[newbie] Iterating a list in reverse ?

[Andy Dingley]

Python newbie: I've got this simple task working (in about ten
different ways), but I'm looking for the "favoured" and "most Python
like" way.

Forwards I can do this
for t in listOfThings:
print t

Now how do I do it in reverse? In particular, how might I do it if I
only wanted to iterate part-way through (with a conditional test and a
break), or if I had a large list ?

reverse( listOfThings )
for t in listOfThings:
print t

As reverse() operates in-place I often can't do this. I'm also
(slightly) concerned about possible inefficiency issues of manipulating
a big list just to scan a peek at its tail.

Currently I'm doing this:

for i in range( len( listOfThings )-1, 0, -1):
t = listOfThings
print t

Is this the optimum ? Would xrange() be a better choice (and when is
it a "big" list) ?



Thanks for any guidance

 

[Fredrik Lundh]

Python newbie: I've got this simple task working (in about ten
different ways), but I'm looking for the "favoured" and "most Python
like" way.

Forwards I can do this
for t in listOfThings:
print t

Now how do I do it in reverse? In particular, how might I do it if I
only wanted to iterate part-way through (with a conditional test and a
break), or if I had a large list ?

reverse( listOfThings )
for t in listOfThings:
print t

for item in reversed(listOfThings):
...
Help on class reversed in module __builtin__:

class reversed(object)
| reversed(sequence) -> reverse iterator over values of the sequence
|
| Return a reverse iterator

....

As reverse() operates in-place I often can't do this.

given that lists only hold references to objects, reversing a *copy* of
the list is a lot more efficient than you may think...

</F>

 

[Tim Chase]

Python newbie: I've got this simple task working (in about ten

different ways), but I'm looking for the "favoured" and "most Python
like" way.

Forwards I can do this
for t in listOfThings:
print t

Now how do I do it in reverse?

Then general process would be to use the reversed() iterator:

for t in reversed(listOfThings):
print t

Python provides a sorted() wrapper of the same non-in-place form.

-tkc

 

[=?iso-8859-1?q?Luis_M._Gonz=E1lez?=]

Python newbie: I've got this simple task working (in about ten
different ways), but I'm looking for the "favoured" and "most Python
like" way.

Forwards I can do this
for t in listOfThings:
print t

Now how do I do it in reverse? In particular, how might I do it if I
only wanted to iterate part-way through (with a conditional test and a
break), or if I had a large list ?

reverse( listOfThings )
for t in listOfThings:
print t

listOfThings = [1,2,3,4,5,6]

for i in listOfThings:
print i # print from 1 to 6


for i in listOfThings[::-1]:
print i # prints from 6 to 1

 

[Andy Dingley]

for item in reversed(listOfThings):

Thanks! I was staring so hard at reverse() that I'd completely missed
reversed()

I think I prefer this to listOfThings[::-1]: as it's a little more
readable.
Not that I'm reacting to past bad experience of Perl, you understand