title: FFT interpretation
body: <p>I am wokring on an Android project where I am using FFT for processing accelerometer data and I have problems understanding how are these things actually working.
  I am using jTransform library by Piotr Wendykier in the following way:</p>
  
  <pre><code>        int length = vectors.length;
          float[] input = new float[length*2];
          for(int i=0;i&lt;length;i++){
              input[i]=vectors[i];
          }
  
          FloatFFT_1D fftlib = new FloatFFT_1D(length);
          fftlib.complexForward(input);
  
          float outputData[] = new float[(input.length+1)/2];
          if(input.length%2==0){
              for(int i = 0; i &lt; length/2; i++){
  
                  outputData[i]= (float) Math.sqrt((Math.pow(input[2*i],2))+(Math.pow(input[2*(i)+1], 2)));
              }
          }else{
              for(int i = 0; i &lt; length/2+1; i++){
  
                  outputData[i]= (float) Math.sqrt((Math.pow(input[2*i],2))+(Math.pow(input[2*i+1], 2)));
              }
          }
  
          List&lt;Float&gt; output = new ArrayList&lt;Float&gt;();
          for (float f : outputData) {
              output.add(f);
          }
  </code></pre>
  
  <p>the result is an array with following data <img src="https://i.stack.imgur.com/eCZN9.png" alt="output data visualization">.</p>
  
  <p>I have problem with interpreting the output data..The data are from 10 seconds long interval, and the sampling frequency is 50Hz..While capturing I was moving the phone up and down cca each 3/4 second in my hand, so is possible that the extreme which is about x value 16 could be the period of the strongest component of the signal?
  I need to obtain the frequency of the strongest component in the signal..</p>