Selection (OCR A Level Computer Science)

Selection

What is Selection?

• Selection is used within the branching (selection) programming construct 

• They are used to test conditions and the outcome of this condition will then determine which lines or block of code is run next

• There are two ways to write selection statements:

  • if... elseif... else... 

  • switch... case... 

Writing a condition

• Think of it like writing a yes/no question

• For example

  • Is the number bigger than 10?

  • Is the number between 50 and 100?

  • Is the answer Paris?

• If the question can’t be answered with yes/no then it needs to be rewritten in this way

Using more than one operator

• Using one operator is most common but sometimes two are needed. More than two operations can be used but it gets more complicated as it involves using Boolean operators

• Imagine a program where the user has to enter a number based on the role of a dice. The number needs to be between 1 and 6

• The yes/no question could be is the number between 1 and 6 - but it would be structured slightly differently in the code

• if number >=1 and number <=6 then

• The first check is if the number is greater than or equal to 1

• The second check is if the number is less than or equal to 6

• The final check is if both sides are true

If Statements

• An if elseif else statement will let you choose a line/lines of code to run if a condition is true or false 

• Below are three ways to use if statements:

1. if...

2. if... else...

3. if... elseif... else...

Syntax of an if... statement

The syntax of an if the statement consists of the if keyword, followed by a condition, and a code block that is executed if the condition evaluates to true.

if condition then
  // Code to be executed if the condition is true
endif

Pseudocode example

if statement example pseudocode

Python example

number = 5
if number > 0:
    print('The number is positive.')

• In this example, the if statement checks if the value of the variable number is greater than 0. If the condition is true, the message 'The number is positive.' is output

Java example

public class Main {
    public static void main(String[] args) {
        int number = 5;
        if (number > 0) {
            System.out.println("The number is positive.");
        }
    }

}

Syntax of an if... else... statement

The if statement can be extended with an else clause to specify an alternative block of code that is executed when the condition evaluates to false.

if condition then
  // Code to be executed if the condition is true
else
// Code to be executed if the condition is false
endif

Pseudocode example

if else statement example pseudocode

Python example

number = -3
if number > 0:
    print('The number is positive.')
else:
    print('The number is not positive.')

In this example, if the value of number is greater than 0, the message 'The number is positive.' is output. Otherwise, the message 'The number is not positive.' is output.

Java example

public class Main {
    public static void main(String[] args) {
        int number = -3;
        if (number > 0) {
            System.out.println("The number is positive.");
        } else {
            System.out.println("The number is not positive.");
        }
    }
}

Syntax of an if... elseif... else... statement

The elseif clause specifies additional conditions to check if the initial if condition is false. This allows the handling of multiple scenarios in a more complex decision-making process:

if condition then
   // Code to be executed if the condition is true
elseif condition then
   // Code to be executed if the condition is true
else
   // Code to be executed if all conditions are false
endif

Pseudocode example

if elseif else statement example pseudocode

Python example

score = 85
if score >= 90:
    print('Excellent!')
elif score >= 80:
    print('Good.')
elif score >= 70:
    print('Fair.')
else:
    print('Needs improvement.')

In this example, the if elseif else statement evaluates the value of score to determine the corresponding message based on the score range.

Java example

public class Main {
    public static void main(String[] args) {
        int score = 85;
        if (score >= 90) {
            System.out.println("Excellent!");
        } else if (score >= 80) {
            System.out.println("Good.");
        } else if (score >= 70) {
            System.out.println("Fair.");
        } else {
            System.out.println("Needs improvement.");
        }
    }
}

Case Statements

• switch/case is a type of conditional statement that provides an alternative way to perform multiple comparisons based on the value of an expression

• These statements are particularly useful when you have a single expression that you want to compare against multiple possible values

Syntax of a switch case statement

• The syntax of a switch case statement consists of the switch keyword followed by an expression

• This expression is evaluated, and its value is then compared against various case labels. If a match is found, the corresponding block of code is executed

• The default keyword is optional and specifies a block of code to be executed if none of the case labels match the expression

switch expression
  case value1:
    // Code to be executed if the expression matches value1
  case value2:
    // Code to be executed if the expression matches value2
  // more case statements if needed
  default:
    // Code to be executed if no case matches the expression
endswitch

Python example

grade='b'

match (grade):

      case 'a':

         print("Excellent")

      case 'b':

         print("Good")

      case 'c':

         print("Fair")

      case_:

         print("Needs improvement")

Java example

public class Main {
    public static void main(String[] args) {
        char grade = 'b';
        switch (grade) {
            case 'a':
                System.out.println("excellent");
                break;
            case 'b':
                System.out.println("good");
                break;
            case 'c':
                System.out.println("fair");
                break;
            default:
                System.out.println("needs improvement");
        }
    }
}