Whats it doing?

[hellowworld]

Hi I've very little c++ experience and was wondering could someone explain to me what this bit of code is doing? It's mainly just what the x array is doign which is confusing me. The code is meant to be doing a fourier transform on an array of image data. Also is it adding all details to position 0 and nFFT2 in the array?

TAnalysis::FFT(float* x, float*& re, float*& im)
{
    // Real part
    re[0] = 0.;
    for(int n = 0; n < nFFT; n++) 
           re[0] += x[n];
    re[nFFT2] = 0.;
    for(int n = 0; n < nFFT; n++) {
         float a = (n % 2)? -x[n] : x[n];
         re[nFFT2] += a;
    }
        for(int k = 1; k < nFFT2; k++) {
         re[k] = 0.;
         for(int n = 0; n < nFFT; n++)
                 re[k] += x[n] * Cos[k][n];
    }
 
    // Image part
    im[0] = im[nFFT2] = 0.;
    for(int k = 1; k < nFFT2; k++) {
         im[k] = 0.;
         for(int n = 0; n < nFFT; n++)
                 im[k] += x[n] * Sin[k][n];
    }
}

[hellowworld]

can nobody help me with this?

[therockon7throw]

please edit your post(codes) and take them inside code syntax, so that people can read them, it is not easy to read code looking like text

[hellowworld]

sorry, that should be easier to read now

[therockon7throw]

for(int n = 0; n < nFFT; n++) re[0] += x[n]; is an addition operation , add each elements of x_i to the first element of re. after that operation the first element of re is updated to sum of all x_i

than re_nFFT2 is set to 0 and

for(int n = 0; n < nFFT; n++) {
         float a = (n % 2)? -x[n] : x[n];
         re[nFFT2] += a;
    }

if n is an even number x_n element is added to re_nFFT2

if n is an odd number x_n element is subtracted from re_nFFT2

for(int k = 1; k < nFFT2; k++) {
         re[k] = 0.;
         for(int n = 0; n < nFFT; n++)
                 re[k] += x[n] * Cos[k][n];
    }

than for each element between 1 and nFFT2
the elements of re_k where-> 1 <= k < nFFT2 is updated to the total sum of
x_n * Cos_kn where -> 0<= n < nFFT

and in the last loops the elements of im_k where-> 1 <= k < nFFT2 is set to
the total sum of x_n * Sin_kn where ->0 <= n < nFFT


Dont ask me about the variable, and what they stand for , no idea

[hellowworld]

thanks very much for your help, its been of great benefit.

just to confirm, on this piece of code:

 
for(int k = 1; k < nFFT2; k++) 
{
         re[k] = 0.;
         for(int n = 0; n < nFFT; n++)
                 re[k] += x[n] * Cos[k][n];
}

this is doing x[n] + Cos[k][n] rather than adding the contents of whats currently in re[k] to x[n] * Cos[k][n]?

Thanks again

[therockon7throw]

I've updated the error which you've just seen ->

for(int k = 1; k < nFFT2; k++) {
         re[k] = 0.;
         for(int n = 0; n < nFFT; n++)
                 re[k] += x[n] * Cos[k][n];
    }

than for each element between 1 and nFFT2
the elements of re_k where-> 1 <= k < nFFT2 is updated to the total sum of
x_n * Cos_kn where -> 0<= n < nFFT

[hellowworld]

Thanks, also to confirm is:

for(int n = 0; n < nFFT; n++) {
         float a = (n % 2)? -x[n] : x[n];
         re[nFFT2] += a;
    }

this definately subtracting if n is an odd number (i.e. if n%2!=0 then -x[n]) ? rather than subtracting if n is an even number

[therockon7throw]

float a = (n % 2)? -x[n] : x[n]; it says that

if n is an integer that is "evenly divisible" by 2, i.e., divisible by 2 without remainder than

a = x_n,

and if n is an integer that is not evenly divisible by 2 than a = -x_n


(n % 2)? -x[n] : x[n]; means

if ( n % 2 == 1 )  than a = -xn
otherwise

a = xn