Pesky reverse()

[engsolnom@ipns.com]

I need to use the built-in method 'reverse', but notice strange behavior.
Foe example:

print my_list.reverse () doesn't work.

new_list = my_list.reverse () doesn't work.

my_list.reverse ()
print my_list does work. (displays reversed list)

But I want both a 'forward' and 'reverse' list:

new_list = my+list # should save a 'forward' copy, right? Nope
my_list.reverse () actually reverses both copies, since Python is a bit too
helpful sometimes, and I understand why.

So the question is, how do I get a forward and reverse list?

Thanks......Nor m.

[Paul Rubin]

Re: Pesky reverse()

engsolnom@ipns. com writes:[color=blue]
> print my_list.reverse () doesn't work.[/color]

The reverse() method reverses the list in place and returns None.
[color=blue]
> But I want both a 'forward' and 'reverse' list:
>
> new_list = my_list # should save a 'forward' copy, right? Nope[/color]

No, both new_list and my_list are bound to the same list, like in C
you might have two pointers to the same structure. To make a copy, use

new_list = my_list[:]
[color=blue]
> my_list.reverse () actually reverses both copies, since Python is a bit too
> helpful sometimes, and I understand why.
>
> So the question is, how do I get a forward and reverse list?[/color]

new_list = my_list[:]
new_list.revers e()

[Sean Ross]

Re: Pesky reverse()

<engsolnom@ipns .com> wrote in message
news:3t3svvs7ni 8l8jnn1l1v60oog irqoaa85f@4ax.c om...[color=blue]
> I need to use the built-in method 'reverse', but notice strange behavior.[/color]

[snip]
[color=blue]
> new_list = my+list # should save a 'forward' copy, right? Nope
> my_list.reverse () actually reverses both copies, since Python is a bit too
> helpful sometimes, and I understand why.
>
> So the question is, how do I get a forward and reverse list?[/color]
[color=blue][color=green][color=darkred]
>>> forward = range(10)
>>> reverse = forward[:] # copy
>>> reverse.reverse ()
>>> print forward[/color][/color][/color]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][color=blue][color=green][color=darkred]
>>> print reverse[/color][/color][/color]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

HTH
Sean

[Elaine Jackson]

Re: Pesky reverse()

>>> list1=[1,2,3][color=blue][color=green][color=darkred]
>>> list2=list1[:] ## "cloning"
>>> list1.reverse() ==None[/color][/color][/color]
1[color=blue][color=green][color=darkred]
>>> list1[/color][/color][/color]
[3, 2, 1][color=blue][color=green][color=darkred]
>>> list2[/color][/color][/color]
[1, 2, 3]


<engsolnom@ipns .com> wrote in message
news:3t3svvs7ni 8l8jnn1l1v60oog irqoaa85f@4ax.c om...
|
| I need to use the built-in method 'reverse', but notice strange behavior.
| Foe example:
|
| print my_list.reverse () doesn't work.
|
| new_list = my_list.reverse () doesn't work.
|
| my_list.reverse ()
| print my_list does work. (displays reversed list)
|
| But I want both a 'forward' and 'reverse' list:
|
| new_list = my+list # should save a 'forward' copy, right? Nope
| my_list.reverse () actually reverses both copies, since Python is a bit too
| helpful sometimes, and I understand why.
|
| So the question is, how do I get a forward and reverse list?
|
| Thanks......Nor m.
|
|