Python indexing issue (converting MATLAB code)

Question

I'm trying to convert a piece of Matlab code into Python and am running into a problem.

t = linspace(0,1,256);
s = sin(2*pi*(2*t+5*t.^2));
h = conj(s(length(s):-1:1));

The above line for h is meant to calculate the impulse response, but my Python code:

import numpy as np

t = np.linspace(0,1,256)
s = np.sin(2*np.pi*(2*t+5*t**2))

h = np.conj(s[len(s),-1,1])

gives me an error IndexError: index 256 is out of bounds for axis 0 with size 256. I know that this has to do with indexing the s array, but how can I fix it?

nneonneo · Accepted Answer · 2014-06-24 01:59:25Z

6

Remember that Python is zero-indexed, while MATLAB is 1-indexed. Note too that the MATLAB slice notation includes the endpoint, whereas the Python slice notation excludes the endpoint.

s(length(s):-1:1) is a common MATLAB idiom for reversing a vector. Python actually has a nicer syntax: s[::-1]. A direct translation would be s[len(s)-1:-1:-1].

Also note that MATLAB start:step:stop corresponds to the Python start:stop:step; the position of the step argument is different.

edited Jun 24, 2014 at 1:59

answered Jun 24, 2014 at 1:43

nneonneo

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CT Zhu · Accepted Answer · 2014-06-24 01:52:17Z

1

The python way to do it is even simpler:

In [242]:

a=np.arange(10)
print a
print a[::-1]
[0 1 2 3 4 5 6 7 8 9]
[9 8 7 6 5 4 3 2 1 0]

simply: s[::-1]

This was first introduced in python 2.3

answered Jun 24, 2014 at 1:52

CT Zhu

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