The copy of a list is not being iterated through all the elements

Question

t=[1,2,3,5,2,1]
j=t.copy()

for f in j:
    print(f)
    j.remove(f)
    print(j)

Output

1
[2, 3, 5, 2, 1]
3
[2, 5, 2, 1]
2
[5, 2, 1]

I have a list t and I create a copy of it in j by calling t.copy().

But when I use the looping constructs, it does not traverse through all the elements. For example, in the output it selects 1, removes it and then prints the list without that element. After that it should choose 2 which is the next element and remove the same, but it selects 3 instead and 5 in the next iteration.

What could possibly be the reason for this alternate selection? Perhaps this is quite simple, but I am unable to figure the reason out.

Answer 1

The issue is that while you're iterating through the list you're also editing it leading to errors. Instead of iterating through j iterate through t since it'll remain the same throughout iterations.

Code:

t = [1, 2, 3, 5, 2, 1]
j = t.copy()

for f in t:
    print(f)
    j.remove(f)
    print(j)

Output:

1
[2, 3, 5, 2, 1]
2
[3, 5, 2, 1]
3
[5, 2, 1]
5
[2, 1]
2
[1]
1
[]