How to make a reverse for loop in python?

[Alex Snast]

Hello

I'm new to python and i can't figure out how to write a reverse for
loop in python

e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)

[Thoma]

Alex Snast a écrit :

Hello

I'm new to python and i can't figure out how to write a reverse for
loop in python

e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)

for (i = 0; i < 10; i--) -for i in range(10):

for (i = 10; i >= 0; --i) -for i in range(10,-1,-1):

Thoma

[Mensanator]

On Sep 20, 11:16�am, Alex Snast <asn...@gmail.c omwrote:

Hello

I'm new to python and i can't figure out how to write a reverse for
loop in python

e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)

>>for i in xrange(10,-1,-1): print i,

10 9 8 7 6 5 4 3 2 1 0

Note the starting number is 10, the ending
number is -1 because you want to include 0
and the step size is -1.

[Duncan Booth]

Alex Snast <as****@gmail.c omwrote:

Hello

I'm new to python and i can't figure out how to write a reverse for
loop in python

e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)

The exact equivalent would be:

for i in range(10, -1, -1): print i

except you virtually never want to do that in Python. Don't expect just to
translate statement by statement from one language to another: normally in
Python you will iterate directly over the sequence you want to process
rather than trying to count loop indices with all the telegraph pole errors
that result.

The usual way to iterate over a sequence in reverse is:

for x in reversed(seq): print x

although if you know it is a list, string or other object that supports
extended slicing you can also do:

for x in seq[::-1]: print x

this may be less clear than using 'reversed', but does allow you to specify
an explicit start, stop and step if you want to do only part of the
sequence.

[Simon Brunning]

2008/9/20 Alex Snast <as****@gmail.c om>:

I'm new to python and i can't figure out how to write a reverse for
loop in python

e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)

for i in range(10, 0, -1):
print i

--
Cheers,
Simon B.

[Fredrik Lundh]

Alex Snast wrote:

I'm new to python and i can't figure out how to write a reverse for
loop in python

e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)

use range with a negative step:

for i in range(10-1, -1, -1):
...

or just reverse the range:

for i in reversed(range( 10)):
...

(the latter is mentioned in the tutorial, and is the second hit if you
google for "python reverse for loop")

</F>

[Gary Herron]

Alex Snast wrote:

Hello

I'm new to python and i can't figure out how to write a reverse for
loop in python

e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)
--
http://mail.python.org/mailman/listinfo/python-list

What are you trying to loop through?

If it's the contents of a list, you can reverse the list (in place) first:

L = [1,2,3]
L.reverse()
for item in L:
print item

Or you can create a new reversed (copy of the original) list and iterate
through it

for item in reversed(L):
print item

If it's just a sequence of numbers you want to generate:

range(3) generates a forward list [0,1,2], and
range(3,0,-1) generates a backward list [2,1,0]

so

for i in range(11,0,-1):

might be what you want.
If your list is huge, consider xrange rather than range.
And as always, you could just roll your own index manipulation:

i = 10
while i >=0:
# do whatever
i -= 1


Gary Herron

[Fredrik Lundh]

Fredrik Lundh wrote:

>e.g. the python equivalent to the c++ loop

for (i = 10; i >= 0; --i)

use range with a negative step:

for i in range(10-1, -1, -1):
...

or just reverse the range:

for i in reversed(range( 10)):
...

(and to include the 10 in the range, add one to the 10 above)

</F>

[Peter Otten]

Gary Herron wrote:

Or you can create a new reversed (copy of the original) list and iterate
through it

for item in reversed(L):
Â* print item

It's not a copy, it's a view:

>>items = [1,2,3]
r = reversed(items)
items[:] = "abc"
for item in r: print item

....
c
b
a

Peter

[bearophileHUGS]

Duncan Booth:

e.g. the python equivalent to the c++ loop
for (i = 10; i >= 0; --i)

The exact equivalent would be:
for i in range(10, -1, -1): print i

I'd use xrange there. Anyway, I have always felt that Python syntax
not easy to understand at first sight, expecially when you try to
convert a bit more complex inverted for loops from/to C to/from
Python. It's one of the few cases where (for example) Pascal (loop)
syntax wins a bit over Python syntax :-)

Bye,
bearophile