Python for loop to while loop for unique value

string = "Python, program!"

result = []
for x in string:
    if x not in result and x.isalnum():
        result.append(x)
print(result)

This program makes it so if a repeat letter is used twice in a string, it’ll appear only once in the list. In this case, the string “Hello, world!” will appear as

[‘H’, ‘e’, ‘l’, ‘o’, ‘w’, ‘r’, ‘d’]

My question is, how would I go about using a while loop instead of a for loop to achieve the same result? I know there’s really no need to change a perfectly good for loop into a while loop but I would still like to know.

Answer

Here is one way you could do it in a while loop:

text = "Python, program!"

text = text[::-1]  # Reverse string
result = []
while text:
    if text[-1] not in result:
        result.append(text[-1])
    text = text[:-1]

print(result)