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Home KS3 GCSE A-Level Programming

Lesson 3. Iteration

Lesson Notes

Iteration

Iteration is the process of repeating code until a condition is met.

Definite Iteration:
When code it repeated for a set number of times.

Indefinite Iteration:
When code is repeated for an unknown number of times.

FOR Loops (Definite Iteration)

FOR loops are examples of definite iteration. They loop a set number of times and can be executed in different ways.

Iterating a range

name = input(“Enter your name: ”)
for i in range (8):
   print (name)
print (“Your name has been displayed 8 times.”)

Enter your name: Hamiela
Hamiela
Hamiela
Hamiela
Hamiela
Hamiela
Hamiela
Hamiela
Hamiela
Your name has been displayed 8 times  

Iterating through a list

fruits = ["apple", "banana", "cherry"]
for x in fruits:
   print(x)

apple
banana
cherry 

Iterating through a string

for x in “banana”:
   print(x)

b
a
n
a
n
a  

WHILE Loops (Indefinite Iteration)

While loops are examples of indefinite iteration. They repeat lines on code for an unknown number of times (until a condition is met).

choice = input(“What you you like to eat?: ”)
while choice == “I dont know”:
   print (“No!!”)
   choice = input(“What you you like to eat?: ”)
print (“Awesome, let’s go eat.”)

What do you want to eat: I dont know
No!!
What do you want to eat: I dont know
No!!
What do you want to eat: I dont know
No!!
What do you want to eat: Ice Cream
Awesome, let’s go eat.    

?

name = input(“Enter your name: ”)
while name != “Mr Ahmed”:
   print (“Invalid User!!”)
   name = input(“Enter your name: ”)
print (“Welcome Mr Ahmed”)

Enter your name: Hamiela
Invalid User!!
Enter your name: Jamie
Invalid User!!
Enter your name: Mr Vince
Invalid User!!
Enter your name: Mr Ahmed
Welcome Mr Ahmed    

?

toads = input(“Do you like toads? ‘y’ or ‘n’: ”)
while toads != “y” AND toads != “n”:
   print (“Invalid Response!!”)
   toads = input(“Do you like toads? ‘y’ or ‘n’: ”)
print (“Interesting response...”)

Enter your name: yea
Do you like toads? ‘y’ or ‘n’: 
Enter your name: yeas
Do you like toads? ‘y’ or ‘n’: 
Enter your name: yes
Do you like toads? ‘y’ or ‘n’: 
Enter your name: y
Interesting response...

Nested Loop

A nested loop is a loop inside a loop. The "inner loop" will be executed one time for each iteration of the "outer loop".

adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
   for y in fruits:
      print(x, y)

red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry