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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):
206
[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.
Meaning of X = X[:, 1] in Python is: X is a dataset or a array. Say Here X have n rows and n columns. so by doing x=x[:,1] we get all the rows in x present at index 1.
Python also allows you to index from the end of the list using a negative number, where [-1] returns the last element. This is super-useful since it means you don't have to programmatically find out the length of the iterable in order to work with elements at the end of it.
This is just standard Python list comprehension. It's a different way of writing a longer for loop. You're looping over all the characters in your string and putting them in the list if the character is a digit.
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
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