Conditional loops with while

Another control flow statement is the conditional loop with while. While loops combine the properties for and if. The loop is executed as long as the conditional expression at the beginning holds. The conditional expressions work in exactly the same way as in if statements.

Counting until a certain value

A simple usage of while is to count until an exit condition is met. The following loop calculates the sum of all numbers from 1 through 10:

i = 0
total = 0
while i < 10:
    print(i)
    i = i + 1
    total = total + i

Searching through data

With a while loop you can perform search operations - although many times the methods on lists and dictionaries will give you a shortcut.

The following loop finds the first name starting with an 'E':

data = ['Alice', 'Bob', 'Charlie', 'Emily', 'Fred']
i = 0
while i < len(data) and not data[i].startswith('E'):
    i += 1
print(i)

Waiting for user input

A while loop is also useful to let a user stop the program:

number = 0
while input('press [Enter] to continue or [x] to exit') != 'x':
    number = number +1
    print(number)

Endless loops

With while it is possible to build loops that never stop. Most of the time this happens by accident. In the following loop, the instruction to decrease a is missing. It runs endlessly:

a = 10
b = 1
while a > 0:
    b = 1 - b
    print(b)