need help with fftw..

**kris.c** · 07-11-2006

I am trying to get a complex to complex fourier transform using fftw..
I first allocate space for 16 complex numbers. I initialise them to random numbers and then i perfrom the transform...
I am aware of the fact that in a complex to real mapping, the size of the output array is half the size of hte input array..
take a look at the output i have pasted here. i am getting all elements whose index is greater than (input array size)/2 to be zeros...
is this a valid output or have i done anything wrong..
Also, I have never used a complex array before, have i dealt with the initialisation correctly?

Code:

# include<complex.h>
# include<fftw3.h>
# include<time.h>
# include<math.h>
# define m 16


int main(int argc ,char *argv[])
 {
                                                                                                                             
 int rc,i,j,sender,tag,number=0;
 fftw_complex *in;
 fftw_complex *out,*indexin,*indexout;
 int count=0,sta,end,myid,nprocs;
 double time1,time2;
 fftw_plan p;
                                                                                                                             
                                                                                                                             
                                                                                                                             
 in=(complex *)(fftw_malloc(sizeof(complex)*m));
 out=(complex *)(fftw_malloc(sizeof(complex)*m));
 indexin=in;
 indexout=out;
                                                                                                                             
                                                                                                                             

                                                                                                                             
       for(i=0;i<m;i++)
        {
            *in=rand();
             printf("%d\t%f + i" ,i,*in++);
                                                                                                                             
            *in=rand();
             printf(" %f\n",*in++);
        }
                                                                                                                             
                                                                                                                             
  in=indexin;
    p=fftw_plan_dft_1d(m,in,out,FFTW_FORWARD,FFTW_ESTIMATE);
  printf("\noutput array \n");
  fftw_execute(p);
                                                                                                                             
//  out=indexout;
                                                                                                                             
  for(j=0;j<m;j++)
   {
      printf("%d\t%f +i ",j,*out++);
      printf("%f\n",*out++);
   }
                                                                                                                             
   printf("time consumed =%f\n",time2-time1);
                                                                                                                             
   fftw_destroy_plan(p);
       fftw_free(indexin);
       fftw_free(indexout);
                                                                                                                             
                                                                                                                             
return 0;
}

this is the output i am getting :
output:

Code:

 0       1804289383.000000 + i 846930886.000000
1       1681692777.000000 + i 1714636915.000000
2       1957747793.000000 + i 424238335.000000
3       719885386.000000 + i 1649760492.000000
4       596516649.000000 + i 1189641421.000000
5       1025202362.000000 + i 1350490027.000000
6       783368690.000000 + i 1102520059.000000
7       2044897763.000000 + i 1967513926.000000
8       1365180540.000000 + i 1540383426.000000
9       304089172.000000 + i 1303455736.000000
10      35005211.000000 + i 521595368.000000
11      294702567.000000 + i 1726956429.000000
12      336465782.000000 + i 861021530.000000
13      278722862.000000 + i 233665123.000000
14      2145174067.000000 + i 468703135.000000
15      1101513929.000000 + i 1801979802.000000
 
output array
0       20859332864.000000 +i 2984769599.594296
1       410609648.520185 +i -1165990454.966789
2       -329755773.000000 +i 779267800.640235
3       -1091230550.520185 +i 2233043990.732258
4       367868742.000000 +i 2233043990.732258
5       -1091230550.520185 +i 779267800.640235
6       -329755773.000000 +i -1165990454.966789
7       410609648.520185 +i 2984769599.594296
8       0.000000 +i 0.000000
9       0.000000 +i 0.000000
10      0.000000 +i 0.000000
11      0.000000 +i 0.000000
12      0.000000 +i 0.000000
13      0.000000 +i 0.000000
14      0.000000 +i 0.000000
15      0.000000 +i 0.000000

**Cactus_Hugger** · 07-11-2006

Are you clobbering memory? Take this snippet:

Code:

       for(i=0;i<m;i++)
        {
            *in=rand();
             printf("%d\t%f + i" ,i,*in++);
            *in=rand();
             printf(" %f\n",*in++);
        }

Unless I've missed something, you increment variable in 32 times, but you've only allocated space for 16.
Also, can you do a (complex) = rand() ? What is type complex?

