On Jun 30, 3:29 pm, python_newbie <[EMAIL PROTECTED]> wrote:
> On 25 Haziran, 17:44, MRAB <[EMAIL PROTECTED]> wrote:
>
>
>
> > On Jun 25, 11:37 am, "A.T.Hofkamp" <[EMAIL PROTECTED]> wrote:
>
> > > On 2008-06-25, python_newbie <[EMAIL PROTECTED]> wrote:
>
> > > > On 24 Haziran, 04:33, Terry Reedy <[EMAIL PROTECTED]> wrote:
> > > > Thanks for all answers. At the end i ve only one point. If a decide to
> > > > copy list to iterate when will i have to do this ? Before the
> > > > iteration ? And then iterate through one list and change value of the
> > > > other ?
>
> > > Before starting the iteration would be a good point....
>
> > > I usually do in such cases:
>
> > > for x in mylist[:]:
> > > ...
>
> > > making a copy just before the for loop starts.
>
> > > Lately, I have started avoiding in-place modification of lists. Instead, I
> > > construct a new list from scratch inside the for-loop, and replace the
> > > old list
> > > with the newly constructed list afterwards like:
>
> > > new_list = []
> > > for x in mylist:
> > > ....
> > > new_list.append(x)
>
> > > mylist = new_list
>
> > > by appending a different value than the original or by not appending, you
> > > can
> > > influence the contents of the new list.
>
> > > I find this solution nicer than in-place modification of an existing list.
>
> > > Sincerely,
> > > Albert
>
> > And if you were originally doing in-place modification because there
> > were also other references to the list then you could just do:
>
> > mylist[:] = new_list
>
> Thanks again. I see that you use two different syntax for lists
> "mylist = new_list" and "mylist[:] = new_list" these are same i
> think ?
The difference is that "mylist = new_list" makes mylist refer to the
same list as new_list:
Before:
otherlist ───┐
├───► [1, 2, 3]
mylist ──────┘
new_list ────────► [4, 5, 6]
After:
otherlist ───────► [1, 2, 3]
mylist ──────┐
├───► [4, 5, 6]
new_list ────┘
whereas "mylist[:] = new_list" modifies the list to which mylist
already refers to contain the same items as new_list:
Before:
otherlist ───┐
├───► [1, 2, 3]
mylist ──────┘
new_list ────────► [4, 5, 6]
After:
otherlist ───┐
├───► [4, 5, 6]
mylist ──────┘
new_list ────────► [4, 5, 6]
(I hope the characters come out OK! :-))
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