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Selection (Cambridge (CIE) O Level Computer Science)

Revision Note

Robert Hampton

Written by: Robert Hampton

Reviewed by: James Woodhouse

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

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 <condition>
    THEN
    <statement>
    ENDIF

  • A detailed look at a Python IF statement:

if-else-if-else-statement-pseudocode-computer-science-revision-notes

Example

Concept

Pseudocode

Python

IF-THEN-ELSE

IF Answer ← "Yes"
  THEN
      OUTPUT "Correct"
  ELSE
      OUTPUT "Wrong"
ENDIF
if answer == "Yes":
   print("Correct")
elif answer == "No":
   print("Wrong")
else:
   print("Error")

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

Pseudocode

IF Player2Score > Player1Score 
    THEN 
         IF Player2Score > HighScore 
             THEN 
                  OUTPUT Player2, " is champion and highest scorer" 
             ELSE 
                  OUTPUT Player2, " is the new champion" 
         ENDIF 
    ELSE 
         OUTPUT Player1, " is still the champion" 
         IF Player1Score > HighScore 
             THEN 
                  OUTPUT Player1, " is also the highest scorer" 
         ENDIF 
ENDIF

Python

# Prompt the user to enter a number
print("Enter a number: ")
test_score = int(input())

# Outer statement to check if the test_score is above 40
if test_score > 40:
    # Inner statement to assign the result from the test
    if test_score > 70:
        result = "Distinction"
    elif test_score > 55:
        result = "Merit"
    else:
        result = "Pass" 
else:
    result = "Fail"

# Output the result
print(result)

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 <identifier>
    <value 1> : <statement>
    <value 2>: <statement>
    ....
    OTHERWISE <statement>
    ENDCASE

Concept

Pseudocode

Python

CASE

INPUT Move

CASE OF Move
    'W' : Position ← Position - 10
    'E' : Position ← Position + 10
    'A' : Position ← Position - 1
    'D' : Position ← Position + 1
    OTHERWISE 
        OUTPUT "Beep"
ENDCASE
day = "Sat"
match day:
    case "Sat":
        print("Saturday")
    case "Sun":
        print("Sunday")
    case _:
        print("Weekday")

Examiner Tips and Tricks

  • 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

INPUT A
INPUT B
INPUT C

IF A > B 
    THEN 
        IF A > C
            THEN
                OUTPUT A
            ELSE
                OUTPUT C
        ENDIF
    ELSE
        OUTPUT B
ENDIF

Python

if A > B and A > C:
    print(A)
elif B > C:
    print(B)
else:
    print(C)

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    Author
    Reviewer
    Robert Hampton

    Author: Robert Hampton

    Expertise: Computer Science Content Creator

    Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.

    James Woodhouse

    Author: James Woodhouse

    Expertise: Computer Science

    James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.