fftw/c++ computes fft wrong, compared to matlab -

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i trying fftw c++. want test works correct. implemented simple 

ifft(fft(shift(data)) - data == 0

test, fails completely.

the testdata rect function, amplitude , phase 1. matlab code comparison works same test.

the basic question is: doing wrong?

here matlab code (which using fftw...) fftw dll/.h latest.

data = zeros(1, 64); halfsize = numel(data)/2; data(halfsize-10:halfsize+10) = 1;  phase = ones(size(data)); data =  data.*exp(phase*sqrt(-1));  ft = fft(fftshift(data));

original data fourier transform

in c++ code (not complete)

std::vector<complex<double>,fftalloc<complex<double> > > data(n); std::vector<complex<double>,fftalloc<complex<double> > > datafourier(n); ... create data int nfft = data.size(); fftw_plan plan = fftw_plan_dft_1d(nfft,fftw_cast(&data[0]),fftw_cast(&datafourier[0]), fftw_forward, fftw_estimate|fftw_preserve_input); fftw_execute(plan); //fftw_execute_dft( plan, fftw_cast(&data[0]),fftw_cast(&datafourier[0])); cout << datafourier[0] << datafourier.back() << endl;

the output different c++ plots

the first complex value different last

(59.8627,7.57324)(-4.00561,7.33222)

whereas in matlab similar. phase different:

11.3463 +17.6709i  10.8411 +13.7128i

for higher n these values same (here n = 64)

fftw , matlab don't compute same things. the fftw tutorial:

fftw computes unnormalized dft. thus, computing forward followed backward transform (or vice versa) results in original array scaled n. definition of dft, see what fftw computes.


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