palindrome function

[kdt]

Hi all,

Can someone please explain to me why the following evaluates as false?

>>>list=['a','n','n','a']
>>>list==list.r everse()
>>>False

I'm stumped :s

[Denis Kasak]

Re: palindrome function

On Sat, Jul 12, 2008 at 12:22 AM, kdt <dorjetarap@goo glemail.comwrot e:

Hi all,
>
Can someone please explain to me why the following evaluates as false?
>

>>>>list=['a','n','n','a']
>>>>list==list. reverse()
>>>>False

>
I'm stumped :s

Read the documentation on list.reverse().

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome "

Also, 'list' is a really bad name for a list, since this is the name of
the builtin type object for the list type.

--
Denis Kasak

[kdt]

Re: palindrome function

On Jul 11, 11:34 pm, Denis Kasak <denis.kasak271 8281...@gmail.c om>
wrote:

On Sat, Jul 12, 2008 at 12:22 AM, kdt <dorjeta...@goo glemail.comwrot e:
>
 Hi all,
 >
 Can someone please explain to me why the following evaluates as false?
 >
 >>>>list=['a','n','n','a']
 >>>>list==list. reverse()
 >>>>False
 >
 I'm stumped :s
>
Read the documentation on list.reverse().
>
Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:
>
spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
    print "Palindrome "
>
Also, 'list' is a really bad name for a list, since this is the name of
the builtin type object for the list type.
>
--
Denis Kasak

thanks for the explanation :D

[bearophileHUGS@lycos.com]

Re: palindrome function

Denis Kasak:

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome "

An alternative version:

>>txt = "anna"
>>txt == txt[::-1]

True

>>txt = "annabella"
>>txt == txt[::-1]

False

Bye,
bearophile

[Mensanator]

Re: palindrome function

On Jul 11, 5:34 pm, Denis Kasak <denis.kasak271 8281...@gmail.c om>
wrote:

On Sat, Jul 12, 2008 at 12:22 AM, kdt <dorjeta...@goo glemail.comwrot e:
>
 Hi all,
 >
 Can someone please explain to me why the following evaluates as false?
 >
 >>>>list=['a','n','n','a']
 >>>>list==list. reverse()
 >>>>False
 >
 I'm stumped :s
>
Read the documentation on list.reverse().
>
Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:
>
spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
    print "Palindrome "

You could also do

>>spam = ['a','n','n','a']
>>if spam == [i for i in reversed(spam)]:

print "Palindrome "

>
Also, 'list' is a really bad name for a list, since this is the name of
the builtin type object for the list type.
>
--
Denis Kasak

[Paul McGuire]

Re: palindrome function

On Jul 11, 6:20 pm, Mensanator <mensana...@aol .comwrote:

Try this:

>

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
    print "Palindrome "

>
You could also do
>

>spam = ['a','n','n','a']
>if spam == [i for i in reversed(spam)]:

>
        print "Palindrome "
>

Or instead of this:

[ i for i in generator_expre ssion]

just use the list constructor:

list(generator_ expression)

-- Paul

[Peter Otten]

Re: palindrome function

Denis Kasak wrote:

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:
>
spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
print "Palindrome "

Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome ", either.

Peter

[Denis Kasak]

Re: palindrome function

Peter Otten wrote:

Denis Kasak wrote:
>

>Basically, it reverses the list in place, so it modifies the list which
>called it. It does not return a /new/ list which is a reversed version
>of the original, as you expected it to. Since it doesn't return anything
>explicitly, Python makes it return None. Hence, the comparison you are
>doing is between the original list and a None, which is False, naturally.
>Try this:
>>
>spam = ['a', 'n', 'n', 'a']
>eggs = spam[:]
>if spam.reverse() == eggs:
> print "Palindrome "

>
Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome ", either.

Of course. Thank you for the correction. I guess you know your caffeine
has started to wear off when you start making the same mistakes you were
trying to fix. :-)

--
Denis Kasak

[Terry Reedy]

Re: palindrome function



Peter Otten wrote:

Denis Kasak wrote:
>

>Basically, it reverses the list in place, so it modifies the list which
>called it. It does not return a /new/ list which is a reversed version
>of the original, as you expected it to. Since it doesn't return anything
>explicitly, Python makes it return None. Hence, the comparison you are
>doing is between the original list and a None, which is False, naturally.
>Try this:
>>
>spam = ['a', 'n', 'n', 'a']
>eggs = spam[:]
>if spam.reverse() == eggs:
> print "Palindrome "

>
Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome ", either.

I don't know if this was posted yet, but 'seq.reversed() == seq' is the
simple way to test for 'palindomeness' .

[Mensanator]

Re: palindrome function

On Jul 12, 2:18�pm, Terry Reedy <tjre...@udel.e duwrote:

Peter Otten wrote:

Denis Kasak wrote:

>

Basically, it reverses the list in place, so it modifies the list which
called it. It does not return a /new/ list which is a reversed version
of the original, as you expected it to. Since it doesn't return anything
explicitly, Python makes it return None. Hence, the comparison you are
doing is between the original list and a None, which is False, naturally.
Try this:

>

spam = ['a', 'n', 'n', 'a']
eggs = spam[:]
if spam.reverse() == eggs:
� � print "Palindrome "

>

Your explanation is correct, but your example code compares None to
['a', 'n', 'n', 'a'] and therefore won't print "Palindrome ", either.

>
I don't know if this was posted yet,

It hasn't. and here's why:

IDLE 2.6b1

>>seq=['a','n','n','a']
>>seq.reversed( )

Traceback (most recent call last):
File "<pyshell#3 >", line 1, in <module>
seq.reversed()
AttributeError: 'list' object has no attribute 'reversed'

but 'seq.reversed() == seq' is the
simple way to test for 'palindomeness' .

Not in 2.x.

[Terry Reedy]

Re: palindrome function



Mensanator wrote:

It hasn't. and here's why:
>
IDLE 2.6b1

>>>seq=['a','n','n','a']
>>>seq.reversed ()

>
Traceback (most recent call last):
File "<pyshell#3 >", line 1, in <module>
seq.reversed()
AttributeError: 'list' object has no attribute 'reversed'

My apologies. reversed() is a builtin func, not a method, and it
produces an iterator, not a seq. SO, corrected,

>>for s in ((1,2,3,2,1), [1,2,3,2,1]):

.... type(s)(reverse d(s)) == s
....
True
True

>>s = '12121'
>>''.join(rever sed(s)) == s

True