Using For Loop To Remove Elements From A List Is Not Working, But Using List Comprehension Is Fine

OK, this problem is actually solved, but I still want to know WHY. I am trying to remove elements from a 2-dimensional array, and here's the code: print('adj in the while loop: ',

Solution 1:

These two methods are not identical. In the list comprehension, you never call adj.remove(z).

The list comprehension is creating a new list as it iterates over adj, then you assign that (empty) list back to adj once it's done. It doesn't change adj while iterating, only after the last iteration.

Solution 2:

You can definitely use .remove(), but iterate over a copy of the original and not the original itself:

adj = [[1, True], [0, 2, True], [1, True]]

for z in adj[:]:
    print("for loop: ", z)
    if z[-1] is True:
        adj.remove(z)
result += 1
print("adj after all execution: ", adj)

which prints out an empty list.

Here, we iterate over the copy, but remove elements from the original. This avoids discrepancy that you see with your code.

Solution 3:

Iteration over the list and at same time with it's inner block, if you use method remove or append for an element is not right way to do it. Basically better to change the coding to other way. As like Austin suggested in his code or use separate index instead of iterative index or element.

That is

adj = [[3, 4, 5, False], [8, 7, False], [0, True], [-1, False], [1, True], [0, 2, False], [0, 2, True], [1, True], [4, False]]

del_index = 0
for i in range(len(adj)):
    print("for loop: ", adj[del_index])
    if adj[del_index][-1] is True:
        adj.remove(adj[del_index])
    else:
        del_index+=1
print("adj after all execution: ", adj)

Filter can applied for this purpose

filter(lambda elem: elem[-1] is True, adj)

This may crash your system since the append element to the list and same time iterating over the same.

crash_my_system = [0]
for i in crash_my_system: crash_my_system.append(i+1)

This is not only true for List. this is common to all mutable data structure such as Dict. Refer: Christoph Zwerschke's blog