Selection (Cambridge International IGCSE Computer Science)

What is Selection?

• Selection is when the flow of a program is changed, depending on a set of conditions

• The outcome of this condition will then determine which lines or block of code is run next

• Selection is used for validation, calculation and making sense of a user's choices

• There are two ways to write selection statements:

  • if... then... else...
  • case...

If Statements

Spec point — "(b) Understand and use the concept of selection 5 Understand and use nested statements"

If Statements

What is an If statement?

• As If statements allow you to execute a set of instructions if a condition is true
• They have the following syntax: IF THEN ENDIF
• A detailed look at a Python IF statement:

Example

Nested Selection

Spec point — "(b) Understand and use the concept of selection 5 Understand and use nested statements"

Nested Selection

What is nested selection?

• Nested selection is a selection statement within a selection statement, e.g. an If inside of another If
• Nested means to be 'stored inside the other'

Example

Case Statements

Spec point — "(b) Understand and use the concept of selection 5 Understand and use nested statements"

Case Statements

What is a case statement?

• A case statement can mean less code but it only useful when comparing multiple values of the same variable
• **If **statements are more flexible and are generally used more in languages such as Python
• The format of a CASE statement is: CASE OF <value 1> : ** <value 2>: ** .... OTHERWISE ENDCASE

Exam Hint

• Make sure to include all necessary components in the selection statement:

  • the condition,
  • the statement(s) to execute when the condition is true
  • any optional statements to execute when the condition is false

• Use proper indentation to make the code easier to read and understand

• Be careful with the syntax of the programming language being used, as it may differ slightly between languages

• Make sure to test the selection statement with various input values to ensure that it works as expected

Worked Example Write an algorithm using pseudocode that:

• Inputs 3 numbers
• Outputs the largest of the three numbers

[3]

Exemplar answer

Pseudocode

Python