**kris.c** · 07-11-2006

from the man pages of fftw:

[ code]

The data is an array of type fftw_complex, which is by default a double[2] composed of the real (in[i][0]) and imaginary (in[i][1]) parts of a complex number.

[/code]

But, they havent described as to how complex types are stored in memory..
I think extending the way normal arrays are allocated , this would be happening :
the real part of the firlst complex number will be at in,
the imaginary part of the first would be at (in + 1)

the real part of the 2nd complex number will be ar (in+2) so on...

is this wrong??
if not ,if i have created an array of 16 complex numbers, there should be 32 reals in this array..

**kris.c** · 07-11-2006

I am not too sure about doing (complex)= rand() or something of that kind.. ok will try.. thats why i am loading the real and imaginary parts separately..

**Salem** · 07-12-2006

> in=(complex *)(fftw_malloc(sizeof(complex)*m));
Do you really need that cast?

> for(i=0;i<m;i++)
Instead of messing about with pointer increment, and perhaps also messing up the save/restore of your pointers, how about

Code:

for(i=0;i<m;i++) {
  in[i][0]=rand();
  in[i][1]=rand();
}

Use the same idea when you print them as well.

**kris.c** · 07-12-2006

surprisingly ,this is what i got ,when i did what u asked me to do:

Code:

 
       fourier_sc.c:37: error: subscripted value is neither array nor pointer

it is pointing to the line

Code:

      in[i][0]=rand();

referencing "malloc"ed arrays like this is legal, isnt it??
and i removed the type casting thing.. i still get the same thing..

I have included stdlib.h ..so, that should not be the problem

**Salem** · 07-12-2006

OK, how about pasting what those types really are?

**kris.c** · 07-12-2006

come again..

**kris.c** · 07-12-2006

will this suffice:

"
The data is an array of type fftw_complex, which is by default a double[2] composed of the real (in[i][0]) and imaginary (in[i][1]) parts of a complex number.

If you have a C compiler, such as gcc, that supports the recent C99 standard, and you #include <complex.h> before <fftw3.h>, then fftw_complex is the native double-precision complex type and you can manipulate it with ordinary arithmetic. "

**Salem** · 07-12-2006

> which is by default a double[2]
Which is why I suggested [0] and [1] subscripts.

But then you turn around and say that doesn't work.

So what is it?

I guess you need to go read up on C99 and find out how the C99 complex type works.

**kris.c** · 07-12-2006

well ,this is what i cud dig up :

Code:

 complex number 


/* complex in C99 is a built-in data type.
   complex in C++ is a class. They are not compatible.
   A program using complex conforming either C99 or C++ can readily run
   in Ch without modification. */
        
#include <stdio.h> 
#include <complex.h>
        
int main() {
    double complex z1 = 1+I*2;                // C99 and Ch
    double_complex z2 = double_complex(1, 2); // C++ and Ch
    double complex z3 = complex(1, 2);        // Ch
    double complex sz1, sz2, sz3;
          
    sz1 = csqrt(z1);                              // C99 and Ch
    sz2 = sqrt(z2);                               // C++ and Ch
    sz3 = sqrt(z3);                               // C++ and Ch
    printf("sz1 = %f, %f\n", creal(sz1), cimag(sz1)); // C99 and Ch
    printf("sz2 = %f, %f\n", real(sz2),  imag(sz2));  // C++ and Ch
    printf("sz3 = %f\n", sz3);                 // Ch, added for users's convenience
                       
}


--------------------------------------------------------------------------------
The output is: 
sz1 = 1.272020, 0.786151
sz2 = 1.272020, 0.786151
sz3 = complex(1.272020,0.786151)

i cant do any kind of testing for the next 8 hrs or so.. will doing something like this help :

Code:


# include<complex.h>
# include<fftw3.h>

.............................
............................

int j;
fftw_complex *in,*out;

in=malloc(sizeof(fftw_complex)*16);

for( j=0;j<16;j++)

  in[j] = rand() + I * rand();


.................................
................................   / * doing all the fftw stuff here */

/* the out put is stored in out */


for(j=0;j<16;j++)

  printf("%d\t%f + I * %f\n", j,creal(out[j]), cimag(out[j]);

..............................
..................................

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