>
Chris@42: <hr>
Chris@42: <a name="Dynamic-Arrays-in-C-1"></a>
Chris@42: <h4 class="subsection">3.2.4 Dynamic Arrays in C</h4>
Chris@42:
Chris@42: <p>We recommend allocating most arrays dynamically, with
Chris@42: <code>fftw_malloc</code>. This isn’t too hard to do, although it is not as
Chris@42: straightforward for multi-dimensional arrays as it is for
Chris@42: one-dimensional arrays.
Chris@42: </p>
Chris@42: <p>Creating the array is simple: using a dynamic-allocation routine like
Chris@42: <code>fftw_malloc</code>, allocate an array big enough to store N
Chris@42: <code>fftw_complex</code> values (for a complex DFT), where N is the product
Chris@42: of the sizes of the array dimensions (i.e. the total number of complex
Chris@42: values in the array). For example, here is code to allocate a
Chris@42: 5 × 12 × 27 rank-3 array:
Chris@42: <a name="index-fftw_005fmalloc-2"></a>
Chris@42: </p>
Chris@42: <div class="example">
Chris@42: <pre class="example">fftw_complex *an_array;
Chris@42: an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex));
Chris@42: </pre></div>
Chris@42:
Chris@42: <p>Accessing the array elements, however, is more tricky—you can’t
Chris@42: simply use multiple applications of the ‘<samp>[]</samp>’ operator like you
Chris@42: could for fixed-size arrays. Instead, you have to explicitly compute
Chris@42: the offset into the array using the formula given earlier for
Chris@42: row-major arrays. For example, to reference the <em>(i,j,k)</em>-th
Chris@42: element of the array allocated above, you would use the expression
Chris@42: <code>an_array[k + 27 * (j + 12 * i)]</code>.
Chris@42: </p>
Chris@42: <p>This pain can be alleviated somewhat by defining appropriate macros,
Chris@42: or, in C++, creating a class and overloading the ‘<samp>()</samp>’ operator.
Chris@42: The recent C99 standard provides a way to reinterpret the dynamic
Chris@42: array as a “variable-length” multi-dimensional array amenable to
Chris@42: ‘<samp>[]</samp>’, but this feature is not yet widely supported by compilers.
Chris@42: <a name="index-C99"></a>
Chris@42: <a name="index-C_002b_002b-2"></a>
Chris@42: </p>
Chris@42:
Chris@42:
Chris@42:
Chris@42: </body>
Chris@42: </html>