unexpected output for a list loop problem

Question

The output must be an empty list but I am getting 3. Literaly got confused in this silly problem.

j = [2,3,4]
lk = []
for i in j:
 if i>=0:
 j.remove(i)
 
print(j)

output = [3]

expected output = []

what is the issue here

• Arihant: you shouldn't edit the list within it's own iterator

Answer 1

When you are looping it is iterating with reference to it's index.

In the first iteration -> Index is 0 and Value is 2: 2 > 0, the value is getting removed from the list (j). Result: j=[3,4]

In the second iteration -> Index is 1 and value becomes 4 (since you have removed 2 from j): 4 > 0, the value is getting removed. Result: j=[3]

Now the size of the list is 1 (which is less than the current iterator value(2)), so the iteration stops and the list j still has value 3 j=[3]

Trial code:

j = [2,3,4]
lk = []
for i in j:
 if i>=0:
 j.remove(i)
 print(j)

Output:

[3, 4]
[3]

So, you shouldn't edit the list within it's own iterator. Instead create a copy and perform the operation:

Suggested code:

j = [2,3,4]
j_cpy = j.copy()
lk = []
for i in j_cpy:
 if i>=0:
 j.remove(i)
 print(j)
print("Final j value: ", j)

Here you are iterating on j_cpy and editing the list j.

Output:

[3, 4]
[4]
[]
Final j value: []

Answer 2

Use a lambda function, they are much simpler, and always work when removing items from lists according to boolean checks.

Example:

j = [2, 3, 4]
lk = []
j = list(filter(lambda x: not x >= 0, j))
print(j)

Here's a small tutorial on them: https://www.geeksforgeeks.org/lambda-filter-python-examples/