The while loop

The while loop in python acts a bit differently than the for. While the for operates over the items of an Iterable, the while loop acts on a boolean condition.

To be more precise, a while loop may be used when the exact number of iterations is unknown. known.

For example, looping until the user gives a valid input:

# uci_bootcamp_2021/examples/while_loops.py
... # code omitted for brevity (see source file in repository)
    # initialize valid to False.
    valid = False
    result = -1

    # loop while the user hasn't given us a suitable value.
    while not valid:
        # get the user's untrusted input.
        untrusted_input = input(prompt)
        # check if the input is numeric.
        if untrusted_input.isnumeric():
            # if its numeric, we can safely interpret it as an integer
            result = int(untrusted_input)
            # then we can check the bounds
            valid = valid_minimum <= result <= valid_maximum
        if not valid:
            print("invalid input, please try again!")
... # code omitted for brevity (see source file in repository)

This example has a decent bit of complexity so let's unpack it a bit.

    # loop while the user hasn't given us a suitable value.
    while not valid:

Here we define our while loop. We specifically loop while valid is False.

• We don't know how many times the user will give us invalid input; only that the input will be eventually valid given enough iterations.

        # get the user's untrusted input.
        untrusted_input = input(prompt)

Here we actually prompt the user for an input, using the built-in input function.

input() always returns a string. To accept an integer, the result of input needs to be cast or otherwise parsed.

Speaking of converting the input to a string, that is the next step!

Before we can safetly cast the string to an integer, we should first check that the contents of the string is actually numerical.

str.isnumeric returns a boolean, True if the string is nothing but numerical digits.

        if untrusted_input.isnumeric():
            # if its numeric, we can safely interpret it as an integer
            result = int(untrusted_input)

Now that the result is an integer, we can do the bounds check.

            # then we can check the bounds
            valid = valid_minimum <= result <= valid_maximum

Conversely, if the numerical or bounds checks fail, we should tell the user off!

        if not valid:
            print("invalid input, please try again!")

Note that the level of indentation decreased. the if not valid is at the same level of indentation as the if untrusted_input.isnumeric()!

Python has significant whitespace!