Iteration (Cambridge (CIE) A Level Computer Science): Revision Note

What is iteration?

• Iteration is repeating a line or a block of code using a loop

• Iteration can be:

  • Count-controlled

  • Condition-controlled

Count-controlled loops

What is a count-controlled loop?

• A count-controlled loop is when the code is repeated a fixed number of times (e.g. using a FOR loop)

• A count-controlled loop can be written as:

FOR <identifier> ← <value1> TO <value2>
  <statements(s)>
NEXT <identifier>

FOR <identifier> ← <value1> TO <value2> STEP <increment>
  <statements(s)>
NEXT <identifier>

• In a FOR loop, the increment must be an expression that evaluates to an integer

  • The loop starts at value1

  • The identifier is updated by the increment value on each iteration

  • The loop continues until the identifier passes value2

• The increment can be positive or negative, depending on the direction you want the loop to count

Nested count-controlled loops

FOR i ← 1 TO 3
    FOR j ← 1 TO 3
        OUTPUT i, " x ", j, " = ", i * j
    NEXT j
NEXT i

• The outer loop runs i from 1 to 3

• For each value of i, the inner loop runs j from 1 to 3

• It outputs all combinations of i × j

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
2 x 1 = 2
...
3 x 3 = 9

Count-controlled loops in different languages

for i in range(1, 6):
    print(i)

For i As Integer = 1 To 5
    Console.WriteLine(i)
Next

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

Condition-controlled loops

What is a condition controlled loop?

• A condition controlled loop is when the code is repeated until a condition is met

• There are two types of condition controlled loops:

  • Post-condition (REPEAT)

  • Pre-condition (WHILE)

Post-condition loops (REPEAT)

• A post-condition loop is executed at least once

• The condition must be an expression that evaluates to a Boolean (True/False)

• The condition is tested after the statements are executed and only stops once the condition is evaluated to True

• It can be written as:

REPEAT
  <statement(s)>
UNTIL <condition>

Pre-condition loops (WHILE)

• The condition must be an expression that evaluates to a Boolean (True/False)

• The condition is tested and statements are only executed if the condition evaluates to True

• After statements have been executed the condition is tested again

• The loop ends when the condition evaluates to False

• It can be written as:

WHILE <condition> DO
  <statement(s)>
ENDWHILE

Nested condition-controlled loops

DECLARE Username : STRING
DECLARE Count : INTEGER

Count ← 0

WHILE Count < 3
    Username ← ""
    
    WHILE Username = ""
        OUTPUT "Enter a username: "
        INPUT Username

        IF Username = "" THEN
            OUTPUT "Username cannot be blank."
        ENDIF
    ENDWHILE

    OUTPUT "Username accepted: ", Username
    Count ← Count + 1
ENDWHILE

• Ask the user to enter up to 3 usernames

• For each one, keep asking until a non-empty name is entered

• Outer WHILE runs 3 times (for 3 usernames)

• Inner WHILE ensures that each username is not blank

Condition-controlled loops in different languages

password = ""

while password != "admin":
    password = input("Enter the password: ")
    
    if password != "admin":
        print("Incorrect, try again.")

print("Access granted!")

Module PasswordCheck
    Sub Main()
        Dim password As String = ""

        While password <> "admin"
            Console.Write("Enter the password: ")
            password = Console.ReadLine()

            If password <> "admin" Then
                Console.WriteLine("Incorrect, try again.")
            End If
        End While

        Console.WriteLine("Access granted!")
        Console.ReadLine() ' Keeps the console open
    End Sub
End Module

import java.util.Scanner;

public class PasswordCheck {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String password = "";

        while (!password.equals("admin")) {
            System.out.print("Enter the password: ");
            password = scanner.nextLine();

            if (!password.equals("admin")) {
                System.out.println("Incorrect, try again.");
            }
        }

        System.out.println("Access granted!");
        scanner.close();
    }
}

When to use each type of loop