[Tutor] Sort pointers -

[Liam Clarke]

Hi all,

Thanks Kent. Question - is Python 2.4 release 1 considered a stable release?

Brian - Thanks very much for the illuminating functions, they help indeed.

So when it returns cmp(x,y) it's returning a negative for x < y, 0 for
x == y, and a positive for x > y...

eep, a little confused, just trying to understand how Python sorts
using that. I may go check out Python 2.4...

Regards,


Liam Clarke


On Wed, 24 Nov 2004 11:30:15 -0500, Brian van den Broek
<bvande at po-box.mcgill.ca> wrote:
> Kent Johnson said unto the world upon 2004-11-24 06:05:
> 
> <SNIP helpful explanations>
> 
> 
> 
> >
> > In Python 2.3 you have two choices:
> > - Use the decorate - sort - undecorate idiom (aka Schwartzian Transform)
> > - Define your own comparison function
> >
> > I'm out of time for explaining right now, maybe someone else can pick up
> > the thread or I'll come back to it later...
> >
> > Kent
> >
> > Liam Clarke wrote:
> >
> >> Kia ora,
> >>
> >> Just exploring the wonderful world of dictionaries, and I'm trying to
> >> comprehend the sort() method, because as I found, dictionaries are not
> >> sorted.
> >>
> 
> <SNIP examples from question>
> 
> 
> 
> >> That's just an example of output. I'm not too hung up on the output at
> >> the moment, just sort().
> >>
> >> It says to use a comparison function...
> >>
> >> And I have no idea how to work that.
> >>
> >> ??? Very confused. I've tried searching for info, there's some very
> >> complex ways out there to sort data, usually with proper first names
> >> (Schwartzian transformations...), but I can't seem to find the simple.
> >>
> >> Could anyone post some useful links for my education?
> >>
> >> Much appreciated.
> >>
> >> Liam Clarke
> 
> Hi Liam,
> 
> maybe this will help:
> 
>  >>> def length_sort(first, second):
>      '''A custom cmp function to pass to sort for sort on item length'''
>      # We do a standard sort of the length of the items compared.
>      return cmp(len(first), len(second))
> 
>  >>> def alphabetical_sort(first, second):
>      '''A custom cmp function to pass to sort for case insensitive sort'''
>      first = first.lower()
>      second = second.lower()
>      # We do a standard sort on lower-cased versions of the items
>      # compared.
>      return cmp(first, second)
> 
> a_list = ['BC', 'def', 'GHIJ', 'a', 'kH', 'AH']
>  >>> print a_list.sort()
> None
>  >>> # sort modifies in place, so printing a_list.sort() gives None
>  >>> # Probably not what was wanted!
>  >>>
>  >>> a_list.sort()
>  >>> print a_list
> ['AH', 'BC', 'GHIJ', 'a', 'def', 'kH']
>  >>> # quite likely not what is wanted either, as all CAP letters come
>  >>> # before lower-case when the standard cmp(x,y) function is used.
>  >>>
>  >>> a_list.sort(alphabetical_sort)
>  >>> print a_list
> ['a', 'AH', 'BC', 'def', 'GHIJ', 'kH']
>  >>> # now, via alphabetical_sort(), it is sorted in true alpha-order
>  >>>
>  >>> a_list.sort(length_sort)
>  >>> print a_list
> ['a', 'AH', 'BC', 'kH', 'def', 'GHIJ']
>  >>> # Since length_sort only cares about the len of an object, for
>  >>> # any two string of equal length, it leaves their order unchanged
>  >>> # from the way it was before the list was sorted.
>  >>>
>  >>> # Now let's try something a bit silly:
>  >>> def goofy_H_sort(first, second):
>      '''A custom cmp function to pass to sort for 'H'-based sorting'''
>      if 'H' in first and 'H' not in second:
>          return first, second
>      if 'H' in second and 'H' not in first:
>          return second, first
>      return cmp(first, second)
> 
>  >>> a_list.sort(goofy_H_sort)
> 
> Traceback (most recent call last):
>    File "<pyshell#76>", line 1, in -toplevel-
>      a_list.sort(goofy_H_sort)
> TypeError: comparison function must return int
>  >>>
>  >>> # Well, that didn't work! Checking section 2.1 of the Lib Ref
>  >>> # for the cmp(x,y) built-in suggests we do:
>  >>> def goofy_H_sort(first, second):
>      '''A custom cmp function to pass to sort for 'H'-based sorting'''
>      if 'H' in first and 'H' not in second:
>          return -1
>          # cmp(x,y)'s way of saying x < y, so by returning -1, we
>          # model that behaviour and have our custom sort function
>          # say that first comes before second.
>      if 'H' in second and 'H' not in first:
>          return 1
>          # cmp(x,y)'s way of saying x > y, so by returning 1, we
>          # model that behaviour and have our custom sort function
>          # say that first comes after second.
>      return cmp(first, second)
> 
>  >>> a_list.sort(goofy_H_sort)
>  >>> print a_list
> ['AH', 'GHIJ', 'kH', 'BC', 'a', 'def']
>  >>>
> 
> HTH,
> 
> Brian vdB
> 
> 


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