title: The Absolute Value of a Complex Number with Numpy
body: <p>I have the following script in Python. I am calculating the Fourier Transform of an array. When I want to plot the results (Fourier transform) I am using the absolute value of that calculation.
  However, I do not know how the absolute value of complex numbers is being produced.
  Does anyone know how it calculates? I need this to reproduce in Java.</p>
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  from numpy import fft
  
  inp = [1,2,3,4]
  
  res = fft.fft(inp)
  
  print(res[1]) # returns (-2+2j) complex number
  
  print(np.abs(res[1])) # returns 2.8284271247461903
  </code></pre>