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PODifficulty with zeros from 2D FFTW
 

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  1. PODifficulty with zeros from 2D FFTW
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    <p>I have a real matrix represented by the structure <code>double input[N][M]</code> where I want to take a 1D FT for each column j = 0..M along the N direction. My attempt at this is as follows:</p> <pre><code>#include &lt;fftw3.h&gt; #include &lt;math.h&gt; #define M_PI 3.14159265358979323846264338327 int main(void) { int N = 16; int M = 2; double input[N][M]; // input data double *in = (double *) input; fftw_complex output[N][M]; // output data fftw_complex *out = (fftw_complex *) output; for (int i = 0; i &lt; N; ++i) { for (int j = 0; j &lt; M; ++j) { double x = (double) i / (double) N; input[i][j] = sin(2*(j+1) * M_PI * x); fprintf (stderr, "input[%d][%d] = %.9f, ", i, j, input[i][j]); } fprintf (stderr, "\n"); } fprintf (stderr, "\n"); // setup plans int rank = 1; int howmany = M; int istride = M; int ostride = M; int idist = 1; int odist = 1; fftw_plan fp = fftw_plan_many_dft_r2c(rank, &amp;N, howmany, in, NULL, istride, idist, out, NULL, ostride, odist, FFTW_MEASURE); // take the forward transform fftw_execute(fp); fprintf (stderr, "FFT complete\n"); for (int j = 0; j &lt; M; ++j) for (int i = 0; i &lt; N/2; ++i) fprintf (stderr, "OUT[%3d] = (%.4f + %.4fi)\n", i, output[i][j][0], output[i][j][1]); fprintf (stderr, "\n"); fftw_destroy_plan(fp); return 0; } </code></pre> <p>which I'm compiling with <code>gcc fft.c -std=c99 -g -lfftw3 -lm</code>. However the code doesn't appear to be working: FT output is all zeros (see <a href="http://pastebin.com/Km2nZx6H" rel="nofollow">here</a>)</p> <p>Documentation for the function is <a href="http://www.fftw.org/doc/Advanced-Real_002ddata-DFTs.html#Advanced-Real_002ddata-DFTs" rel="nofollow">here</a>.</p> <p><strong>EDIT</strong>: update, so it only seems to work for FFTW_ESTIMATE and not any of the <a href="http://www.fftw.org/doc/Planner-Flags.html#Planner-Flags" rel="nofollow">other flags</a>. Any idea why this might be?</p>
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    1. PO
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    1. USHemmer
    1. USHemmer
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
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    1. COMm the valgrind output is confusing: http://pastebin.com/4jYw5c3G? Or should I use gdb? (relatively new to this!)
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      1. PODifficulty with zeros from 2D FFTW
      1. USHemmer
    2. COIf you check the Valgrind output, you see it prints the stack-trace, with the bottom of the stack being line 32 in the file `fftw.c`. Start by examining this line, its context and its variables.
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      1. PODifficulty with zeros from 2D FFTW
      1. USSome programmer dude
    3. COI'm not familiar with the FFTW library, but to me the declaration `int n[] = {N}` looks suspicious too. This will create an array containing only a single entry. Accessing (reading or writing) beyond this single entry is undefined behavior.
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      1. PODifficulty with zeros from 2D FFTW
      1. USSome programmer dude
 

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