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I have a question

say I have two list, and each of them contains couple of strings

a = ['x', 'y', 'z', 't'] b = ['xyz', 'yzx', 'xyw']

I want to delete xyw in list b, because w is not in list a.

I tried 

for s in a:
    k = [t for t in b if t.find(s)]

but it didn't work Does anyone know how to do this in Python? Thank you!

Ningxi
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    I'm sorry but as SO is not a *do it for me* site you need to show us that what you have tried so far! then we can help you on your problems! – Mazdak Feb 17 '15 at 16:22
  • @KasraAD thanks for your reminder, I just join this website. I did have a try, I changed my question. – Ningxi Feb 17 '15 at 16:35
  • welcome! OK, always remember that as you explain more about your question as you get a complete answer! – Mazdak Feb 17 '15 at 16:37

2 Answers2

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You could check that all of the letters in each string are contained in your list a then filter out strings using a list comprehension.

>>> a = ['x', 'y', 'z']
>>> b = ['xyz', 'yzx', 'xyw']
>>> [i for i in b if all(j in a for j in i)]
['xyz', 'yzx']
Cory Kramer
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  • Great! But if a = ['x', 'y', 'z', 't'], how to change the code? thanks! – Ningxi Feb 17 '15 at 16:24
  • I tried , but the answer is [] ... I also tried [i for i in b if any(j in i for j in a)], the answer is ['xyz', 'yzx', 'xyw'] ... – Ningxi Feb 17 '15 at 16:28
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>>> a = ['x', 'y', 'z']
>>> b = ['xyz', 'yzx', 'xyw']
>>> for element in b:
...     if not all(i in a for i in element):
...         b.remove(element)
... 
>>> b
['xyz', 'yzx']
>>>

Correcttion: I shouldn't delete during iterating. So following like the solution above fits

>>> a = ['x', 'y', 'z']
>>> b = ['xyz', 'yzx', 'xyw']
>>> b = [i for i in b if all(j in a for j in i)]
>>> b
['xyz', 'yzx']
>>>
BigTailWolf
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  • This fails for e.g. `b = ['xyz', 'yzx', 'xyw', 'xyq']`, because you're removing items from the list as you iterate over it. – Zero Piraeus Feb 17 '15 at 17:03
  • @ZeroPiraeus Oh, I got. I shouldn't do that. I should follow the above solution and reassign to b. Is that correct? – BigTailWolf Feb 17 '15 at 19:40