Iteration: While loops

Repeat a procedure while some condition holds

Control: Iteration with While Loops

Loops allow us to repeat the execution of code. We discuss two types of Python loops:

The for loop. For each element of this sequence, repeat this code.

for <elem> in <sequence>:
    <for body>

The while loop. While this condition is True, repeat this code.

while <boolean expression>:
    <while body>

Repeat while the condition is True.

On each iteration of the below while loop, we first check if counter is greater than zero. If it is, then we to run the loop body, which involves printing counter, then decrementing it. If it is not, we exit the loop.

counter = 10
while counter > 0:
    print(counter)
    counter -= 1
print("blast off!")

10
9
8
7
6
5
4
3
2
1
blast off!

For Loop or While Loop?

Compare the two functions below. Why might they do the same thing?

def sum_squares_for(values):
    total = 0
    for v in values:
        total += v ** 2
    return total

sum_squares_for(make_array(3, 4, 5))

50

def sum_squares_while(values):
    total = 0
    i = 0
    while i < len(values):
        total += values.item(i) ** 2
        i += 1
    return total

sum_squares_while(make_array(3, 4, 5))

50

Which do you prefer?

Every for-loop can be written as a while-loop, but not vice versa. You will see and effectively prove this claim in a future computer science class, but now is not the time.

Instead, we recommend you use the following heuristics: * If you need to check/modify/read every element in a sequence, use a for loop. for loops are often more readable. * Use while loops when you don’t know how many iterations to run in advance.