the important conceptual thing to note here is that mylist[:] is a shallow
*copy* of mylist
for example:
mylist = [1,2,3,4,5]
for i in mylist:
print mylist.pop(-1)
this code prints:
5
4
3
this is because we are modifying mylist during the loop: with each
iteration it is getting shorter..
but the following will print every element:
mylist = [1,2,3,4,5]
for i in mylist[:]:
print mylist.pop(-1)
5
4
3
2
1
so, you may see that construct used when the list being looped over is also
being modified in place. it can also be used to simply make a copy.
l1 = [1,2,3,4,5]
l2 = l1[:]
l2.pop(-1)
print l1
print l2
[1, 2, 3, 4, 5]
[1, 2, 3, 4]
-chad
On Fri, Mar 1, 2013 at 9:11 AM, Chad Fox <[email protected]> wrote:
> The colon ':' is used to return list items above, below or in between any
> index numbers your provide. This is also known as index slicing. However
> when you don't provide any index numbers it simply returns every item in
> the list.
>
> Usage eg.
>
> >> testList = [0,1,2,3,4,5]
> >> print testList[:] # all items
> [0,1,2,3,4,5]
>
> >> print testList[3:] #all above and including index 3
> [3,4,5]
>
> >> print testList[:3] # all below, but not including index 3
> [0,1,2]
>
> >> print testList[2:5] # returns indices 2,3 and 4, but not 5
> [2,3,4]
>
> You can also use negative numbers which will count back from the end of
> the list.
>
> >> print testList[-2] # returns 2nd to last item in the list
> [4]
>
> >> print testList[:-2] # returns items up to, but not including the 2nd to
> last item.
> [0,1,2,3]
>
> Additionally, as Dorian pointed out, arrays is the list object itself,
> but arrays[:] is the contents of the list object.
>
> In your loop example the result is the same because both the arrays and
> arrays[:]
> return the entire list to be looped over.
>
> For more information refer to this python documentation page. Both the
> Stings and Lists subjects have many examples and explanations.
> http://docs.python.org/2/tutorial/introduction.html#strings
>
> Cheers
>
>
> -Chad
>
>
> On Fri, Mar 1, 2013 at 8:33 AM, Panupat Chongstitwattana <
> [email protected]> wrote:
>
>> for a in arrays:
>> and
>> for a in arrays[:]:
>> seem to do exactly the same thing. Is there any difference made by the
>> [:] ?
>>
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