Making a While loop out of a For loop

Question

I made this piec of code and it works just fine for the following:

Write the code to check whether a string given through input()contains only valid characters, but stop once you reach one invalid character. So if the string is valid print valid string. If the string is invalid print invalid string till that point.

But now i have to use a while loop for this and i can't get it done, can you guys help me or explain to me how convert this code to a while loop, while maintaining the exact same function.

file = input()

invalid_letter_found = False
correct_letters = []

for current_letter in file:
    if current_letter in ['A', 'T', 'G', 'C']:
        correct_letters.append(current_letter)
        continue
    elif current_letter != ['A', 'T', 'G', 'C']:
        invalid_letter_found = True
        break
if invalid_letter_found == True:
    print(f'invalid {"".join(correct_letters)}')
else:
    print(f'valid {"".join(correct_letters)}')

Answer 1

This program will give the exact same output as above program in the query

file = input()
n=len(file)
lst=[]
for alp in file:  # this will convert the string to list, you can use any other method if you don't want for loop
    lst.append(alp) #Here the conversion ends
invalid_letter_found = False
correct_letters = []
i= 0
while i < n:
    if lst[i] in ["A", "T", "G", "C"]:
        correct_letters.append(lst[i])
        i += 1
        continue
    elif lst[i] != ["A", "T", "G", "C"]:
        invalid_letter_found = True
        break
if invalid_letter_found == True:
    print(f'invalid {"".join(correct_letters)}')
else:
    print(f'valid {"".join(correct_letters)}')

Answer 2

Although the only reason you would NEED to use a while loop is if this is a homework question - here's a nudge in the right direction:

1. You could progressively remove letters from the string using string slicing

current_letter = file[0]
file = file[1:]

2. You could use the string length as your condition for your while loop - once the string length is zero, you are done:

while len(file):