Arrayfun in Python?

Question

I'm trying to convert some code from Matlab to Python. Any idea how I would go about converting this line? I'm very new to python and I've never seen arrayfun before. Thanks. Much appreciated.

 zj=arrayfun(@sigmoid,aj);

Answer 1

A generic way, use a loop:

 zj=[sigmoid(x) for x in aj]

Answer 2

You will want to use the numerical library numpy whenever you're working with numerical data.

In it, the Matlab function called arrayfun is simply the vectorized form of that function. E.g.

Matlab:

>> a = 1:4

a =

     1     2     3     4

>> arrayfun(@sqrt, a)

ans =

    1.0000    1.4142    1.7321    2.0000

>> sqrt(a)

ans =

    1.0000    1.4142    1.7321    2.0000

Whereas in numpy, you'd do:

>>> import numpy as np
>>> a = np.arange(4)
>>> np.sqrt(a)
array([ 0.        ,  1.        ,  1.41421356,  1.73205081])

Most functions can be vectorized, a sigmoid is no exception to that. For example, if the sigmoid were defined as 1./(1 + exp(-x)), then you could write in Python:

def sigmoid(x):
    return 1./(1 + np.exp(-x))

zj = sigmoid(aj)