Selection And Iteration Code Guides: FOR Loops: Structure, Use Cases And Pitfalls (Copy)

FOR Loops: Structure, Use Cases And Pitfalls

What A FOR Loop Is (Count-Controlled Iteration)

• A FOR loop repeats a block of code a known number of times
• You use it when:
  • The number of repetitions is fixed (e.g., 10 times)
  • You must process a fixed range of values (e.g., 1 to n)
  • You want to traverse an array/list by index safely
• Core idea
  • A counter variable automatically changes each iteration until the loop finishes

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Standard FOR Loop Structure (Pseudocode)

• Basic form
  • FOR counter ← start TO end
  • <statements>
  • NEXT counter
• With step
  • FOR counter ← start TO end STEP stepValue
  • <statements>
  • NEXT counter
• Key exam-safe assumptions
  • start and end are treated as inclusive
  • If no STEP is given, default is usually STEP 1

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Counter, Start, End, Step (Roles + Mistake Traps)

Component	Purpose	Correct Use	Pitfall That Loses Marks
Counter	Tracks iteration number	i, index, attempt	Changing counter inside loop
Start	First value for counter	1 or 0 depending on indexing	Starting at wrong bound
End	Last value to reach	n, 10, LEN(arr) type logic	Off-by-one (missing last item)
STEP	Increment/decrement size	2, -1, etc.	Wrong sign when counting down

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When A FOR Loop Is The Best Choice (Vs Other Loops)

Task	Best Loop	Why
Repeat exactly 20 times	FOR	Count known
Traverse array indices	FOR	Controlled index range
Keep asking until valid input	WHILE / REPEAT UNTIL	Repetitions unknown
Stop as soon as condition met	WHILE (or FOR with flag)	FOR needs safe early-exit logic

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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

Structure (How To Write It Correctly Every Time)

Template 1: Simple Count Loop

• Use when you need a fixed number of repetitions

FOR i ← 1 TO 10
  OUTPUT i
NEXT i

• What it does
  • Outputs: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

Template 2: Range With A Step

• Use to skip values or count backwards

FOR i ← 2 TO 20 STEP 2
  OUTPUT i
NEXT i

• What it does
  • Outputs: 2, 4, 6, 8, … , 20

Template 3: Counting Down

FOR i ← 10 TO 1 STEP -1
  OUTPUT i
NEXT i

• What it does
  • Outputs: 10, 9, 8, … , 1

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Use Cases (Code + Application-Based Examples)

Use Case 1: Repeat A Task Exactly N Times

• Scenario
  • A system needs to display a message exactly 5 times

FOR i ← 1 TO 5
  OUTPUT "Welcome"
NEXT i

• Marks usually come from
  • Correct bounds
  • Correct output statement inside loop

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Use Case 2: Summation (Adding Many Values)

• Scenario
  • Input 10 numbers, output their sum

total ← 0
FOR i ← 1 TO 10
  INPUT num
  total ← total + num
NEXT i
OUTPUT total

• Common examiner marking points
  • total ← 0 before the loop
  • update uses total ← total + num not overwriting
• Pitfall
  • Writing total ← num (kills the sum)

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Use Case 3: Counting Matches (Selection Inside Iteration)

• Scenario
  • Input 8 numbers, count how many are even

countEven ← 0
FOR i ← 1 TO 8
  INPUT num
  IF num MOD 2 = 0 THEN
    countEven ← countEven + 1
  ENDIF
NEXT i
OUTPUT countEven

• Why this matters
  • Shows controlled repetition + IF logic (common paper pattern)
• Pitfalls
  • Using / instead of MOD
  • Forgetting ENDIF

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Use Case 4: Processing A 1D Array By Index

• Scenario
  • Marks stored in marks[1..5], find total

total ← 0
FOR index ← 1 TO 5
  INPUT marks[index]
  total ← total + marks[index]
NEXT index
OUTPUT total

• Pitfall (array out of range)
  • Accidentally looping 1 TO 6

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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

Use Case 5: Find Maximum (Safe Exam Pattern)

• Scenario
  • Find largest number in an array arr[1..n]
• Safe method
  • Set max to first element (works even if all values are negative)

INPUT n
FOR i ← 1 TO n
  INPUT arr[i]
NEXT i

max ← arr[1]
FOR i ← 2 TO n
  IF arr[i] > max THEN
    max ← arr[i]
  ENDIF
NEXT i

OUTPUT max

• Pitfall
  • max ← 0 (fails if values are all negative)

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Use Case 6: Average Of Values

• Scenario
  • Input n values, output average

INPUT n
total ← 0
FOR i ← 1 TO n
  INPUT num
  total ← total + num
NEXT i
average ← total / n
OUTPUT average

