Same input ,different ans? matlab Vs FFTW

For learning use of the FFTW, i wrote a sample program,but campared with
the result of Matlab,i find the ans are totally different. how can i solve
this problem ? thank u!  i'm looking forward for ur answer.

The result of Matlab:
a=[1 2 3 1 2 3 1 2];
ans =

  15.0000          
  -1.0000 + 0.5858i
  -1.0000 + 2.0000i
  -1.0000 - 3.4142i
  -1.0000          
  -1.0000 + 3.4142i
  -1.0000 - 2.0000i
  -1.0000 - 0.5858i

The result of FFTW is 
===============================
[0]real=0.000000 img=-1.000000
[1]real=-0.585786 img=-1.000000
[2]real=-2.000000 img=-1.000000
[3]real=3.414214 img=-1.000000
[4]real=0.000000
img=-6277438562204192500000000000000000000000000000000000000000
000000000.000000
[5]real=-6277438562204192500000000000000000000000000000000000000000000000000.000
000
img=-6277438562204192500000000000000000000000000000000000000000000000000.000
000
[6]real=-6277438562204192500000000000000000000000000000000000000000000000000.000
000
img=-6277438562204192500000000000000000000000000000000000000000000000000.000
000
[7]real=-6277438562204192500000000000000000000000000000000000000000000000000.000
000
img=-6277438562204192500000000000000000000000000000000000000000000000000.000
000
===============================




====================================the program==================
int main(int argc, char* argv[])
{
	fftw_complex *out;
	double in[]={1,2,3,1,2,3,1,2};
	fftw_plan p;
	//in=(fftw_complex *)fftw_malloc(sizeof(fftw_complex)*8);
	out=(fftw_complex *)fftw_malloc(sizeof(fftw_complex)*10);
	

	p=fftw_plan_dft_r2c_1d(8,in,out,FFTW_ESTIMATE);
	fftw_execute(p);

	for(int i=0;i<8;i++)
	{
		printf("[%d]real=%f img=%f \n",i,out[i][1],out[i][2]);
	}
}

"Vincent2046" <cui.z.w@gmx.net> wrote in message
news:obWdnXUK6b2prbrZnZ2dnUVZ_vednZ2d@giganews.com...
> For learning use of the FFTW, i wrote a sample program,but campared with
> the result of Matlab,i find the ans are totally different. how can i solve
> this problem ? thank u!  i'm looking forward for ur answer.
>
> The result of Matlab:
> a=[1 2 3 1 2 3 1 2];
> ans =
>
>   15.0000
>   -1.0000 + 0.5858i
>   -1.0000 + 2.0000i
>   -1.0000 - 3.4142i
>   -1.0000
>   -1.0000 + 3.4142i
>   -1.0000 - 2.0000i
>   -1.0000 - 0.5858i
>
> The result of FFTW is
> ===============================
> [0]real=0.000000 img=-1.000000
> [1]real=-0.585786 img=-1.000000
> [2]real=-2.000000 img=-1.000000
> [3]real=3.414214 img=-1.000000
> [4]real=0.000000
> img=-6277438562204192500000000000000000000000000000000000000000
> 000000000.000000
>
[5]real=-6277438562204192500000000000000000000000000000000000000000000000000
.000
> 000
>
img=-6277438562204192500000000000000000000000000000000000000000000000000.000
> 000
>
[6]real=-6277438562204192500000000000000000000000000000000000000000000000000
.000
> 000
>
img=-6277438562204192500000000000000000000000000000000000000000000000000.000
> 000
>
[7]real=-6277438562204192500000000000000000000000000000000000000000000000000
.000
> 000
>
img=-6277438562204192500000000000000000000000000000000000000000000000000.000
> 000
> ===============================
>
>
>
>
> ====================================the program==================
> int main(int argc, char* argv[])
> {
> fftw_complex *out;
> double in[]={1,2,3,1,2,3,1,2};
> fftw_plan p;
> //in=(fftw_complex *)fftw_malloc(sizeof(fftw_complex)*8);
> out=(fftw_complex *)fftw_malloc(sizeof(fftw_complex)*10);
>
>
> p=fftw_plan_dft_r2c_1d(8,in,out,FFTW_ESTIMATE);
> fftw_execute(p);
>
> for(int i=0;i<8;i++)
> {
> printf("[%d]real=%f img=%f \n",i,out[i][1],out[i][2]);
> }
> }
>
>
Pick a simpler case where you can hand-calculate the answer. Eg take

[1,0,1,0] or whatever.


Tam

Vincent2046 wrote:

> For learning use of the FFTW, i wrote a sample program,but campared with
> the result of Matlab,i find the ans are totally different. how can i solve
> this problem ? thank u!  i'm looking forward for ur answer.
/*snip*/
> ====================================the program==================
> int main(int argc, char* argv[])
> {
> fftw_complex *out;
> double in[]={1,2,3,1,2,3,1,2};
> fftw_plan p;
> //in=(fftw_complex *)fftw_malloc(sizeof(fftw_complex)*8);
> out=(fftw_complex *)fftw_malloc(sizeof(fftw_complex)*10);
> 
> 
> p=fftw_plan_dft_r2c_1d(8,in,out,FFTW_ESTIMATE);
> fftw_execute(p);
> 
> for(int i=0;i<8;i++)
> {
> printf("[%d]real=%f img=%f \n",i,out[i][1],out[i][2]);
> }
> }

In C (or C++) the indexing starts from 0. So you have to change your
printing to
for(int i=0;i<8;i++)
{
        printf("[%d]real=%f img=%f \n",i,out[i][0],out[i][1]);
}


-- 
Jani Huhtanen
Tampere University of Technology, Pori