While loops

Learning outcomes¶

• Write pieces of code that run an indeterminate amount of times.

• Prevent infinite loops.

Prerequisites¶

• First steps

• Variables

• If statements

• For loops

While loop syntax¶

With for loops, we learned how to repeat an operation for a pre-determined amount of iterations. However, in some circumstances, we may want to keep repeating until certain conditions are met. The solution for this, is the while loop. Here is the generic syntax:

while condition:
    some action

• while initiates the loop.

• condition should be replaced with an expression that amounts to a Boolean. If the Boolean is True, the loop will repeat. If it ever becomes False, the loop stops.

• some action is to be replaced by whatever action you want to repeat, just like a for loop.

For example:

n_protons = 1

while n_protons < 6:
    print(f"The number of protons is {n_protons}.")
    n_protons = n_protons + 1 # add 1 to n_proton

Note how the number 6 never appears because as soon as n_protons becomes 6, the condition is not fulfilled and the while loop stops. Exchange the order of the last two lines. Check the output and ensure that it makes sense.

This example showed how small errors can accidentally cause infinite loops, which keep your computer busy until they are solved. while loops are the primary cause of infinite loops in programming, but thankfully most problems can be resolved with a safer for loop in Python.

# initiate Fibonacci sequence
fibo_old = 0 # penultimate number
fibo_new = 1 # current final number

# set ending condition
while fibo_new < 1000:
    # calculate the new number
    new_sum = fibo_new + fibo_old
    # update the penultimate number
    fibo_old = fibo_new
    # update the new final number
    fibo_new = new_sum
    print(fibo_new)

Summary¶

• The syntax of a while loop is: while condition:, and the code within the loop is on the lines immediately following it, indented using <tab>.

• while loops are liable to looping infinitely if their condition never becomes False.

• while loops can be combined with if statements and for loops by using indentation.