title: Build sample data for apache commons Fast Fourier Transform algorithm
body: <p>I wanted to use Apache math commons implementation for FFT (<strong>FastFourierTransformer</strong> class) to process some dummy data whose 8 data samples are contributing to one complete sinusoidal wave. The maximum being amplitude 230. The code snippet that I tried is below :</p>
  
  <pre><code>private double[] transform() 
  {   
      double [] input = new double[8];
      input[0] = 0.0;
      input[1] = 162.6345596729059;
      input[2] = 230.0;
      input[3] = 162.63455967290594;
      input[4] = 2.8166876380389125E-14;
      input[5] = -162.6345596729059;
      input[6] = -230.0;
      input[7] = -162.63455967290597;
  
      double[] tempConversion = new double[input.length];
  
      FastFourierTransformer transformer = new FastFourierTransformer();
      try {           
          Complex[] complx = transformer.transform(input);
  
          for (int i = 0; i &lt; complx.length; i++) {               
              double rr = (complx[i].getReal());
              double ri = (complx[i].getImaginary());
  
              tempConversion[i] = Math.sqrt((rr * rr) + (ri * ri));
          }
  
      } catch (IllegalArgumentException e) {
          System.out.println(e);
      }
  
      return tempConversion;
  }
  </code></pre>
  
  <p>1) Now the data returned by method <strong>transform</strong> is an array of complex number. Does that array contains the frequency component information about input data? or the tempConversion array that I created will contain the frequency information? The values in tempConversion array is :</p>
  
  <pre><code> 2.5483305001488234E-16
   920.0
   4.0014578493024757E-14
   2.2914314707516465E-13
   5.658858581079313E-14
   2.2914314707516465E-13
   4.0014578493024757E-14
   920.0
  </code></pre>
  
  <p>2) I searched a lot but at most of the places there is no clear documentation on what format of data algorithm expects (in terms of sample code to understand better) and how do I use the array of results to calculate the frequencies contained in the signal?</p>