Selection And Iteration Code Guides: Loop Tracing And Dry-Run Techniques (Copy)

Loop Tracing And Dry-Run Techniques

What Loop Tracing (Dry-Run) Means

• Loop tracing = manually simulating loop execution step-by-step
• Purpose
  • Predict final output
  • Detect logic errors
  • Avoid examiner traps in trace / predict-output questions
• Examiners expect:
  • Clear understanding of execution order
  • Correct handling of loop conditions
  • Accurate updates of control variables

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Golden Rule Of Dry-Running

• Never jump steps
• Always follow this order:
  • Check condition (if applicable)
  • Execute body
  • Update control variables
  • Re-evaluate condition

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Universal Tracing Table Method (Exam-Safe)

Step 1: Identify Control Variables

• Loop counters (i, index, count)
• Condition variables (choice, num, found)
• Accumulators (total, sum)
• Flags (found, valid)

Step 2: Create A Trace Table

• Columns typically include:
  • Iteration number
  • Loop variable(s)
  • Key variables
  • Condition result
  • Output (if any)

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Tracing FOR Loops

Execution Order (FOR Loop)

• Counter initialised
• Condition implicitly checked
• Body executes
• Counter automatically updated
• Stops after end value exceeded

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FOR Loop Trace Example 1 (Summation)

total ← 0
FOR i ← 1 TO 4
  total ← total + i
NEXT i
OUTPUT total

Iteration	i	total before	total after
1	1	0	1
2	2	1	3
3	3	3	6
4	4	6	10

• Final output: 10

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FOR Loop Trace Example 2 (STEP Value)

FOR i ← 2 TO 10 STEP 2
  OUTPUT i
NEXT i

Iteration	i	Output
1	2	2
2	4	4
3	6	6
4	8	8
5	10	10

• Common trap
  • Forgetting STEP value when tracing

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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 AS Level Computer Science Full Scale Course

Tracing WHILE Loops (Entry-Controlled)

Execution Order (WHILE Loop)

• Condition checked before body
• If FALSE → loop skipped entirely
• If TRUE → body runs
• Control variable updated
• Condition checked again

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WHILE Loop Trace Example 1

i ← 1
WHILE i < 4 DO
  OUTPUT i
  i ← i + 1
ENDWHILE

Step	i before	Condition (i < 4)	Output	i after
Start	1	TRUE	–	–
1	1	TRUE	1	2
2	2	TRUE	2	3
3	3	TRUE	3	4
End	4	FALSE	–	–

• Output: 1 2 3

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WHILE Loop Trace Example 2 (Sentinel)

total ← 0
INPUT num
WHILE num <> -1 DO
  total ← total + num
  INPUT num
ENDWHILE
OUTPUT total

Assume inputs: 4, 6, -1

Step	num	total
Start	4	0
Loop 1	4	4
Loop 2	6	10
Exit	-1	10

• Sentinel value is not added

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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 AS Level Computer Science Full Scale Course

Tracing REPEAT UNTIL Loops (Exit-Controlled)

Execution Order (REPEAT UNTIL)

• Body executes first
• Condition checked after
• Loop repeats while condition is FALSE
• Stops when condition becomes TRUE

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REPEAT UNTIL Trace Example 1

i ← 1
REPEAT
  OUTPUT i
  i ← i + 1
UNTIL i = 4

Iteration	i before	Output	i after	Condition (i = 4)
1	1	1	2	FALSE
2	2	2	3	FALSE
3	3	3	4	TRUE

• Output: 1 2 3

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REPEAT UNTIL Trace Example 2 (Validation)

REPEAT
  INPUT num
UNTIL num >= 1 AND num <= 5

Assume inputs: -3, 7, 4

Attempt	num	Condition
1	-3	FALSE
2	7	FALSE
3	4	TRUE

• Loop always runs at least once

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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 AS Level Computer Science Full Scale Course

Tracing Nested Loops

Nested Loop Rule

• Inner loop completes fully for each outer loop iteration
• Total executions = outer × inner

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Nested Loop Trace Example

FOR i ← 1 TO 2
  FOR j ← 1 TO 3
    OUTPUT i * j
  NEXT j
NEXT i

i	j	Output
1	1	1
1	2	2
1	3	3
2	1	2
2	2	4
2	3	6

• Output order matters (row-major)

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Tracing With Flags (Boolean Control)

Flag-Controlled Loop Example

found ← FALSE
index ← 1
WHILE index <= n AND found = FALSE DO
  IF arr[index] = target THEN
    found ← TRUE
  ELSE
    index ← index + 1
  ENDIF
ENDWHILE

Trace focus:

• When does found change?
• Does loop stop immediately after TRUE?
• Does index exceed bounds?

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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 AS Level Computer Science Full Scale Course

High-Frequency Examiner Traps In Dry-Runs

Trap 1: Forgetting Initial Condition Check (WHILE)

• Loop may not run at all
• Always check starting condition first

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Trap 2: Misreading REPEAT UNTIL Condition

• UNTIL condition = stop condition
• Students often trace it like WHILE (wrong)

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Trap 3: Off-By-One Errors

• Loop bounds incorrect
• Missing first or last iteration

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Trap 4: Updating Variable In Wrong Place

• Update before output vs after output
• Order changes results

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Trap 5: Ignoring STEP Direction

• Negative STEP not applied mentally
• Loop assumed to increment

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Examiner-Approved Dry-Run Checklist

• Identify loop type
• Write start values
• Create trace table
• Follow execution order strictly
• Update variables only where code says
• Stop exactly when condition fails / succeeds

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Rapid Practice: Spot The Final Output

Practice 1

i ← 1
WHILE i <= 3 DO
  OUTPUT i * 2
  i ← i + 1
ENDWHILE

• Output: 2 4 6

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Practice 2

REPEAT
  OUTPUT "X"
  count ← count + 1
UNTIL count = 3

• Output: X X X

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Practice 3

FOR i ← 5 TO 1 STEP -2
  OUTPUT i
NEXT i

• Output: 5 3 1

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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 AS Level Computer Science Full Scale Course

One-Line Dry-Run Rules To Memorise

• FOR → count-controlled, predictable
• WHILE → condition checked first
• REPEAT UNTIL → condition checked last
• Nested → inner completes fully
• STEP → direction matters
• Order of statements matters more than intuition