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While Loops

While Loops

A while loop is a type of loop that runs as long as a logical condition is True. When the logical condition becomes False, the loop stops running. The general form of a while loop in Python is below:

while <logical_condition>:
    <code>

The keyword while must be included, as well as a <logical_condition> which can be evaluated as True or False. The <code> after the while statement must be indented. Each line of code runs in the while loop needs to be indented the same number of spaces. (Many code editors, including Jupyter notebooks, auto-indent after a while statement) If you add indentation manually, four space spaces is the Python standard.

An example of a while loop is below:

In [1]:
i = 0
while i<4:
    print(i)
    i = i+1

The first line i=0 creates the variable i and assigns it the value 0. The next line declares the logical condition needed to keep the loop running. The statement i<4 is True or False depending on the variable i. Since i=0, the statement i<4 is True and the while loop starts to run. The code inside while the loop prints the value of i then increases i by 1. When i=4, the statement i<4 is False and the while loop ends.

Using a while loop to validate user input

While loops can be used to validate user input. Say you want to insist that a user inputs positive number. You can code this into a while loop that keeps repeating 'Enter a positive number: ' until the user enters valid input.

The code below continues to ask a user for a positive number until a positive number is entered.

In [ ]:
num_input = -1
while num_input < 0:
    str_input = input('Enter a positive number: ')
    num_input = float(str_input)

In the section of code above, it is important to initialize the variable num_input with a value that causes the statement num_input < 0 to evaluate as True. num_input = -1 causes the statement num_input < 0 to evaluate as True. Besides num_input = -1, any other negative number would have worked.

If the while statement can't be evaluated as True or False, Python throws an error. Therefore, it is necessary to convert the user's input from a string to a float. The statement '5' < 0 does not evaluate to True or False, because the string '5' can't be compared to the number 0.