need help with fftw..

This is a discussion on need help with fftw.. within the C Programming forums, part of the General Programming Boards category; I am trying to get a complex to complex fourier transform using fftw.. I first allocate space for 16 complex ...

  1. #1
    Eager young mind
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    need help with fftw..

    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
    Last edited by kris.c; 07-11-2006 at 10:30 PM.

  2. #2
    int x = *((int *) NULL); Cactus_Hugger's Avatar
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    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?
    long time; /* know C? */
    Unprecedented performance: Nothing ever ran this slow before.
    Any sufficiently advanced bug is indistinguishable from a feature.
    Real Programmers confuse Halloween and Christmas, because dec 25 == oct 31.
    The best way to accelerate an IBM is at 9.8 m/s/s.
    recursion (re - cur' - zhun) n. 1. (see recursion)

  3. #3
    Eager young mind
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    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..

  4. #4
    Eager young mind
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    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..

  5. #5
    and the hat of wrongness Salem's Avatar
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    > 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.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  6. #6
    Eager young mind
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    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
    Last edited by kris.c; 07-12-2006 at 01:39 AM.

  7. #7
    and the hat of wrongness Salem's Avatar
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    OK, how about pasting what those types really are?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
    Eager young mind
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    come again..

  9. #9
    Eager young mind
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    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. "
    Last edited by kris.c; 07-12-2006 at 02:15 AM.

  10. #10
    and the hat of wrongness Salem's Avatar
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    > 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.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  11. #11
    Eager young mind
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    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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