Creating list from list in python based on an if condition

Question

I want to create a new_list which will contain only the items of my old_list which satisfy the condition that the index in an array of labels is 3. I am trying something like this:

new_list = [x for x in old_list if idx[x] == 3]
IndexError: arrays used as indices must be of integer (or boolean) type

But i am getting the following error because idx is an array. How can i solve this problem?

edited: Idx is an array of equal size with my original data which contains labels for them. So basically i want to create a new list which will contain only the items of my original list which e.g have the label 3.

I want to do something like this: cluster_a = [old_list[x] for x in idx if x == 3]

Clarification: my old list is a list containing 3d arrays and the idx is an equal size array containing a label for each 3d array of my list as i aforementioned. I am trying my best to explain the problem. If something is needed please tell me.

This is the list with the 3d arrays

and this is the array with the labels

what you mean by index in an array of labels is 3 ? what index ? can you show an example ? — Kasravnd
– Kasravnd, Commented Nov 8, 2014 at 17:52
And why do you have to do it with list comprehension? By making it 3 lines longer this will become a very easy task, and at the same time code readability will skyrocket. — Aida Paul
– Aida Paul, Commented Nov 8, 2014 at 17:53
Code readability will probably not be an issue if the correct code is written. — chepner
– chepner, Commented Nov 8, 2014 at 17:55

WeaselFox · Accepted Answer · 2014-11-08 18:15:08Z

1

The problem is not that idx is a list but probably that x is an array - old_list must contain a list as an element. You need to reference the index, and not the item itself:

[old_list[x] for x in range(len(old_list)) if idx[x] == 3]

here's a minimal example:

>>> old_list = [4,5,6]
>>> idx = [3,2,3]
>>> [old_list[x] for x in range(len(old_list)) if idx[x] == 3]
[4, 6]

edited Nov 8, 2014 at 18:15

answered Nov 8, 2014 at 17:54

WeaselFox

7,3989 gold badges53 silver badges77 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

chepner Over a year ago

This identifies the (likely) problem, but does not provide a solution.

WeaselFox Over a year ago

The solution in this case is to rethink the logic. Its impossible to tell what can be done differently with the code exposed.

chepner Over a year ago

Which is why you need clarification of the actual problem before providing an answer.

Aida Paul Over a year ago

@WeaselFox which would mean that, currently, there is no possible answer and we should probably flag the question appropriately and/or ask in comments. But this isn't really an answer.

WeaselFox Over a year ago

@Puciek - I dont agree, but in any case the question was updated, so that irrelevant.

|

Kasravnd · Accepted Answer · 2014-11-08 17:58:08Z

0

What about this ? :

new_list = [x for x in old_list if idx.index(x) == 3 ]

answered Nov 8, 2014 at 17:58

Kasravnd

108k19 gold badges167 silver badges195 bronze badges

2 Comments

azal Over a year ago

when i try that i get an AttributeError:'numpy.ndarray' object has no attribute 'index'

WeaselFox Over a year ago

idx.index(x) will return the first index in idx where the value is x, but according to the OP, idx is a list of labels, so I dont think this is right.

Collectives™ on Stack Overflow

Creating list from list in python based on an if condition

2 Answers 2

9 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

9 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related