Sorting troubles

[Steven D'Aprano]

On Mon, 14 May 2007 09:49:56 -0700, Thomas Nelson wrote:

> The thing is that [x for x in List[1:]...] is a brand new list created
> by iterating over the old one.
> How about:
> qSortHelp(List):
>     newlist = qSort(List)
>     for i, val in enumerate(newlist):
>         List[i] = val
> You have to iterate over one more time, but this sorts the list in
> place.

A better way of spelling that would be:

def qSortHelp(List):
    List[:] = qSort(List)
    return List


but the helper function is redundant, as it is easy enough to make the 
qSort function behave the same way. We can also make the code a smidgen 
more concise by reversing the sense of the test at the start of the code:


def qSort(List):
    if List:
        List[:] = qSort([x for x in List[1:] if x< List[0]]) \
        + List[0:1] + qSort([x for x in List[1:] if x>=List[0]])
    return List



-- 
Steven.