Remove an element from a lists of lists in Python

Question

How can I remove from a list of list every list in which a '5' is placed before a '3' given an initial list of list like the following one?

[('3', '3', '3'), ('3', '3', '5'), ('3', '5', '3'), ('3', '5', '5'), ('5', '3', '3'), ('5', '3', '5'), ('5', '5', '3'), ('5', '5', '5')]

I tried with

for i in list_ck:
   for j in range(0,2):
      if (i[j]=='5' and i[j+1]=='3'):
         list_ck.remove(i)

but it doesn't work

Welcome to StackOverflow. See [minimal, reproducible example](https://stackoverflow.com/help/minimal-reproducible-example). We cannot effectively help you until you post your MRE code and accurately specify the problem. We should be able to paste your posted code into a text file and reproduce the problem you specified. Your given code doesn't run due to undefined variables; "doesn't work" is not a problem specification. — Prune, Dec 18 '19 at 21:32
What is `ck`? That variable is not defined anywhere in your code. — John Gordon, Dec 18 '19 at 21:33
I think you're guilty of DWI - Deleting While Iterating. See https://stackoverflow.com/q/1207406/10077 — Fred Larson, Dec 18 '19 at 21:36
You don't want to modify the list while iterating over it. That leads to nasty behaviours. Instead, create a new list while iterating the original — Tomerikoo, Dec 18 '19 at 21:38

kaya3 · Answer 1 · 2019-12-18T21:41:37.357

It's easier to test whether a 5 occurs before a 3 if they are strings, rather than tuples; so we can use ''.join to convert them to strings.

>>> data = [('3', '3', '3'), ('3', '3', '5'), ('3', '5', '3'), ('3', '5', '5'), ('5', '3', '3'), ('5', '3', '5'), ('5', '5', '3'), ('5', '5', '5')]
>>> [r for r in data if '53' not in ''.join(r)]
[('3', '3', '3'), ('3', '3', '5'), ('3', '5', '5'), ('5', '5', '5')]

This assumes you only want to test for a 5 immediately before a 3, and won't work for more general cases where the strings in the tuple could e.g. be '53' themselves. But it's sufficient for your example.

A more general solution is to use a regex, and join on a character like , which none of the strings will contain:

>>> data = [('5', '3', '1'), ('53', '1', '1'), ('5', '1', '3')]
>>> import re
>>> pattern = re.compile('(^|,)5,(.*,)*3(,|$)')
>>> [r for r in data if not pattern.search(','.join(r))]
[('53', '1', '1')]

Here the pattern (^|,)5,(.+,)*3(,|$) matches a 5 either at the start or after a ,, followed by a comma, followed by any number of things ending with commas, followed by a 3 which is either before a comma or the end of the string.

Thank you, yes I only have single '5' and '3' – cakeBoy Dec 18 '19 at 22:01 — cakeBoy, Dec 18 '19 at 22:01

Alexander · Accepted Answer · 2019-12-18T22:12:19.053

You could use a conditional list comprehension:

# List of Tuples (lot).
list_of_tups = [('3', '3', '3'), ('3', '3', '5'), ('3', '5', '3'), ('3', '5', '5'), ('5', '3', '3'), ('5', '3', '5'), ('5', '5', '3'), ('5', '5', '5')]

>>> [tup for tup in list_of_tups 
     if not any((a == '5' and b == '3') for a, b in zip(tup, tup[1:]))]
[('3', '3', '3'), ('3', '3', '5'), ('3', '5', '5'), ('5', '5', '5')]

To modify the list inplace rather than create a new list, create an index of items that need to be removed and then pop them off in reverse order.

idx = [n for n, tup in enumerate(list_of_tups) 
       if any((a == '5' and b == '3') for a, b in zip(tup, tup[1:]))]
for i in reversed(idx):
    list_of_tups.pop(i)

>>> list_of_tups
[('3', '3', '3'), ('3', '3', '5'), ('3', '5', '5'), ('5', '5', '5')]

Thank you, this worked. I'm new to python – cakeBoy Dec 18 '19 at 22:01 — cakeBoy, Dec 18 '19 at 22:01

score -1 · Answer 3 · edited Dec 18 '19 at 23:43

Try to create another list with the same arguments (dont copy them, create new) and remove() from the second list

tab = [('3', '3', '3'), ('3', '3', '5'), ('3', '5', '3'), ('3', '5', '5'), ('5', '3', '3'), ('5', '3', '5'), ('5', '5', '3'), ('5', '5', '5')]    
tab2 = [('3', '3', '3'), ('3', '3', '5'), ('3', '5', '3'), ('3', '5', '5'), ('5', '3', '3'), ('5', '3', '5'), ('5', '5', '3'), ('5', '5', '5')]
for i in tab:
    for j in range(0,2):
        if i[j]=='5' and i[j+1]=='3':
            tab2.remove(i)
            break

Remove an element from a lists of lists in Python

3 Answers3