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While Loops

A while loop is similar to a for loop in that it is a tool used to automate a repetive task. The core difference between while and for loops is as follows:

  • A for loop executes a predetermined number of times. A for loop that begins with "for i in range(a,b)" will execute exactly once for each value of the counter i that falls within the given range.

  • A while loop executes as long as some supplied condition is true. Depending on the nature of this condition, we do not necessarily know ahead of time how many times the loop will execute.

The syntax for creating a while loop is as follow:

while condition:
    code to be executed each iteration

The condition in a while statement should be an expression that evaluates to a Boolean value. When Python encounters a while loop, it will check to see if the condition evaluates to True. If it does, then it will step into the loop and perform the first iteration. Each time the loop executes, Python returns to the start of the loop and checks the condition again. As long as the condition evaluates to True, Python will continue executing iterations of the loop. If the condition ever evaluates to False, then Python will stop executing the body of the loop, and will move on to the next lines of code.

The following cell illustrates a simple example of a while loop. The cell prints out the squares of the first five positive integers.

n = 1
while n <= 5:
    print(n**2)
    n += 1

1
4
9
16
25

The task above could have been accomplished more succinctly using a for loop:

for n in range(1,6):
    print(n**2)

1
4
9
16
25

Mention Break and Continue

break and continue Comparing while and for Loops