• Pitfalls
  • Forgetting to divide by n
  • Dividing inside loop (logic still possible but messy and risky)

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Use Case 7: Building A Frequency Count

• Scenario
  • Count how many numbers are greater than 50

countHigh ← 0
FOR i ← 1 TO n
  INPUT num
  IF num > 50 THEN
    countHigh ← countHigh + 1
  ENDIF
NEXT i
OUTPUT countHigh

• Why examiners like it
  • Clear count-controlled loop + clear condition

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Nested FOR Loops (When + How)

What Nested FOR Loops Are Used For

• Traversing a 2D array
• Creating a grid output (tables, matrices)
• Checking every combination (not always efficient, but common in exam tasks)

Example: 2D Array Input And Output

• Scenario
  • Read a 3×4 grid and output it

FOR row ← 1 TO 3
  FOR col ← 1 TO 4
    INPUT grid[row][col]
  NEXT col
NEXT row

FOR row ← 1 TO 3
  FOR col ← 1 TO 4
    OUTPUT grid[row][col]
  NEXT col
NEXT row

Nested Loop Pitfalls

• Reusing the same counter name
  • Wrong:
    • FOR i ← 1 TO 3
    • FOR i ← 1 TO 4
• Incorrect NEXT placement (closing wrong loop)
• Misunderstanding total iterations
  • Total = rows × cols

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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 A FOR Loop (Dry Run Method That Always Works)

Tracing Checklist

• Create a table with:
  • counter value
  • important variable(s) (total/count/max)
  • output if any
• Execute loop body line-by-line
• After body ends, counter updates automatically

Worked Trace Example

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

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

• Output: 10

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Pitfalls (The Ones That Constantly Destroy Marks)

Pitfall 1: Off-By-One Errors

• Example mistake
  • Array arr[1..n] but loop does:
    • FOR i ← 1 TO n - 1
  • Misses last element
• Fix
  • Write the array bounds first, then match the loop

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Pitfall 2: Array Index Out Of Range

• Example mistake
  • FOR i ← 1 TO n
  • Accessing arr[i + 1]
  • When i = n, arr[n + 1] is invalid
• Fix options
  • Loop only to n - 1
  • Or change logic to avoid i + 1

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Pitfall 3: Wrong STEP Sign When Counting Down

• Wrong
  • FOR i ← 10 TO 1 STEP 1
• Correct
  • FOR i ← 10 TO 1 STEP -1

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Pitfall 4: Changing Counter Inside Loop

• Wrong

FOR i ← 1 TO 10
  i ← i + 1
  OUTPUT i
NEXT i

• Why wrong
  • Skips values unpredictably
• Correct approach
  • Use STEP:

FOR i ← 1 TO 10 STEP 2
  OUTPUT i
NEXT i

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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

Pitfall 5: Not Initialising Variables

• Common fails
  • Using total without setting total ← 0
  • Using count without setting count ← 0
• Fix
  • Always initialise before loop begins

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Pitfall 6: Using FOR When Repetitions Are Unknown

• Bad scenario
  • Input validation:
    • “Keep asking until correct”
• Better pattern
  • Pre-condition or post-condition loop instead (FOR is not natural here)

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“Copy-Paste Patterns” Students Should Memorise

Pattern A: Total Of N Inputs

INPUT n
total ← 0
FOR i ← 1 TO n
  INPUT num
  total ← total + num
NEXT i
OUTPUT total

Pattern B: Count Items That Match A Condition

count ← 0
FOR i ← 1 TO n
  IF <condition> THEN
    count ← count + 1
  ENDIF
NEXT i
OUTPUT count

Pattern C: Maximum Value In Array

max ← arr[1]
FOR i ← 2 TO n
  IF arr[i] > max THEN
    max ← arr[i]
  ENDIF
NEXT i
OUTPUT max

Pattern D: Reverse Traversal

FOR i ← n TO 1 STEP -1
  OUTPUT arr[i]
NEXT i

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Quick “Spot The Mistake” Mini Practice (Exam-Style)

Q1 (Bug In Summation)

total ← 0
FOR i ← 1 TO 5
  INPUT num
  total ← num
NEXT i
OUTPUT total

• What’s wrong
  • Overwrites total instead of adding
• Fix
  • total ← total + num

Q2 (Bug In Reverse Loop)

FOR i ← 5 TO 1
  OUTPUT i
NEXT i

• What’s wrong
  • Missing STEP -1
• Fix
  • FOR i ← 5 TO 1 STEP -1

Q3 (Bug In Array Bounds)

• Array is arr[1..10], loop:
  • FOR i ← 0 TO 9
• What’s wrong
  • Wrong range for this indexing system
• Fix
  • FOR i ← 1 TO 10