Loop Backwards

[Dave]

This should be simple, but I can't get it:

How do you loop backwards through a list?

For example, in, say, javascript:

for (var i = list.length - 1; i >=0; i--) {
do_stuff()
}

I mean, I could reverse the list, but I don't want to. I want it to
stay exactly the same, but I want to start at the end and end at the
beginning.

Thanks!

- Dave

[mensanator]

Dave wrote:

This should be simple, but I can't get it:

How do you loop backwards through a list?

For example, in, say, javascript:

for (var i = list.length - 1; i >=0; i--) {
do_stuff()
}

I mean, I could reverse the list, but I don't want to. I want it to
stay exactly the same, but I want to start at the end and end at the
beginning.
Negative indexes start at the opposite end than positive indexes.

s = [1,2,3,4,5]
for i in range(len(s)): print s[-i-1],

5 4 3 2 1

Thanks!

- Dave

[Dave]

Thanks! I knew it was simple...

[Carl Cerecke]

Dave wrote:

This should be simple, but I can't get it:

How do you loop backwards through a list?

For example, in, say, javascript:

for (var i = list.length - 1; i >=0; i--) {
do_stuff()
}

I mean, I could reverse the list, but I don't want to. I want it to
stay exactly the same, but I want to start at the end and end at the
beginning.

Thanks!

- Dave

for i in reversed(ls):
do_stuff()

reversed doesn't reverse the list. It returns an iterator that iterates
through the list backwards.

Cheers,
Carl.

[bonono]

me********@aol.com wrote:

Dave wrote:

This should be simple, but I can't get it:

How do you loop backwards through a list?

For example, in, say, javascript:

for (var i = list.length - 1; i >=0; i--) {
do_stuff()
}

I mean, I could reverse the list, but I don't want to. I want it to
stay exactly the same, but I want to start at the end and end at the
beginning.

Negative indexes start at the opposite end than positive indexes.

s = [1,2,3,4,5]
for i in range(len(s)):

print s[-i-1],

5 4 3 2 1

I thought the python way is to use slice :

range(10)[::-1]

[skip]

Dave> This should be simple, but I can't get it:
Dave> How do you loop backwards through a list?

Standard idiom:

for i in range(len(mylist)-1, -1, -1):
do_stuff()

Contrast that with the standard forward loop:

for i in range(len(mylist)):
do_stuff()

So, to go backwards, just add "-1, -1, -1" to the range function in the
forward loop version.

Skip

[bruno at modulix]

Dave wrote:

This should be simple, but I can't get it:

How do you loop backwards through a list?

For example, in, say, javascript:

for (var i = list.length - 1; i >=0; i--) {
do_stuff()
}

I mean, I could reverse the list, but I don't want to. I want it to
stay exactly the same, but I want to start at the end and end at the
beginning.

for item in reversed(alist):
do_something_with(item)

or (more perlish at first sight):

for item in alist[::-1]:
do_something_with(item)

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom.gro'.split('@')])"

[Felipe Almeida Lessa]

Em Ter, 2006-02-14 Ã*s 10:08 +0100, bruno at modulix escreveu:

for item in reversed(alist):
do_something_with(item)

or (more perlish at first sight):

for item in alist[::-1]:
do_something_with(item)

No "or" here. The [::-1] version creates a whole new list in memory,
it's silly to believe both will behave equally (well, strictly speaking
they will, but one will use twice more memory than the other).

--
"Quem excele em empregar a força militar subjulga os exércitos dos
outros povos sem travar batalha, toma cidades fortificadas dos outros
povos sem as atacar e destrói os estados dos outros povos sem lutas
prolongadas. Deve lutar sob o Céu com o propósito primordial da
'preservação'. Desse modo suas armas não se embotarão, e os ganhos
poderão ser preservados. Essa é a estratégia para planejar ofensivas."

-- Sun Tzu, em "A arte da guerra"

[Dave]

>> or (more perlish at first sight):

for item in alist[::-1]:
do_something_with(item)

No "or" here. The [::-1] version creates a whole new list in memory,
it's silly to believe both will behave equally (well, strictly speaking
they will, but one will use twice more memory than the other).

Thank you for your reply.

Please be mindful of making statements such as:
"it's silly to believe both will behave equally"

One of the greatest weaknesses of Python is the less than friendly
attitude Pythonistas display towards one another. Bruno's suggestions
were valid, given the original question. Even if I had asked for the
most efficient method of iterating through a list in reverse, this
response is unwarranted.

This example is why everyone is more excited about Ruby than Python. If
we become more friendly to each other, Python will win!

[Tim Roberts]

"Dave" <da*********@gmail.com> wrote:

Please be mindful of making statements such as:
"it's silly to believe both will behave equally"

One of the greatest weaknesses of Python is the less than friendly
attitude Pythonistas display towards one another.
I don't know how much Usenet experience you've had, but comp.lang.python is
one of the friendliest places in the comp.lang universe. Newbie questions
get answered without derision, and disagreements are handled with spirited
discussion, instead of flames and insults. Compare, for example, to
comp.lang.c and comp.lang.c++.
This example is why everyone is more excited about Ruby than Python.

That statement is just not true. I've looked at Ruby, but syntactically it
will never sway me from Python.
--
- Tim Roberts, ti**@probo.com
Providenza & Boekelheide, Inc.

[john_sips_tea]

Tim Roberts wrote:

"Dave" <da*********@gmail.com> wrote:

Please be mindful of making statements such as:
"it's silly to believe both will behave equally"

One of the greatest weaknesses of Python is the less than friendly
attitude Pythonistas display towards one another.
I don't know how much Usenet experience you've had, but comp.lang.python is
one of the friendliest places in the comp.lang universe. Newbie questions
get answered without derision, and disagreements are handled with spirited
discussion, instead of flames and insults. Compare, for example, to
comp.lang.c and comp.lang.c++.

It's possible that english may not be Felipe's native language,
and Dave may not've thought of that. To a native english speaker,
Felipe's comment (which was very helpful!) could've also been
construed as being slightly rude. Perhaps Felipe meant to write
"mistaken" instead of "silly".

Also, as an aside, smileys are always helpful on usenet. :)
[snip] I've looked at Ruby, but syntactically it
will never sway me from Python.

Same here.