Conditional loops

Explain what is wrong with each program

for x = 0 to 5:
  print(x)

for x = 5 to 3:
  print(x)

What a loop is in computer science
How to implement a conditional loop in Python

Be able to explain benefit of loops in computer programming
Be able to develop a conditional loop in Python

Conditional loops

There are two kinds of loops: fixed loops and conditional loops
We've already looked at fixed loops and we've also already looked at conditions.

A conditional loop

A condition will always evaluate to True or False.
Conditional loops use a condition to loop whilst a condition evaluates to true.
Some conditional loops never evaluate to True and therefore are considered to be infinite loops.
Python has several types of conditional loops, but the one we are interested in is the while loop.

For each of the following, state whether it is true or false:

Task

x = True
y = False

print(x and y)

x = 54
y = 24

print(x > y)

x = 54
y = 24

print(x < y or y > 5)

x = True
y = True

print(not(x or y))

A while loop

While loops will continue to loop while a condition is True.
If the condition is initially false, then the loop will never run.

while True:
  print("This loop is never gonna end!")

n = 0
while n < 100:
  print(n)
  n = n + 1

While loops

counter = 0
while counter != 10:
  print ("Hello")

counter = counter + 1

A login system

print("Please insert your username")
username = input()
print("Please insert your password")
password = input()

if username == "Hamish" and password == "pass":
  print("Welcome!")
else:
  print("Login incorrect")

A login system - using loops

print("Please insert your username")
username = input()
print("Please insert your password")
password = input()
attempts = 0

while username != "Hamish" and password != "pass":
  print("Login incorrect")
  print("Please insert your username")
  username = input()
  print("Please insert your password")
  password = input()
  attempts = attempts + 1

print("Welcome", "It took you", attempts, "to login")

Conditional Loops

Functional requirements

Pseducode for a conditional loop is shown below. Write the Python code based on this pseudocode:

SET number TO 0
SET total TO 0
WHILE total <= 100:
ASK user to enter number
Total = Total + number
DISPLAY Total
END WHILE
DISPLAY Total

Complete the Conditional Loops worksheet

Task

Input validation

Input validation is a means of checking what a user has typed in is valid.
For example, if a program requires a number between 1 and 100 and the user typed 830, the data entered would be invalid.

Validation icons created by Vectors Tank - Flaticon

Input validation in Python

The steps to validate input are:
- Get the input
- Loop while the input is not valid
  - Display an error message saying that the input is invalid
  - Ask the user to re-enter the value

print("Please insert a quantity")
quantity = int(input())

while quantity < 1 or quantity > 10:
  print("You must order at least one and fewer than ten items!")

print("You ordered " + str(quantity))

Presentation Overview

Conditional loops

23:58 | 18-01-2026

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