Testing palindrome with nested loops

Question

I understand that this is the easiest and most simplest way to check:

if val == val[::-1]:
    print "yes"
else:
    print "no"

but just for practice, I wanted to test it with nested for loops (using the reversed() function -- I spent hours but couldn't quite figure where to continue and break to get it right:

for i in val:
    for j in reversed(val):
        if i == j:
            break
    break

Why are you comparing each letter of the word with every letter of the palindrome? — jamylak, May 18 '12 at 07:46
@jamylak cause I want to learn continue/break. I want to use for loop inside for loop for this excercise. — eozzy, May 18 '12 at 08:10
@Nimbuz This is not a good exercise to use a nested loop since each letter of the word needs to only compare with one other letter. — jamylak, May 18 '12 at 08:16
To avoid `break`/`continue` hassles, just make it a function and `return` where appropriate. — georg, May 18 '12 at 09:31

Daren Thomas · Accepted Answer · 2012-05-18T08:12:27.900

3

this should do it:

for i, j in zip(val, reversed(val)):
    if i != j:
        print 'no'
        break
else:
    print 'yes'

EDIT: many thanks to @jamylak for pointing out the else clause on for loops. I keep forgetting that!

But then, I'd probably just do:

print 'yes' if list(val) == list(reversed(val)) else 'no'

edited May 18 '12 at 08:12

answered May 18 '12 at 07:46

Daren Thomas

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2

To print yes with this for loop and an `else` to the for loop. Alternatively you can use `all(i==j for i,j in zip(text,reversed(text)))` – jamylak May 18 '12 at 07:53
Using a for/else seems like intentional obfuscation. – sneak May 18 '12 at 08:01
1

@sneak, but the OP already had the one true way to do it. he's looking for a more cumbersome approach ;) – Daren Thomas May 18 '12 at 08:10
@sneak Yeah a lot of Pythonistas don't like `for`/`else` – jamylak May 18 '12 at 08:11

gefei · Answer 2 · 2012-05-18T08:05:26.823

2

for i in range(len(val)/2):
    if val[i] != val[-i-1]:
        return False
return True

edited May 18 '12 at 08:05

answered May 18 '12 at 07:46

gefei

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score 0 · Answer 3 · answered May 18 '12 at 09:16

0

This is not a usecase of nested loops. Nested loops are useful when you want to compare each element of something with each element of something else. In your case you only want to compare the first position with the last, the second with the second last and so on. You need to do this:

for i in range(len(val)/2):
    if val[i] != val[-i-1]:
        continue
    else:
        break

answered May 18 '12 at 09:16

acid_crucifix

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12

It is the same as the above answer and that should be given preference as the correct answer.. I have simply used it to show continue and break and also explain why nested loops are not apt – acid_crucifix May 18 '12 at 09:17

Testing palindrome with nested loops

3 Answers3