C++ Perform to a 2D FFT Inplace Given a Complex 2D Array

Fast Fourier transform (FFT) is an algorithm to compute the discrete Fourier transform (DFT) and its inverse. Basically Fourier analysis converts time (or space) to frequency and vice versa. A FFT rapidly computes transformations by factorizing the DFT matrix into a product of sparse (mostly zero) factors.

Algorithm

Begin
   Declare the size of the array
   Take the elements of the array
   Declare three arrays
   Initialize height =size of array and width=size of array
   Create two outer loops to iterate on output data
   Create two outer loops to iterate on input data
   Compute real, img and amp.
End

Example Code

#include <iostream>
#include <math.h>
using namespace std;
#define PI 3.14159265
int n;
int main(int argc, char **argv) {
   cout << "Enter the size: ";
   cin >> n;
   double Data[n][n];
   cout << "Enter the 2D elements ";
   for (int i = 0; i < n; i++)
      for (int j = 0; j < n; j++)
         cin >> Data[i][j];
   double realOut[n][n];
   double imgOut[n][n];
   double ampOut[n][n];
   int height = n;
   int width = n;
   for (int yWave = 0; yWave < height; yWave++) {
      for (int xWave = 0; xWave < width; xWave++) {
         for (int ySpace = 0; ySpace < height; ySpace++) {
            for (int xSpace = 0; xSpace < width; xSpace++) {
               realOut[yWave][xWave] += (Data[ySpace][xSpace] * cos(2 *
                  PI * ((1.0 * xWave * xSpace / width) + (1.0 * yWave * ySpace /
                  height)))) / sqrt(width * height);
               imgOut[yWave][xWave] -= (Data[ySpace][xSpace] * sin(2 * PI
                  * ((1.0 * xWave * xSpace / width) + (1.0 * yWave * ySpace / height)))) /
                  sqrt( width * height);
               ampOut[yWave][xWave] = sqrt(
                  realOut[yWave][xWave] * realOut[yWave][xWave] +
                  imgOut[yWave][xWave] * imgOut[yWave][xWave]);
            }
            cout << realOut[yWave][xWave] << " + " <<
            imgOut[yWave][xWave] << " i (" << ampOut[yWave][xWave] << ")\n";
         }
      }
   }
}

Output

Enter the size: 2
Enter the 2D elements
4 5
6 7
4.5 + 6.60611e-310 i (4.5)
11 + 6.60611e-310 i (11)
-0.5 + -8.97448e-09 i (0.5)
-1 + -2.15388e-08 i (1)
4.5 + 6.60611e-310 i (4.5)
-2 + -2.33337e-08 i (2)
-0.5 + -8.97448e-09 i (0.5)
0 + 5.38469e-09 i (5.38469e-09)