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<title>Guru vector and transform sizes - FFTW 3.1.2</title>
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This manual is for FFTW
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(version 3.1.2, 23 June 2006).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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<a name="Guru-vector-and-transform-sizes"></a>
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Next: <a rel="next" accesskey="n" href="Guru-Complex-DFTs.html#Guru-Complex-DFTs">Guru Complex DFTs</a>,
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<h4 class="subsection">4.5.2 Guru vector and transform sizes</h4>
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<p>The guru interface introduces one basic new data structure,
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<code>fftw_iodim</code>, that is used to specify sizes and strides for
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multi-dimensional transforms and vectors:
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<pre class="example"> typedef struct {
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<p><a name="index-fftw_005fiodim-234"></a>
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Here, <code>n</code> is the size of the dimension, and <code>is</code> and <code>os</code>
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are the strides of that dimension for the input and output arrays. (The
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stride is the separation of consecutive elements along this dimension.)
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<p>The meaning of the stride parameter depends on the type of the array
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that the stride refers to. <em>If the array is interleaved complex,
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strides are expressed in units of complex numbers
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(</em><code>fftw_complex</code><em>). If the array is split complex or real, strides
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are expressed in units of real numbers (</em><code>double</code><em>).</em> This
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convention is consistent with the usual pointer arithmetic in the C
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language. An interleaved array is denoted by a pointer <code>p</code> to
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<code>fftw_complex</code>, so that <code>p+1</code> points to the next complex
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number. Split arrays are denoted by pointers to <code>double</code>, in
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which case pointer arithmetic operates in units of
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<code>sizeof(double)</code>.
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<a name="index-stride-235"></a>
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The guru planner interfaces all take a (<code>rank</code>, <code>dims[rank]</code>)
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pair describing the transform size, and a (<code>howmany_rank</code>,
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<code>howmany_dims[howmany_rank]</code>) pair describing the “vector” size (a
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multi-dimensional loop of transforms to perform), where <code>dims</code> and
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<code>howmany_dims</code> are arrays of <code>fftw_iodim</code>.
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<p>For example, the <code>howmany</code> parameter in the advanced complex-DFT
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interface corresponds to <code>howmany_rank</code> = 1,
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<code>howmany_dims[0].n</code> = <code>howmany</code>, <code>howmany_dims[0].is</code> =
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<code>idist</code>, and <code>howmany_dims[0].os</code> = <code>odist</code>.
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<a name="index-howmany-loop-236"></a><a name="index-dist-237"></a>(To compute a single transform, you can just use <code>howmany_rank</code> = 0.)
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<p>A row-major multidimensional array with dimensions <code>n[rank]</code>
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(see <a href="Row_002dmajor-Format.html#Row_002dmajor-Format">Row-major Format</a>) corresponds to <code>dims[i].n</code> =
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<code>n[i]</code> and the recurrence <code>dims[i].is</code> = <code>n[i+1] *
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dims[i+1].is</code> (similarly for <code>os</code>). The stride of the last
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(<code>i=rank-1</code>) dimension is the overall stride of the array.
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e.g. to be equivalent to the advanced complex-DFT interface, you would
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have <code>dims[rank-1].is</code> = <code>istride</code> and
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<code>dims[rank-1].os</code> = <code>ostride</code>.
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<p>In general, we only guarantee FFTW to return a non-<code>NULL</code> plan if
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the vector and transform dimensions correspond to a set of distinct
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indices, and for in-place transforms the input/output strides should
added
removed
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