Question

I am stuck with this problem for 2 days. Can someone help me with the logic ?

I am working on C++ programs for good algorithms. I am now working on the Danielson-Lanczos Algorithm to compute the FFT of a sequence.

Looking at

mmax=2;
while (n>mmax) {
    istep = mmax<<1;
    theta = -(2*M_PI/mmax);
    wtemp = sin(0.5*theta);
    wpr = -2.0*wtemp*wtemp;
    wpi = sin(theta);
    wr = 1.0;
    wi = 0.0;

    for (m=1; m < mmax; m += 2) {
        for (i=m; i <= n; i += istep) {
            j=i+mmax;
            tempr = wr*data[j-1] - wi*data[j];
            tempi = wr * data[j] + wi*data[j-1];

            data[j-1] = data[i-1] - tempr;
            data[j] = data[i] - tempi;
            data[i-1] += tempr;
            data[i] += tempi;
        }
        wtemp=wr;
        wr += wr*wpr - wi*wpi;
        wi += wi*wpr + wtemp*wpi;
    }
    mmax=istep;
}

Source: http://www.eetimes.com/design/signal-processing-dsp/4017495/A-Simple-and-Efficient-FFT-Implementation-in-C–Part-I

Is there any way to logically write a code such that the whole for-loop portion is reduced to just 4 lines of code (or even better)?

Answer 1

Better indentation would go a long way. I fixed that for you. Also, this seems to beg for better locality of the variables. The variable names are not clear to me, but that might be because I don’t know the domain this algorithm belongs to.

Generally, if you want to make complex code easier to understand, identify sub-algorithms and put them into their own (inlined) functions. (Putting a code snippet into a function effectively gives it a name, and makes the passing of variables into and out of the code more obvious. Often, that makes code easier to digest.)
I’m not sure this is necessary for this piece of code, though.

Merely condensing code, however, will not make it more readable. Instead, it will just make it more condensed.