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how do you make a loop run in reverse?

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  how do you make a loop run in reverse? rahulreddy24@hotmail.com - 2013-03-26 16:59 -0700
    Re: how do you make a loop run in reverse? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-03-27 00:18 +0000
    Re: how do you make a loop run in reverse? Xavier Ho <contact@xavierho.com> - 2013-03-27 11:19 +1100
    Re: how do you make a loop run in reverse? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-03-27 01:08 +0000
    Re: how do you make a loop run in reverse? Dave Angel <davea@davea.name> - 2013-03-26 20:08 -0400
    Re: how do you make a loop run in reverse? Arnaud Delobelle <arnodel@gmail.com> - 2013-03-27 21:18 +0000

#41964 — how do you make a loop run in reverse?

So i have a set of for loops that create this :

***************************************
***   ***   ***   ***   ***   ***   ***   
***   ***   ***   ***   ***   ***   ***   
   ***   ***   ***   ***   ***   ***   ***
   ***   ***   ***   ***   ***   ***   ***
***************************************
                  
                 *
                ***
               *****
              *******
             *********

but i want to nest all the loops under one BIG loop that'll run in reverse to make this:

***************************************
***   ***   ***   ***   ***   ***   ***   
***   ***   ***   ***   ***   ***   ***   
   ***   ***   ***   ***   ***   ***   ***
   ***   ***   ***   ***   ***   ***   ***
***************************************
                  
                 *
                ***
               *****
              *******
             *********
              *******
               *****
                ***
                 *
***************************************
   ***   ***   ***   ***   ***   ***   ***
   ***   ***   ***   ***   ***   ***   ***
***   ***   ***   ***   ***   ***   ***   
***   ***   ***   ***   ***   ***   ***   
***************************************

Is this possible?

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#41966

On 26/03/2013 23:59, rahulreddy24@hotmail.com wrote:
> So i have a set of for loops that create this :
>

snipped the art

>
> but i want to nest all the loops under one BIG loop that'll run in reverse to make this:
>

snipped the art

>
> Is this possible?
>

Yes read http://docs.python.org/3/library/stdtypes.html#range and 
http://docs.python.org/3/library/functions.html#reversed

-- 
If you're using GoogleCrap™ please read this 
http://wiki.python.org/moin/GoogleGroupsPython.

Mark Lawrence

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#41968

[Multipart message — attachments visible in raw view] — view raw

There is a built-in function that reverses an iterable.  Have a look at the
documentation.

xav


On 27 March 2013 10:59, <rahulreddy24@hotmail.com> wrote:

> So i have a set of for loops that create this :
>
> ***************************************
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
>                  *
>                 ***
>                *****
>               *******
>              *********
>
> but i want to nest all the loops under one BIG loop that'll run in reverse
> to make this:
>
> ***************************************
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
>                  *
>                 ***
>                *****
>               *******
>              *********
>               *******
>                *****
>                 ***
>                  *
> ***************************************
>    ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
> Is this possible?
> --
> http://mail.python.org/mailman/listinfo/python-list
>

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#41971

> On 27 March 2013 10:59, <rahulreddy24@hotmail.com> wrote:
>>
>> So i have a set of for loops that create this :
>>
[snip]
>> but i want to nest all the loops under one BIG loop that'll run in reverse
>> to make this:
[snip]
>> Is this possible?

On 27 March 2013 00:19, Xavier Ho <contact@xavierho.com> wrote:
> There is a built-in function that reverses an iterable.  Have a look at the
> documentation.

I assume you mean the reversed function. It does not in general
reverse an iterable. From the docs:
"""
reversed(seq)

Return a reverse iterator. seq must be an object which has a
__reversed__() method or supports the sequence protocol (the __len__()
method and the __getitem__() method with integer arguments starting at
0).

New in version 2.4.

Changed in version 2.6: Added the possibility to write a custom
__reversed__() method.
"""

So it reverses a sequence (having the __len__ and __getitem__ methods)
or an object that advertises reversibility (having a __reversed__
method). It does not reverse anything else including generators,
iterators and most non-sequence iterables.

To the OP: To reverse a "set of for loops" as requested is not
possible using the reversed function. It is, however, possible to
reverse a list of strings. So if you have a function that returns the
list of strings you show as output then you can easily reverse that
list with reversed(mylist) or mylist[::-1].


Oscar

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#42015

On 03/26/2013 07:59 PM, rahulreddy24@hotmail.com wrote:
> So i have a set of for loops that create this :
>
> ***************************************
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
>     ***   ***   ***   ***   ***   ***   ***
>     ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
>                   *
>                  ***
>                 *****
>                *******
>               *********
>
> but i want to nest all the loops under one BIG loop that'll run in reverse to make this:
>
> ***************************************
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
>     ***   ***   ***   ***   ***   ***   ***
>     ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
>                   *
>                  ***
>                 *****
>                *******
>               *********
>                *******
>                 *****
>                  ***
>                   *
> ***************************************
>     ***   ***   ***   ***   ***   ***   ***
>     ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
> Is this possible?
>

tes

-- 
DaveA

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#42043

On 26 March 2013 23:59,  <rahulreddy24@hotmail.com> wrote:
> So i have a set of for loops that create this :
>
> ***************************************
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
>                  *
>                 ***
>                *****
>               *******
>              *********
>
> but i want to nest all the loops under one BIG loop that'll run in reverse to make this:
>
> ***************************************
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
>                  *
>                 ***
>                *****
>               *******
>              *********
>               *******
>                *****
>                 ***
>                  *
> ***************************************
>    ***   ***   ***   ***   ***   ***   ***
>    ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
> ***   ***   ***   ***   ***   ***   ***
> ***************************************
>
> Is this possible?

Let's have a look at a simple example.  Imagine you have a function:

>>> def print_pretty_pattern():
...    for i in range(1, 6):
...       print '*'*i
...
>>> print_pretty_pattern()
*
**
***
****
*****

You can't reverse the pattern because it's not data so you need to
turn it into data:

>>> def generate_pretty_pattern():
...     for i in range(1, 6):
...         yield '*'*i

It's the same as above, but the 'print' statement has been replace
with a 'yield' statement, making the function into a generator
function, so that when you call it you get an iterable of all the
lines.  You can now make a function to print a pattern:

>>> def print_pattern(lines):
...     for line in lines:
...         print line

Or if you want to be concise:

>>> def print_pattern(lines):
...     print "\n".join(lines)

So you can print any pattern:

>>> print_pattern(generate_pretty_pattern())
*
**
***
****
*****

So now you can write another generator that makes the mirror pattern
of a given pattern:

>>> def mirror_pattern(pattern):
...     lines = []
...     for line in pattern:
...         yield line
...         lines.append(line)
...     if lines:
...         lines.pop() # don't repeat the last line
...         for line in reversed(lines):
...             yield line
...
>>> print_pattern(mirror_pattern(generate_pretty_pattern()))
*
**
***
****
*****
****
***
**
*

Here's another example:

>>> print_pattern(mirror_pattern(''.join(mirror_pattern("*".ljust(i).rjust(15))) for i in range(1,16,2)))
              *
            *   *
          *       *
        *           *
      *               *
    *                   *
  *                       *
*                           *
  *                       *
    *                   *
      *               *
        *           *
          *       *
            *   *
              *

--
Arnaud

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