A Complete Guide to While loops in Python

In python, we have generally two-loop commands

In this Python article, we will discuss the while loop with some examples.

1. What are while loops in Python?

while loop executes a statement block again and again until the given condition is true or satisfied. But if the condition is encountered to be false, the loop will not be executed any further.

while loop is a type of indefinite iteration, it means that how many times the loop will be executed is not specified explicitly before the loop begins.

If there are lines below the while loop, they will be executed after the program has dealt with the full execution or condition check of the while loop.

Syntax:
while condition/expression :
    statement

When we execute a while loop, the condition is computed in boolean form. If TRUE, the statements inside the loop will be executed.

The condition is checked repeatedly and executed again and again until the condition has become FALSE.

While-loop-flow-diagram
While-loop-flow-diagram

Lets take an example to understand the working of while loop

# An example for while loop
check = 10
while(check > 0):
  check = check - 2
  print(check, end=', ')

print()

# Illustration with list
list = [5, 6, 7, 8, 9]
while list:
  print(list.pop(), end=', ')
Output
8, 6, 4, 2, 0
5, 6, 7, 8, 9

2. Single line while loop

We can write the while loop in a single line, for multiple statements in the block to make the loop body, we can use the semicolon(;) to separate them.

Note: We can write only simple statements in a single line while loop. We cannot combine if condition with a single while loop.

check = 12
while(check > 5): check = check - 2; print(check, end=', ')
Output
10, 8, 6, 4

Let’s see how we can access else after while loop.


3. While loop with else clause

As we now know, when the condition of the while loop becomes FALSE, the loop stops execution. We can write else clause which will be executed when the condition becomes FALSE.

Note: Else block will not work if we use ‘break‘ in the loop or when an exception arises

# while-else loop
check = 0
while check < 10:
  check += 3
  print(check, end=', ')
else:  # This block will be executed since there is no break
  print("\nRunning else block without break")

check = 0
while check < 10:
  check += 2
  print(check, end=', ')
  break
else:  # This block will not be executed since there is break
  print("Running else  block")
Output
3, 6, 9, 12, 
Running else block without break
2

4. Break and Continue in While Loop

As till now, we saw the while loop working on each iteration but we can also terminate the loop iteration with the help of some loop control statements – break and continue.

  • Break statement in Python, which terminates the entire loop immediately
  • continue statement is Python, which terminates a current iteration in the loop

Lets take an example to understand the Python break statement

# while-break statement
count = 3
while count >= 0:
  if count == 1:
    print("Start")
    break
  print(count)
  count = count-1
Output
3
2
Start

Here, when count will be 1, the break statement will be executed and the loop will terminate.

Now let’s take an example to understand the Python continue statement

# while-continue statement
count = 60
while count > 10:
  count -= 10
  if count == 30:
    continue
  print(count)
Output
50
40
20

Here, when the count will be 30, the continue will terminate the current iteration and it will not print 30 and it will move to the next iteration and print 20
To know more about the break and continue statements, we have a post on break and continue in python with a detailed explanation.


5. Nested While Loop

In python, we can use nested while loop which means we can write a while loop inside the body of another while loop.

Here’s an code example to understand how it works.

# nested while loop
list1 = ['fruit', 'colour']
while len(list1):
  print(list1.pop(0))
  list2 = ['orange']
  while len(list2):
    print('--', list2.pop(0))
Output
fruit
--orange
colour
--orange

In addition to the above, we can also nest if/elif/else statements inside the while loop and vice versa. Let’s have a look at the syntax.

While loop inside if-else

if condition_1:
  while condition1:
    statements
  statements
elif condition_2:
  while condition2:
    statements
  statements
if-else blocks inside while loop
while condition:
  if condition_1:
    statements
  elif condition_2:
    statements
  statements

6. Conclusion

In this article we learned about

  • while loop
  • while loop with else block
  • Single line while loop and usage of break and continue
  • Nested while loop

Complete code samples are present on Github project.

An investment in knowledge always pays the best interest. I hope you like the tutorial. Do come back for more because learning paves way for a better understanding

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Happy coding!! 😊

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