Skip to main content

Python while Loop

The while loops are used when you have a block of code which you want to repeat ** until a defined condition is no longer met**.

Syntax​

The while loops repeat a block of code ** until a defined condition is no longer met**.

note

If the condition is initially false, the loop body will not be executed at all.

The syntax of the while loop is as follows

while <condition>:
    <statement>
    <statement>
    ...

where:

  • <condition> is any expression that evaluates to a Boolean
  • All <statements> are executed once for each iteration.

For example:

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

Output

0
1
2
3

while loop with else​

A while loop can also have an optional else block. The else statements are executed only if the while loop ends normally (i.e., if the condition becomes false).

The syntax is the same as a classic while loop, but with the addition of the else branch:

while <condition>:
    <statement>
    <statement>
    ...
else:
    <else-statement>
    ...
warning

The break keyword can be used to break a while loop. In such cases, the else part is ignored.

Nested while Loops​

As with other flow-control structures, such as if and for loops, while loops can also be nested.

For example:

i = 1
while i < 4:
    j = 1
    while j < 4:
        print(f"{i} + {j} = {i+j}")
        j = j + 1
    i = i + 1

Output:

1 + 1 = 2
1 + 2 = 3
1 + 3 = 4
2 + 1 = 3
2 + 2 = 4
2 + 3 = 5
3 + 1 = 4
3 + 2 = 5
3 + 3 = 6
note

All Python control structures can be mixed together to the extent necessary (if, for, while, etc.)

Manipulate while loop execution​

break statement in while loop​

Python break statement is used to exit the loop immediately. It simply jumps out of the while loop and the program continues to execute the code after the loop.

For example, break the loop at yellow

colors = ['red', 'green', 'yellow', 'blue']
i = 0
while i < len(colors):
    i = i + 1
    if colors[i-1] == 'yellow':
        break
    print(colors[i-1])

Output

red
green

continue statement in while loop​

Python continue instruction skips the current iteration of a loop and continues with the next iteration.

For example, continue the loop at yellow

colors = ['red', 'green', 'yellow', 'blue']
i = 0
while i < len(colors):
    i = i + 1
    if colors[i-1] == 'yellow':
        continue
    print(colors[i-1])

Output

red
green
blue

pass statement in while loop​

Python pass statement is a null statement: nothing happens when it is executed.

For example, pass the loop at yellow

colors = ['red', 'green', 'yellow', 'blue']
i = 0
while i < len(colors):
    i = i + 1
    if colors[i-1] == 'yellow':
        pass
    print(colors[i-1])

Output

red
green
yellow
blue

Infinite Loops (while true)​

The while loop is repeated as long as the condition remains True.

If the while condition does not become false, the loop will execute forever. This situation creates an infinite loop (also called endless loop).

note

To terminate an infinite loop, it is necessary to use a break statement

For example, read a number from the user until a number greater than 10 is entered:

while True:
    number = float(input('Enter a number: '))
    if number > 10: break
    print(f'The number is {number}')

Output

Enter a number: 3
The number is 3.0
Enter a number: 10
The number is 10.0
Enter a number: 12

Examples​

Let's look at some examples of frequent situations when programming in python

Iterating a String​

Each string can be iterated character by character, using an auxiliary variable to hold the index and using the [ ] operator.

For example:

string = "Tutorial"
i = 0
while i < len(string): 
    print(string[i])
    i = i + 1

Output

T
u
t
o
r
i
a
l

Iterating a List​

Lists can be iterated with while loop, using an auxiliary variable to hold the index and using the [ ] operator.

For example:

collection = ['a string', 123, 'a', 1.2]
i = 0
while i < len(collection):
    print(collection[i])
    i = i + 1

Output

a string
123
a
1.2

Loop over Lists of lists​

Using nested while loops, lists of lists can be iterated, using auxiliary variables to hold the index and the [ ] operator.

For example:

list_of_lists = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]]
i = 0
while i < len(list_of_lists):
    # list_of_lists[i] returns the i-th nested-list
    j = 0
    while j <  len(list_of_lists[i]):
        # list_of_lists[i][j] returns element j-th of the i-th nested-list of list_of_lists
	    print(list_of_lists[i][j])
	    j = j + 1
    i = i + 1

Output

1
2
3
4
5
6
7
8
9

Table of Contents