Using sets in a list comprehension

Question

what I am trying to do: Create and print a list of words for which the reverse of the word is in the set of lower-case words. For example, "timer" and its reverse, "remit", are both in the lower-case word list.

what i have so far:

     s = set(lowers)
     [word for word in s if list(word).reverse() in s]

i just get an empty list.

-1. Learn to debug. This is at least the third question today where you could try things at the command line and see your error. The key is to break up your problem into smaller testable parts. In this case you could have gone to the command line and tried list('orange').reverse() and found that it returns None. — Steven Rumbalski
– Steven Rumbalski, Commented Nov 12, 2012 at 19:14

Ashwini Chaudhary · Accepted Answer · 2012-11-12 19:20:18Z

4

just use [::-1], using list() is unnecessary here and list(word).reverse() returns None as it changes the list in-place . You can also use "".join(reversed(word)), but I guess just word[::-1] is simple enough:

[word for word in s if word[::-1] in s]

In [193]: word="timer"

In [194]: print list(word).reverse()
None

In [195]: word[::-1]
Out[195]: 'remit'

In [196]: "".join(reversed(word))
Out[196]: 'remit'

edited Nov 12, 2012 at 19:20

answered Nov 12, 2012 at 19:06

Ashwini Chaudhary

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Iguananaut · Accepted Answer · 2012-11-12 19:06:52Z

3

The list.reverse() method reverses a list in place. Its return value is None. If you want to reverse a string just use word[::-1].

answered Nov 12, 2012 at 19:06

Iguananaut

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Óscar López · Accepted Answer · 2012-11-12 19:08:26Z

2

Try this:

s = set(lowers)
[word for word in s if word.lower()[::-1] in s]

answered Nov 12, 2012 at 19:08

Óscar López

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Martin Maillard · Accepted Answer · 2012-11-12 19:47:12Z

2

list.reverse() reverses the list in place and returns nothing. You need "".join(reversed(list)).

edited Nov 12, 2012 at 19:47

answered Nov 12, 2012 at 19:06

Martin Maillard

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1 Comment

Ashwini Chaudhary Over a year ago

reversed(list) returns listreverseiterator, so he need more than just reversed(list).

Tom · Accepted Answer · 2012-11-12 19:10:23Z

0

You should be using [::-1] for reversing the string -

[word for word in s if word[::-1] in s]

answered Nov 12, 2012 at 19:10

Tom

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