Question

A flash question, I’m looking at the following code

from __future__ import division
import math
import time

def dft(x, inverse = False, verbose = False) :
    t = time.clock()
    N = len(x)
    inv = -1 if not inverse else 1
    X =[0] * N
    for k in xrange(N) :
        for n in xrange(N) :
            X[k] += x[n] * math.e**(inv * 2j * math.pi * k * n / N)
        if inverse :
            X[k] /= N
    t = time.clock() - t
    if verbose :
        print "Computed","an inverse" if inverse else "a","DFT of size",N,
        print "in",t,"sec."
    return X

and I’m wondering (I do not know python):

• what does the X =[0] * N line do?
• why the double asterisk ** ?

Answer 1

The x = [0] * n is demonstrated here:

>>> [0]*10
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

It ‘multiplies’ the list elements

>>> [1, 2, 3] * 3
[1, 2, 3, 1, 2, 3, 1, 2, 3]

The ** is the power operator

>>> 3**2
9

Although be careful, it can also be **kwargs (in a different context), see more about that here Proper way to use **kwargs in Python