Re: FFTW versus NRC FFT

I just started to learn something about FFT and try to get familar with FFTW.
I used the following code to calculate fourier transform of data sampled from exp(-t/4) with rate 1s.


#include <iostream>
#include <math.h>
#include <fftw3.h>

using namespace std;

int main()
{ double* in;
fftw_complex* out;
int N=32;
in = (double*) fftw_malloc(sizeof(double) * N);
out = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);

fftw_plan plan;

plan=fftw_plan_dft_r2c_1d(N,in,out,FFTW_MEASURE);
in[0]=0.5;

cout<<"input data"<<endl;
for(int i=1;i<N;i++){in[i]=exp(-double(i)/4);cout<<in[i]<<" "<<endl;}
cout<<endl;

fftw_execute(plan);

cout<<"output data"<<endl;
for(int i=0;i<N/2;i++)
{
for(int j=0;j<2;j++)cout<<out[i][j]<<" "<<" ";
cout<<endl;
}
fftw_free(in);
fftw_free(out);

return 0;

}

the result is about 4 times larger than what i calculated directly from 1/(1+w*w),
not sure why is that.

thanks for your help
.

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