python - Why does del list[0] in a for-loop only delete half the list? -


i have following code , deletes half list using loop. why happen? also, how should delete first element of list while traversing through it?

list = [1,2,3,4,5,6,7,8,9,10] x in list:     print list[0]     del list[0] print list

output:

1  2 3 4 5 [6, 7, 8, 9, 10]

the problem delete list list-iterator doesn't know , happily processes "remaining" list.

so in first iteration iterator @ index 0 , remove index 0, in next iteration iterator returns index 1 (which item @ index 2 before removed first item) , removes index 0. in next iteration item @ index 2 (which @ index 4 since removed 2 items @ index 2) , on. stop index greater items remaining in list, given remove 1 item each item processed that's in middle (half) of original list.

long story short: don't modify list you're iterating over.

if want use while loop:

lst = [1,2,3,4,5,6,7,8,9,10] while lst:   # long lst contains items     print lst[0]     del lst[0] print lst

or iterate on copy:

lst = [1,2,3,4,5,6,7,8,9,10] x in lst[:]:   # [:] makes shallow copy of list     print lst[0]     del lst[0] print lst

note: list name of built-in function of python, shadow function if have variable same name. that's why changed variable name lst.


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