Velocity Reviews > Deleting more than one element from a list

# Deleting more than one element from a list

candide
Guest
Posts: n/a

 04-21-2010
Is the del instruction able to remove _at the same_ time more than one
element from a list ?

For instance, this seems to be correct :

>>> z=[45,12,96,33,66,'ccccc',20,99]
>>> del z[2], z[6],z[0]
>>> z

[12, 33, 66, 'ccccc', 20]
>>>

However, the following doesn't work :

>> z=[45,12,96,33,66,'ccccc',20,99]
>>> del z[2], z[3],z[6]

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range
>>>

Does it mean the instruction

del z[2], z[3],z[6]

to be equivalent to the successive calls

del z[2]
del z[3]
del z[6]

?

Simon Brunning
Guest
Posts: n/a

 04-21-2010
On 21 April 2010 20:56, candide <(E-Mail Removed)> wrote:
> Is the del instruction able to remove _at the same_ time more than one
> element from a list ?

Yup:

>>> z=[45,12,96,33,66,'ccccc',20,99]
>>> del z[:]
>>> z

[]

--
Cheers,
Simon B.

Gary Herron
Guest
Posts: n/a

 04-21-2010
candide wrote:
> Is the del instruction able to remove _at the same_ time more than one
> element from a list ?
>
>
> For instance, this seems to be correct :
>
>
> >>> z=[45,12,96,33,66,'ccccc',20,99]
> >>> del z[2], z[6],z[0]
> >>> z

> [12, 33, 66, 'ccccc', 20]
> >>>

>
>
> However, the following doesn't work :
>
> >> z=[45,12,96,33,66,'ccccc',20,99]
> >>> del z[2], z[3],z[6]

> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> IndexError: list assignment index out of range
> >>>

>
>
> Does it mean the instruction
>
> del z[2], z[3],z[6]
>
> to be equivalent to the successive calls
>
>
> del z[2]
> del z[3]
> del z[6]

Yes, those are equivalent. The reason it fails is that, by the time it
gets around to the third delete, there is no longer in index [6] in the
list. The element you were thinking of is now at index [4].

This, however, will work as you expected:

del z[6], z[3],z[2]

--
Gary Herron, PhD.
Department of Computer Science
DigiPen Institute of Technology
(425) 895-4418

Mensanator
Guest
Posts: n/a

 04-21-2010
On Apr 21, 2:56*pm, candide <(E-Mail Removed)> wrote:
> Is the del instruction able to remove _at the same_ time more than one
> element from a list ?
>
> For instance, this seems to be correct :
>
> *>>> z=[45,12,96,33,66,'ccccc',20,99]
> *>>> del z[2], z[6],z[0]
> *>>> z
> [12, 33, 66, 'ccccc', 20]
> *>>>
>
> However, the following doesn't work :
>
> *>> z=[45,12,96,33,66,'ccccc',20,99]
> *>>> del z[2], z[3],z[6]
> Traceback (most recent call last):
> * *File "<stdin>", line 1, in <module>
> IndexError: list assignment index out of range
> *>>>
>
> Does it mean the instruction
>
> del z[2], z[3],z[6]
>
> to be equivalent to the successive calls
>
> del z[2]
> del z[3]
> del z[6]

That's part of the problem. Let's look at a better example.

>>> z = [0,1,2,3,4,5,6]
>>> del z[0],z[3],z[6]

Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
del z[0],z[3],z[6]
IndexError: list assignment index out of range
>>> z

[1, 2, 3, 5, 6]

Yes, the error was caused by the list shrinking between calls,
so the 6 did not get deleted. But notice that 3 is still there
and 4 is missing.

If you must delete this way, do it bottom up so that the index
remains valid for the subsequent calls:

>>> z = [0,1,2,3,4,5,6]
>>> del z[6],z[3],z[0]
>>> z

[1, 2, 4, 5]

>
> ?

Emile van Sebille
Guest
Posts: n/a

 04-21-2010
On 4/21/2010 12:56 PM candide said...
> Is the del instruction able to remove _at the same_ time more than one
> element from a list ?
>
>
> For instance, this seems to be correct :
>
>
> >>> z=[45,12,96,33,66,'ccccc',20,99]

Not as I see it -- watch your index values - they change after each
delete is completed. It'll work if you order them backwards though.

>>> a = range(10)
>>> del a[0],a[2],a[4],a[6]
>>> a

[1, 2, 4, 5, 7, 8]
>>> a = range(10)
>>> del a[6],a[4],a[2],a[0]
>>> a

[1, 3, 5, 7, 8, 9]
>>>

Emile

candide
Guest
Posts: n/a

 04-21-2010
Thanks for your reponses.

Raymond Hettinger
Guest
Posts: n/a

 04-25-2010
On Apr 21, 12:56*pm, candide <(E-Mail Removed)> wrote:
> Is the del instruction able to remove _at the same_ time more than one
> element from a list ?
>
> For instance, this seems to be correct :
>
> *>>> z=[45,12,96,33,66,'ccccc',20,99]
> *>>> del z[2], z[6],z[0]
> *>>> z
> [12, 33, 66, 'ccccc', 20]
> *>>>
>
> However, the following doesn't work :
>
> *>> z=[45,12,96,33,66,'ccccc',20,99]
> *>>> del z[2], z[3],z[6]
> Traceback (most recent call last):
> * *File "<stdin>", line 1, in <module>
> IndexError: list assignment index out of range
> *>>>
>
> Does it mean the instruction
>
> del z[2], z[3],z[6]
>
> to be equivalent to the successive calls
>
> del z[2]
> del z[3]
> del z[6]
>
> ?

Looks like you got a lot of good answers to the question as asked.

FWIW, successive delete operations on a list are dog slow.
It is better to delete all of the entries in one pass.
There are several ways to do it. Here's one:

>>> z=[45,12,96,33,66,'ccccc',20,99]
>>> targets = [2, 3, 6]
>>> PLACEHOLDER = object()
>>> for i in targets:

.... z[i] = PLACEHOLDER
>>> z[:] = [elem for elem in z if elem is not PLACEHOLDER]

Here's another:

>>> z=[45,12,96,33,66,'ccccc',20,99]
>>> targets = set([2, 3, 6])
>>> z[:] = [elem for i, elem in enumerate(z) if i not in targets]

Besides being scaleable, these two examples have some nice learning
points. Hopefully, you will find them useful.

Raymond