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Question[C++] Fast Fourier Transform algorithm does not work properly Pin
Member 141687012-Mar-19 1:51
memberMember 141687012-Mar-19 1:51 
Hello, there is a pseudocode for recursive radix 2 DIT Fast Fourier Transform(page 11,12) at this link.

I have written a code in c++ which does the same, but it doesn't work properly.
For example when I take 1024 datapoints of sin(0.1*x) where x={0,1,...1023}, I get this.

Reverse transform inserted for this gives initial function multiplied by 1024, so it works.
What could be wrong? I tried with value dir and dir/2 inside "Shift" function too.

Here is the code:
Notation: E2-even part, E3-odd part
I work without complex numbers library, so there are both "_re" and "_im" tables.

void FFT(double *tab_re, double *tab_im, double *results_re, double *results_im, int n, int dir)	//n-data size
{
if(n==1)
{results_re[0]=tab_re[0];	//real values
results_im[0]=tab_im[0];	//imaginary
return;}

int nh=n/2;

double *E2_re=new double [nh];
double *E3_re=new double [nh];
double *E2_im=new double [nh];
double *E3_im=new double [nh];

double *F2_re=new double [nh];
double *F3_re=new double [nh];
double *F2_im=new double [nh];
double *F3_im=new double [nh];

for(int k=0;k<nh;++k)	//even and odd parts
{E2_re[k]=tab_re[2*k];
E2_im[k]=tab_im[2*k];
E3_re[k]=tab_re[2*k+1];
E3_im[k]=tab_im[2*k+1];}

//recursion:
FFT(E2_re,E2_im,F2_re,F2_im,nh,dir);
FFT(E3_re,E3_im,F3_re,F3_im,nh,dir);

Shift(F3_re,F3_im,nh,dir);	//multiplying by phase

for(int k=0;k<nh;++k)
{
results_re[k]=F2_re[k]+F3_re[k];
results_im[k]=F2_im[k]+F3_im[k];
results_re[k+nh]=F2_re[k]-F3_re[k];
results_im[k+nh]=F2_im[k]-F3_im[k];}}




void Shift(double *tab_re, double *tab_im, int n, int v)
{double re,im;
double pi=3.14159265359;
double arg;

for(int i=0;i<n;++i)
{re=tab_re[i];
im=tab_im[i];

arg=v*2.0*pi*i/n;

tab_re[i]=re*cos(arg)-im*sin(arg);
tab_im[i]=re*sin(arg)+im*cos(arg);}}

AnswerRe: [C++] Fast Fourier Transform algorithm does not work properly Pin
Daniel Pfeffer2-Mar-19 22:08
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GeneralRe: [C++] Fast Fourier Transform algorithm does not work properly Pin
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AnswerRe: [C++] Fast Fourier Transform algorithm does not work properly Pin
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