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.

CodePudding user response：

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)

CodePudding user response：

There are several ways of doing this.

Emulating the for loop

This is what a for loop does under the hood:

it = iter(string)
while True:
    try:
        # attempt to get next element
        x = next(it)
    except StopIteration:
        # no next element => get out of the loop
        break

    if x not in result and x.isalnum():
        result.append(x)

The C way

You can simply index into your string:

i = 0
while i < len(string):
    x = string[i]
    if x not in result and x.isalnum():
        result.append(x)

    i  = 1

This is analogous to how one would iterate over a container in the C language - by indexing into it and incrementing the index afterwards:

for (int i = 0; i < strlen(string); i  )
    printf("%c", string[i]);

However, indexing doesn't always work because some objects that one can iterate over don't support indexing:

a_generator = (i for i in range(5))
for elem in a_generator:
    print(a) # works fine

a_generator = (i for i in range(5))
a_generator[0]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'generator' object is not subscriptable