Pseudocode Core Structures (Cambridge Standard): IF, ELSE, ELSE IF And Nested Selection Logic (Copy)

IF, ELSE, ELSE IF And Nested Selection Logic (Cambridge Standard – O Level 2210 + IGCSE 0478)

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Purpose Of Selection Structures In Cambridge Pseudocode

• Selection structures are used to:
  • Make decisions based on conditions
  • Control which parts of an algorithm execute
• Cambridge Paper 2 heavily tests:
  • Correct condition logic
  • Correct placement of selection
  • Correct handling of alternatives
• Selection logic directly affects:
  • Algorithm correctness
  • Trace table accuracy
  • Section B modification success

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Core Selection Structures Recognised By Cambridge

• IF … THEN … ENDIF
• IF … THEN … ELSE … ENDIF
• IF … THEN … ELSE IF … ELSE … ENDIF
• Nested IF structures (IF inside IF)
• All are:
  • Language-independent
  • Fully acceptable in both 2210 and 0478

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Basic IF Statement (Single-Path Selection)

Structure

• IF condition THEN
  • statements
• ENDIF

Meaning

• Statements execute only if the condition is TRUE
• If condition is FALSE:
  • Nothing inside runs
  • Control moves on

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Example (Cambridge-Safe)

• IF score >= 50 THEN
  • OUTPUT “Pass”
• ENDIF
• No ELSE branch
• Used when:
  • Action is required only in one case

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Common Mistakes With Basic IF

• Forgetting ENDIF
• Writing multiple statements without indentation
• Using assignment instead of comparison in condition

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IF … ELSE Structure (Two-Path Selection)

Structure

• IF condition THEN
  • statements when TRUE
• ELSE
  • statements when FALSE
• ENDIF

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Meaning

• Exactly one branch executes
• Condition fully partitions outcomes

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Example

• IF option = 1 THEN
  • OUTPUT “Add record”
• ELSE
  • OUTPUT “Invalid option”
• ENDIF

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Examiner Expectations

• ELSE must handle:
  • All remaining cases
• ELSE branch must:
  • Be logically opposite of IF
• Do not:
  • Leave ELSE empty

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Common IF–ELSE Errors

• Writing two separate IFs instead of IF–ELSE
• Forgetting ELSE when two outcomes exist
• Outputting in both branches unintentionally

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Computer Science Full Scale Course

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ELSE IF Structure (Multiple Conditions)

Structure

• IF condition1 THEN
  • statements
• ELSE IF condition2 THEN
  • statements
• ELSE IF condition3 THEN
  • statements
• ELSE
  • statements
• ENDIF

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Meaning

• Conditions are checked top to bottom
• First TRUE condition executes
• Remaining conditions are skipped

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Example

• IF score >= 80 THEN
  • grade ← “A”
• ELSE IF score >= 60 THEN
  • grade ← “B”
• ELSE IF score >= 40 THEN
  • grade ← “C”
• ELSE
  • grade ← “F”
• ENDIF

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Ordering Rule (Very Important)

• Conditions must be ordered:
  • From most restrictive to least restrictive
• Wrong order:
  • Produces incorrect logic
  • Loses marks even if syntax looks correct

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Common ELSE IF Errors

• Overlapping conditions in wrong order
• Missing final ELSE when required
• Treating ELSE IF as a separate IF

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Nested IF Structures (IF Inside IF)

What Nested Selection Means

• An IF statement placed inside another IF or ELSE block
• Used when:
  • A decision depends on a previous decision

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Structure

• IF condition1 THEN
  • IF condition2 THEN
    • statements
  • ELSE
    • statements
  • ENDIF
• ENDIF

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Example

• IF score >= 50 THEN
  • IF attendance >= 75 THEN
    • OUTPUT “Pass”
  • ELSE
    • OUTPUT “Fail due to attendance”
  • ENDIF
• ELSE
  • OUTPUT “Fail due to marks”
• ENDIF

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When To Use Nested IF Instead Of ELSE IF

• Use nested IF when:
  • Second condition is relevant only if first condition is TRUE
• Use ELSE IF when:
  • Conditions are independent alternatives

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Examiner Trap

• Using ELSE IF when nesting is required
• This changes logic and loses marks

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Computer Science Full Scale Course

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Conditions In IF Statements (Cambridge Rules)

Allowed Condition Types

• Relational:
  • =, ≠, <, >, ≤, ≥
• Logical:
  • AND, OR, NOT
• Boolean variables:
  • IF found THEN

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Examples

• IF score >= 0 AND score <= 100 THEN
• IF NOT valid THEN
• IF found = TRUE THEN

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Best Practice

• Use brackets for clarity:
  • IF (score >= 0 AND score <= 100) THEN
• Avoid long ungrouped conditions

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Selection Logic And Operator Precedence

• Arithmetic evaluated first
• Then relational
• Then logical
• Always use brackets when mixing AND and OR

Example:

• IF (a > b AND c > d) OR (e = f) THEN

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Selection Logic And Validation

Validation Pattern

• INPUT value
• IF value < min OR value > max THEN
  • OUTPUT “Invalid”
• ELSE
  • process value
• ENDIF

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Repeated Validation Pattern

• INPUT value
• WHILE value < min OR value > max DO
  • OUTPUT “Invalid”
  • INPUT value
• ENDWHILE

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Examiner Expectation

• Validation must be:
  • Explicit
  • Traceable
• Silent rejection loses marks

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Computer Science Full Scale Course

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Selection Logic And Trace Tables

• Each IF condition:
  • Affects execution path
• Trace tables must show:
  • Which branch executed
  • Which variables changed

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Trace Safety Rules

• Do not:
  • Skip conditions
• Ensure:
  • Only one branch executes
• Nested IFs:
  • Must be traced step-by-step

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Common Selection Mistakes That Lose Marks

• Using assignment instead of comparison
• Missing ENDIF
• Wrong condition order
• Overlapping ELSE IF conditions
• Changing variable values in conditions
• Over-nesting without need

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Selection Logic In Section B Modifications

• Section B often asks:
  • Add condition
  • Restrict condition
  • Replace condition
• Use:
  • Minimal change approach
• Do not:
  • Rewrite entire selection structure

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Example Section B Modification

Original:

• IF score >= 50 THEN
  • OUTPUT “Pass”
• ELSE
  • OUTPUT “Fail”
• ENDIF

Modified:

• IF score >= 50 AND attendance >= 75 THEN
  • OUTPUT “Pass”
• ELSE
  • OUTPUT “Fail”
• ENDIF

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Computer Science Full Scale Course

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Best-Practice Selection Strategy For Paper 2

• Keep conditions simple
• Use ELSE IF for alternatives
• Use nested IF for dependent checks
• Order conditions carefully
• Always close with ENDIF
• Match logic exactly to pre-release wording

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Final Quality Checklist

• All IFs have matching ENDIF
• ELSE IF order is correct
• Nested IF used only when logically required
• Conditions are clear and bracketed
• No assignment used inside conditions
• Output occurs in correct branch only

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Final Lock-In Rules

• IF controls logic flow
• ELSE handles remaining cases
• ELSE IF handles ordered alternatives
• Nested IF handles dependent decisions
• Correct selection logic = major Paper 2 marks

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Computer Science Full Scale Course