Coding Notes

2.3 While Loop

6 min readAndreas PatuzziAuroville Institute of Applied Technology

A while loop is used to repeat a block of code as long as a specified condition is truthy.

Summary

  • Basic Usage: Similar to an if condition but runs repeatedly. The condition value should eventually change to avoid infinite loops.
  • Break: Exits the loop immediately, regardless of the condition.
  • Continue: Skips the rest of the current iteration, goes back to the top of the loop, and checks the condition again.
  • Else: Runs only if the loop ends normally (not because of break).

Basic

A while loop is similar to a if statement and also has a boolean condition. But instead of executing the code block inside once, it is executed repeatedly as long as the condition is truthy.

while condition:
    # Code to execute while the condition is True

If the condition stays truthy, and there is no other reason for exiting, the loop will run infinitely. So to control the while loop, the condition has to change over time.

count = 0
while count < 5:
    print(f"Count is {count}")
    count += 1
Count is 0
Count is 1
Count is 2
Count is 3
Count is 4

You could explain what is happening similar to this. Create a new variable called “count” of type integer and with the value 0. The condition of the while loop is “count < 5”. Run code inside while because “0 < 5” is True. Inside the loop we output the current value of count, in this case “Count is 0”. Then we increase the value of count by one, in this case to 1. Again we check the condition and go inside the loop, because “1 < 5”. We print the message “Count is 1” on a new line and increase count by one. This will be repeated until count has a value of 5. So the last printed message is “Count is 4”, then we increase count to 5. Then we stop repeating, because 5 < 5 is False, and go after the loop.

total = 0
number = 1
while number <= 10:
	total += number
	print(f"current number: {number} running sum: {total}")
	number += 1
print(f"The sum of all numbers from 1 to 10 is {total}")
current number: 1 running sum: 1
current number: 2 running sum: 3
current number: 3 running sum: 6
current number: 4 running sum: 10
current number: 5 running sum: 15
current number: 6 running sum: 21
current number: 7 running sum: 28
current number: 8 running sum: 36
current number: 9 running sum: 45
current number: 10 running sum: 55
The sum of all numbers from 1 to 10 is 55

Break

The keyword break is used to immediately exit the loop, independent from the condition. If you are inside a nested loop, it will only exit the inner loop.

count = 0
while True:
	if count >= 5:
		break
	print(f"Count is {count}")
	count += 1

This might seem like an infinite loop at first because the condition is always True. But actually, the break statement will exit the loop as soon as the count reaches 5 and because of that, the output will be exactly the same as in the example above.

Count is 0
Count is 1
Count is 2
Count is 3
Count is 4

You should usually handle the main behavior of the loop with the condition and not overuse break.

Continue

The keyword continue is used to skip forward to the next iteration of the loop. So it immediately goes back to the top and checks the condition again. If you are inside a nested loop, it will only skip ahead the inner loop.

count = 1
i = 1
print("start while loop")
while (count < 5) and (i < 10):
    i += 1
    if count % 3 == 0:
        print(f"skipping {count}")
        continue
    print(f"processing {count}")
    count += 1
print(f"end with count={count} and i={i}")

In this example, continue will also skip the count+1. So without the checking if i<10, this would be an infinite loop. For each output, you can see the lines that will be executed in this iteration. So you can see that as soon as count reaches 3 it goes into the if then runs continue, and because of that goes back to the top without incrementing count.

start while loop               # lines: 1, 2, 3
processing 1                   # lines: 4, 5, 6, 9, 10
processing 2                   # lines: 4, 5, 6, 9, 10
skipping 3                     # lines: 4, 5, 6, 7, 8
skipping 3                     # lines: 4, 5, 6, 7, 8
skipping 3                     # lines: 4, 5, 6, 7, 8
skipping 3                     # lines: 4, 5, 6, 7, 8
skipping 3                     # lines: 4, 5, 6, 7, 8
skipping 3                     # lines: 4, 5, 6, 7, 8
skipping 3                     # lines: 4, 5, 6, 7, 8
end with count=3 and i=10      # lines: 4, 10

Else

The keyword else can be used with a while loop. The code inside will only execute if the loop ends normally after all the iterations. If the loop stops because of a break, the code inside else will be ignored.

num = int(input("Please input a integer number: "))
i = 2
 
while i < num:
    if num % i == 0:
        print(f"{num} is not a prime number because it is divisible by {i}")
        break
    i += 1
else:
    print(f"{num} is a prime number")
 
input 5:        5 is a prime number
input 15:       15 is not a prime number because it is divisible by 3
input 29:       29 is a prime number

Questions

  • Explain while in your own words.
  • What differences and similarities between an if statement and a while loop?
  • Explain break in your own words.
  • Explain continue in your own words.
  • What is an infinite loop? How can an infinite loop happen?
  • Explain the following code and its output.
count = 5
while count > -3:
    print(count)
    count -= 1
  • Explain else in combination with a while loop in your own words.
  • How can a while loop be used to sum all even numbers from 1 to 100? Explain and write the code.

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