Pseudocode Core Structures (Cambridge Standard): FOR Loops: Start, End, Step And Boundary Control (Copy)

FOR Loops: Start, End, Step And Boundary Control (Cambridge Standard – O Level 2210 + IGCSE 0478)

---

Purpose Of FOR Loops In Cambridge Pseudocode

• FOR loops are used when:
  • The number of repetitions is known in advance
• Cambridge uses FOR loops to test:
  • Controlled iteration
  • Boundary accuracy
  • Trace precision
• FOR loops are heavily examined in:
  • Array processing
  • Counting records
  • Fixed-range calculations

---

Core FOR Loop Structure (Cambridge Standard)

Basic Structure

• FOR counter ← start TO end
  • statements
• ENDFOR

---

Meaning Of Each Part

• counter
  • Loop control variable
  • Usually INTEGER
• start
  • Initial value assigned to counter
• end
  • Final value at which loop stops
• statements
  • Code executed once per iteration

---

Execution Rule

• The loop:
  • Starts at start
  • Ends when counter reaches end
  • Includes both start and end values

---

Inclusive Nature Of FOR Loops (Very Important)

• Cambridge FOR loops are inclusive
• Example:
  • FOR i ← 1 TO 5
• Loop runs:
  • 5 times (i = 1, 2, 3, 4, 5)

---

Common Examiner Trap

• Treating end value as exclusive
• This causes:
  • One extra iteration
  • Or one missing iteration
• Both result in:
  • Wrong trace tables
  • Incorrect totals

---

FOR Loop With STEP (Increment Control)

Structure With STEP

• FOR counter ← start TO end STEP stepValue
  • statements
• ENDFOR

---

Meaning Of STEP

• STEP controls:
  • How much the counter changes each iteration
• Default STEP:
  • +1 (if STEP not specified)

---

Examples

• FOR i ← 1 TO 10 STEP 1
  • Executes 10 times
• FOR i ← 2 TO 10 STEP 2
  • Values: 2, 4, 6, 8, 10
• FOR i ← 10 TO 1 STEP -1
  • Values: 10, 9, 8, …, 1

---

Examiner Expectations For STEP

• STEP must:
  • Match the logical requirement
  • Be consistent with start and end
• Wrong STEP value:
  • Produces incorrect number of iterations

---

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

---

FOR Loops And Array Traversal

Standard Array Processing Pattern

• FOR i ← 1 TO arraySize
  • PROCESS array[i]
• ENDFOR

---

Examiner Checks

• Loop bounds must:
  • Match array declaration
• Example:
  • DECLARE score[1:30]
  • FOR i ← 1 TO 30

---

Common Array Boundary Errors

• Loop exceeding array size
• Loop starting at 0 when array starts at 1
• Using wrong loop variable for indexing

---

FOR Loops And Accumulators

Accumulator Pattern

• total ← 0
• FOR i ← 1 TO n
  • total ← total + value[i]
• ENDFOR

---

Key Rules

• Accumulator must:
  • Be initialised before the loop
• Accumulator update:
  • Happens inside the loop

---

Examiner Trap

• Initialising total inside the loop
• This resets the accumulator every iteration

---

FOR Loops And Counters

Counting Pattern

• count ← 0
• FOR i ← 1 TO n
  • IF condition THEN
    • count ← count + 1
  • ENDIF
• ENDFOR

---

Difference Between Counter And Loop Variable

• Loop variable (i):
  • Controls repetition
• Counter (count):
  • Tracks events

---

FOR Loops And Conditional Logic

• FOR loops often contain:
  • IF statements
• This allows:
  • Selective processing

Example:

• FOR i ← 1 TO 20
  • IF mark[i] >= 50 THEN
    • passCount ← passCount + 1
  • ENDIF
• ENDFOR

---

Examiner Expectation

• Conditions inside loops:
  • Must not alter loop control
• Never change:
  • Loop counter inside loop body

---

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

---

Boundary Control In FOR Loops (High-Risk Area)

What Boundary Control Means

• Correctly setting:
  • Start value
  • End value
  • STEP
• Ensuring:
  • No extra iterations
  • No missing iterations

---

Boundary Control Checklist

• Start value matches:
  • First valid data item
• End value matches:
  • Last valid data item
• STEP direction matches:
  • Increasing or decreasing logic

---

Example Boundary Analysis

• “Process up to 30 records”
  • Correct:
    • FOR i ← 1 TO 30
• “Process first 10 records only”
  • Correct:
    • FOR i ← 1 TO 10
• “Process every second record”
  • Correct:
    • STEP 2

---

Boundary Trap: “Up To”

• “Up to N” means:
  • Maximum N
• Do NOT assume:
  • Exactly N
• Loop must reflect:
  • Current count if used

---

FOR Loops And Pre-Release Constraints

• FOR loops must respect:
  • Maximum limits
  • Data size constraints
• Ignoring constraints:
  • Loses marks even if loop logic is correct

---

FOR Loops And Trace Tables

Why FOR Loops Dominate Trace Questions

• Each iteration:
  • Produces predictable variable changes
• Trace tables often track:
  • Loop variable
  • Accumulators
  • Counters

---

Trace Table Setup Rule

• Include columns for:
  • Loop counter
  • Variables updated inside loop
• Exclude:
  • Unchanged variables

---

Common Trace Errors With FOR Loops

• Forgetting inclusive end value
• Misapplying STEP
• Updating wrong variable
• Skipping first or last iteration

---

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

---

FOR Loops Vs WHILE Loops (Examiner Comparison)

FOR Loop	WHILE Loop
Known iterations	Unknown iterations
Fixed boundaries	Condition-based
Cleaner traces	More complex traces
Preferred when possible	Used when necessary

• Cambridge prefers:
  • FOR loops when iteration count is fixed

---

FOR Loops In Section B Modifications

• Section B often asks:
  • Restrict loop
  • Extend loop
  • Stop early

---

Correct Modification Approach

• Keep:
  • FOR structure
• Add:
  • IF condition
  • Counter limit
• Do NOT:
  • Replace FOR with WHILE unless asked

---

Example Modification

Original:

• FOR i ← 1 TO 30
  • PROCESS
• ENDFOR

Modified:

• count ← 0
• FOR i ← 1 TO 30
  • IF valid THEN
    • PROCESS
    • count ← count + 1
  • ENDIF
  • IF count = 10 THEN
    • EXIT LOOP
  • ENDIF
• ENDFOR

---

Illegal FOR Loop Practices (Guaranteed Mark Loss)

• Changing loop counter inside loop
• Using non-integer loop counter
• Loop bounds not matching data
• Missing ENDFOR
• Ambiguous start/end values

---

Best-Practice FOR Loop Strategy For Paper 2

• Use FOR loops when:
  • Count is known
• Always:
  • Match array size
  • Initialise accumulators before loop
  • Keep loop body simple
• Use STEP only when required
• Double-check boundaries before moving on

---

Final Quality Checklist

• Start value correct
• End value correct
• STEP correct
• Loop variable not modified inside loop
• Accumulators initialised outside loop
• Loop matches pre-release constraints

---

Final Lock-In Rules

• FOR loops are inclusive
• Boundaries decide correctness
• STEP controls iteration flow
• Boundary errors cost easy marks
• Clean FOR loop logic = high Paper 2 accuracy

---

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