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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))
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