List of non-zero elements in a list in Python

Question

Given the list

a = [6, 3, 0, 0, 1, 0]

What is the best way in Python to return the list

b = [1, 1, 0, 0, 1, 0]

where the 1's in b correspond to the non-zero elements in a. I can think of running a for loop and simply appending onto b, but I'm sure there must be a better way.

Answer 1

For completeness, and since no one else added this answer:

b = map(bool, a)

But it returns Boolean values and not Integers.

Answer 2

b = [int(i != 0) for i in a]

Will give you what you are looking for.

Answer 3

Another way

>>> a = [6, 3, 0, 0, 1, 0]

>>> [cmp(x,0) for x in a]
[1, 1, 0, 0, 1, 0]

Note that cmp(x,0) returns -1 for negative numbers, so you might want to abs that.

Answer 4

You can also, depending on your application, work with numpy array representation:

$ import numpy as np
$ a = [6, 3, 0, 0, 1, 0]
$ (np.array(a)!= 0).astype(int)
array([1, 1, 0, 0, 1, 0])

astype(int) is only necessary if you dont't want a boolean representation