How to skip looping over list elements added to the list from inside a for-loop?

Question

I am iterating over nested lists with a for-loop and would like to insert a new nested lists at the next index if a condition is met. During the next iterations these newly added lists should be skipped. Example:

input_lst = [["a", "b"], ["a", "d"], ["e", "f"]]

for sublst in input_lst:
    if sublst[0] == "a":
        new_sublst = []
        input_lst.insert((input_lst.index(sublst) + 1), new_sublst)
        new_sublst.insert(0, "a")
        new_sublst.insert(1, "z")
    
print(input_lst)

Intended outcome:

[["a", "b"], ["a", "z"], ["a", "d"], ["a", "z"], ["e", "f"]]

Actual outcome - endless loop with:

[["a", "b"], ["a", "z"], ["a", "z"], ["a", "z"], ["a", "z"] ...

Answer 1

You could use a second list:

input_lst = [["a", "b"], ["a", "d"], ["e", "f"]]
new_sublst = ["a", "z"]
output_lst = []
for sublst in input_lst:
    output_lst.append(sublst)
    if sublst[0] == "a":
        output_lst.append(new_sublst)

Output:

[['a', 'b'], ['a', 'z'], ['a', 'd'], ['a', 'z'], ['e', 'f']]

Answer 2

Try this:

input_lst = [["a", "b"], ["a", "d"], ["e", "f"]]
result = []

for sublst in input_lst:
    result.append(sublst)
    if sublst[0] == "a":
        result.append(["a", "z"])

print(result)

Alternatively:

input_lst = [["a", "b"], ["a", "d"], ["e", "f"]]

for sublst in input_lst:
    if sublst == ["a","z"]:
        continue
    if sublst[0] == "a":
        new_sublst = []
        input_lst.insert((input_lst.index(sublst) + 1), new_sublst)
        new_sublst.insert(0, "a")
        new_sublst.insert(1, "z")
    
print(input_lst)

Result:

[['a', 'b'], ['a', 'z'], ['a', 'd'], ['a', 'z'], ['e', 'f']]

Point to note:

1. If you add ["a","z"] as the next element in the list when current sublst start with "a", you created a endless-loop because next iteration is definitely starting with an "a" as in ["a", "z"].

2. Using a new list to store result of each iteration can solve the problem. Or you can skip adding another ["a","z"] when the current sublst is ["a","z"].

Answer 3

Adding to the answer of @ytung-dev, which tests against a specific condition, I came up with a solution that uses a second list as a negative to compare against. Maybe someone else will find this useful:

input_lst = [["a", "b"], ["a", "d"], ["e", "f"]]
input_lst_negative = []

for sublst in input_lst:
    if (sublst[0] == "a") and (sublst not in input_lst_negative):
        new_sublst = []
        input_lst.insert((input_lst.index(sublst) + 1), new_sublst)
        new_sublst.insert(0, "a")
        new_sublst.insert(1, "z")
        input_lst_negative.append(new_sublst)
    
print(input_lst)

Output:

[['a', 'b'], ['a', 'z'], ['a', 'd'], ['a', 'z'], ['e', 'f']]