[Tutor] update a list

[Manprit Singh]

Thanks  a lot sir.

Regards
Manprit Singh

On Sun, Oct 11, 2020 at 3:48 PM dn via Tutor <tutor at python.org> wrote:

> On 11/10/2020 21:44, Manprit Singh wrote:
> > Dear sir ,
> >
> > Consider a problem where I have to update a list , at even places with
> > increment of 1 and at odd places with increment of 2 . Just need to check
> > if this can be done in a better way. My solution is given below:
> >
> > l = [2, 3, 5, 7, 6, 4]
> > for i, j in enumerate(l):
> >      if i % 2 == 0:
> >          l[i] = j+1
> >      else:
> >          l[i] = j+2
> >
> > the updated list is
> >
> > [3, 5, 6, 9, 7, 6]
>
>
>  From where are you finding these problems, or what do you intend to do
> with the results?
>
>
> I am not in-favor of updating lists/list-elements in-place because
> altering iterables inside their iteration can cause big problems in
> certain other scenarios!
>
> Accordingly, I prefer to create a new list (and if necessary, delete the
> old one, afterwards).
>
> Remember that you can slice a sequence. Thus, a faster solution is:
>
>  >>> l = [2, 3, 5, 7, 6, 4]
>  >>> new_l = list()
>  >>> for index in range( 0, len( l ), 2 ):
> ...     new_l.append( l[ index ] + 1 )
> ...     new_l.append( l[ index + 1 ] + 2 )
> ...
>  >>> new_l
> [3, 5, 6, 9, 7, 6]
>  >>>
>
> If you insist upon updating-in-place, replace the append()s with:
>
> l[ index ] += 1
>
>
> Performance comparison:
> No integer-division/remainder comparison, no if, and with half(ish) the
> number of loops!
> --
> Regards =dn
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