24

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 ** ?
Paul
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7 Answers7

35

The [0] * x creates a list with x elements. So,

>>> [ 0 ] * 5
[0, 0, 0, 0, 0]
>>>

Be warned that they all point to the same object. This is cool for immutables like integers but a pain for things like lists. 

>>> t = [[]] * 5
>>> t
[[], [], [], [], []]
>>> t[0].append(5)
>>> t
[[5], [5], [5], [5], [5]]
>>>

The ** operator is used for exponentation. 

>>> 5 ** 2 
25
Noufal Ibrahim
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    +1 for the same mutable object, I would fix "with `x` elements" because you can multiply lists that have multiple elements. – Henry May 15 '11 at 10:47
21

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

Community
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Henry
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  • Do you have a reference to the `[0] * x` syntax in the python documentation? I cannot find it anywhere. I also can't find a standard name for it. – MattCochrane Sep 07 '21 at 11:01
6

  1. X =[0] * N, produces a list of size N, with all N elements being the value zero. for example, X = [0] * 8, produces a list of size 8.

    X = [0, 0, 0, 0, 0, 0, 0, 0]

    Pictorial representation will be like,

The result of command [0] * 8

Technically, all eight cells of the list reference the same object. This is because of the fact that lists are referential structures in python.

and, if you try to assign a new value to list, say X[2] = 10, this does not technically change the value of the existing integer instance. This computes a new integer, with value 10, and sets cell 2 to reference the newly computed value.

Pictorial representation,

enter image description here

  1. ** is power operator and computes the power of a number. for example, 5 ** 2 results in 25.
Sbk3824
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4

X = [0] * N creates an array of zeros of N length. For example:

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

** is the power operator.

>>> 2 ** 2
4
Sam Dolan
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2

what does the X =[0] * N line do?

[0] is a sequence containing a single element – 0. Multiplying a sequence times n means concatenating it n times to itself. That is, the result is a sequence containing n zeros.

why the double asterisk ** ?

It’s the power operator: b ** e = be.

Konrad Rudolph
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0

1) It initialises a list containing N 0's.

2) ** is the exponentiation operator

verdesmarald
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0

[0] * N creates a list of size N which contains only 0's

the ** is a notation for raising the left side to the power of right side

Disclaimer: [a] * N where a is a reference to an object will copy the reference, it won't make copies of a inside the list generated

Mihai Oprea
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