Removing elements from an array in python

Question

Consider the array a= [1, 2, 3, 1, 2, 3]. Now suppose I want to remove all the 2s in this array in python. So I apply a.remove(2). However the result which comes out is [1, 3, 1, 2, 3], i.e the only first 2 is removed. How can I remove all the 2s which appear in an array? In general, given an array and an element p, how can I remove all the elements of the array which are equal to p?

Edit:- I think I should mention this, this question has been inspired from a Brilliant computer science problem.

you can use list(set(myarray)) but it will rearrange the array so that it is in alphabetical/numerical order. — The-IT
– The-IT, Commented Jul 22, 2013 at 11:33
If you do that, you will find that all duplicates in the new list will be removed. list(set([2, 6, 3, 4, 5, 3, 5, 6, 5])) [2, 3, 4, 5, 6] — The-IT
– The-IT, Commented Jul 22, 2013 at 11:52
Yeh, I just reread my comment and realized it was worded really badly. My bad. Sorry. — The-IT
– The-IT, Commented Jul 22, 2013 at 12:07

Martijn Pieters · Accepted Answer · 2013-07-22 11:17:53Z

4

Use a list comprehension to build a replacement list, where all elements are not equal to p:

a = [i for i in a if i != p]

Note that in Python, the datatype is called a list, not an array.

answered Jul 22, 2013 at 11:17

Martijn Pieters

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1 Comment

aba Over a year ago

Thanks, it works! I am just a beginner to python, and I am more used to java, that's why I used the word 'array' instead of 'list'.

Sukrit Kalra · Accepted Answer · 2013-07-22 11:19:11Z

3

You can use filter().

>>> a= [1, 2, 3, 1, 2, 3]
>>> filter(lambda x: x != 2, a)
[1, 3, 1, 3]

In a function :

>>> def removeAll(inList, num):
        return filter(lambda elem: elem != num, inList)

>>> removeAll(a, 2)
[1, 3, 1, 3]

answered Jul 22, 2013 at 11:19

Sukrit Kalra

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5 Comments

tamasgal Over a year ago

Just a comment: afaik filter, map etc. return iterators in python 3. But correct me if I'm wrong…

Sukrit Kalra Over a year ago

List Comprehensions are more readable (although map may be a bit faster in some cases), but I just wanted to provide an alternative functional approach to the problem.

tamasgal Over a year ago

Yes of course, alternatives are great! It was just a comment. I edited it btw. (I meant those builtins don't behave the same in future python versions).

aba Over a year ago

Can you please explain what is the function of the command 'lambda' here? Thanks for contributing, but a much simpler answer has already been posted. Anyways, I prefer remembering the alternatives. Thanks!

Sukrit Kalra Over a year ago

This link should be helpful - secnetix.de/olli/Python/lambda_functions.hawk

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