Deleting more than one element from a list
candide
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
> 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
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
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
> 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
Raymond Hettinger
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