Iteration (OCR AS Computer Science): Revision Note

Iteration

What is iteration?

• Iteration is the process of doing something more than once (repeat), otherwise known as a loop

• A loop can be count controlled which means the code is repeated a fixed number of times

• A loop can also be condition controlled which means the code is repeated until a condition is met

• Three common loops are:

  • for loops (count controlled)

  • while loops (condition controlled)

  • do while loops (condition controlled)

For Loops

• A for loop is a count controlled loop that will repeat a fixed number of times. It provides a concise and structured way to perform repetitive tasks.

Syntax of a for loop

The syntax of a for loop consists of three main parts:

01 for i = x to y
02  // Code to be executed in each iteration
03 next i

1. Initialisation: The initialisation is executed only once at the beginning of the loop. It is used to initialise a counter variable that controls the loop's execution which is i in this example

2. Range: The range that the count variable will increment through

3. Increment/Decrement: The default is to increment by 1 each time unless specified

Pseudocode example

for loop example pseudocode

Python example

01 for i in range(0,6):
02    print i

In Python, the range specifies the numbers used in the loop. The final number (6 in this case) is 1 higher than the number we want to run the loop with.

Java example

01 for (int i = 0; i < 6; i++) {
02     System.out.println(i);
03 }

Iterating over an array

Pseudocode example

for loop iterating over an array and outputting each item

Python example

01 fruits = ["apple", "banana", "orange"]
02 for i in range(len(fruits)):
03     print(fruits[i])

Java example

01 String[] fruits = {"apple", "banana", "orange"};
02 for (int i = 0; i < fruits.length; i++) {
03     System.out.println(fruits[i]);
04 }

While Loops

• A while loop is a condition controlled loop that will repeat until a condition is met 

• While loops provide a flexible and powerful way to handle situations where the number of iterations is unknown in advance

Syntax of a while loop

• The syntax of a while loop consists of a condition and a code block:

01 while condition
02   // Code to be executed as long as the condition is true
03 endwhile

• The condition is evaluated before each iteration. If the condition evaluates to true, the code block is executed

• If the condition evaluates to false, the loop terminates.

Pseudocode example

while loop example pseudocode

Python example

01 i = 0
02 while i <= 5:
03     print(i)
04     i = i + 1

Java example

01 int i = 0;
02 while (i <= 5) {
03     System.out.println(i);
04     i = i + 1;
05 }

Checking if the password is 'secret'

Pseudocode example

while loop checking if the password is correct

Python example

01 password = ""
02 while password != "secret":
03     password = input("What is the password? ")

Java example

01 import java.util.Scanner;
02 public class Main {
03     public static void main(String[] args) {
04         Scanner scanner = new Scanner(System.in);
05         String password = "";
06         while (!password.equals("secret")) {
07             System.out.print("What is the password? ");
08             password = scanner.nextLine();
09         }
10         scanner.close();
11     }
12 }

Do While Loops

• A do while loop is another example of a condition controlled loop that will repeat until a condition is met. 

• Do while loops provide a variation of the while loop with a slightly different behaviour

• The code within a do while loop will always run at least once, unlike a while loop which may not run at all if the condition is already met

Syntax of a do while loop

• The syntax of a do while loop consists of a code block and a condition:

01 do
02   // Code to be executed at least once
03 until condition

• Code Block: The code block is executed first before evaluating the condition

• Condition: The condition is evaluated after executing the code block. If the condition evaluates to false, the loop continues executing. If the condition evaluates to true, the loop terminates

Pseudocode example

Do while example pseudocode

Python example

• It isn't possible to use a do while loop in Python so the code would need to be written to use a while loop

01 import random

02 desired_number = 6
03 roll = 0

04 while roll != desired_number:
05     roll = random.randint(1, 6)
06     print("Rolled:", roll)

Java example

01 import java.util.Random;

02 public class Main {
03     public static void main(String[] args) {
04         Random random = new Random();
05         int desiredNumber = 6;
06         int roll;

07         do {
08             roll = random.nextInt(6) + 1;
09             System.out.println("Rolled: " + roll);
10         } while (roll != desiredNumber);
11     }
12 }