Why the last '-1' isn't removed?

Question

arr = [-1,-1,2,3,4,-1,6,7,8,9,-1]
for i in arr:
if i==-1:
arr.remove(i)
print(arr)

After i print(arr)..i get >>[2, 3, 4, 6, 7, 8, 9, -1]

My question is that, why the last '-1' isn't removed?

Answer 1

When you remove an item from a list, the counter in the next step jumps by one:
At the first iteration i is -1 and you remove it. At the next iteration i is 2 (probably you think it should be -1, but as I said it jumps by one. Because the counter now should be on the second place of arr and now it's 2, and -1 is at the first place). It goes on until it reaches at the third -1. Then it removes the first occurrence of -1 which is at the first position but the current -1 remains unremoved and become the 4th element. Finally it reaches at the last -1 and when it wants to remove the first occurrence of -1, it's now on 4th position and the last -1 remains unremoved.

Answer 2

Removing item while traversing the container is tricky. For that purpose you can remove -1 while the the container still contains a value -1.

while arr.count(-1):
arr.remove(-1)

Answer 3

The reason why there are not all desired elements are removed, is because removing during an iteration on the list "changes" the indexes following a deletion position.
But you can do the job like shown in the following samples

# using reversed direction for iterating
lst = [-1,4,3,-1,7,1,-1]
for i in reversed(lst):
if i == -1:
lst.remove(i)
print(lst)

# using reversed direction for iterating
lst = [-1,4,3,-1,7,1,-1]
x = len(lst)-1
while x >= 0:
if lst[x] == -1:
del lst[x]
x -= 1
print(lst)

# using filter
lst = [-1,4,3,-1,7,1,-1]
print(list(filter(lambda x: x != -1, lst)))

# using list comprehension
lst = [-1,4,3,-1,7,1,-1]
print([i for i in lst if i != -1])

Answer 4

You can always see what the code does. Add print(arr) after "for i in arr:" and "arr.remove(i)".
Another version of code may be:
while -1 in arr:
arr.remove(-1)

Answer 5

The Golden Rule

Never ever delete from a list that you are iterating.

Answer 6

Loop over a copy of the iterable if there is any chance the iterable might change in size.
arr = [-1, -1, 2, 3, 4, -1, 6, 7, 8, 9, -1]
for i in arr[:]: # <-- this is a copy of "arr"
if i == -1:
arr.remove(i)
print(arr)

Answer 7

you might also use this

arr[:] = (x for x in arr if x !=-1)

Answer 8

Tarun that's because of jumping counter in each iteration. For example:

Arr=[0,1,2,3,4]
For i in Arr:
Arr.remove(i)
Print(Arr)
>>> [1,3]

Answer 9

At this point it seems that EVERYTHING I HAVE LEARNT SO FAR IS WRONG.
plz let me know the answer

Answer 10

Change the if condition to

if -1 in arr:

This way you can remove the -1 even if it is all over the list

Answer 11

Amit Dubey you can't delete the last item by the structure you've mentioned. The easiest is what Ipang has said.

Answer 12

The reply with a 'green tick' is the reason behind it...Tarun

Answer 13

I have altered the expression and found that i is not iterating to last of the list.
arr=[-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1]
for i in arr:
If i==-1:
arr.remove(i)
print(i)

Excecute this code and u ll find lots of -1
Although you can simply remove this problem by i+=1 but still unable to find whats going on

Answer 14

I Need debugging.

Answer 15

amit avinoam Thanks for telling me that, but i just wanted to know why it doesn't work..

Answer 16

Qasem so, should i decrease the counter by 1 as to get it removed?

Answer 17

finally i got it why this is happening....
it took me three days to figure out but it was a lesson worth learning.

Answer 18

Use del a[-1] to remove last element

Answer 19

it should be position i so for your if statement it sould be
if arr[i] == 1:
arr.remove(i)