Coding Sheets, Practice & Learning Support: Mistake-Based Coding Practice Sheets (Copy)

Mistake-Based Coding Practice Sheets

How To Use These Sheets (Fast Method)

• For each task:
  • Identify the bug type
  • State why it is wrong (logic reason)
  • Write the minimum fix
  • Run test data mentally using the provided trace table format

---

Sheet 1: Loop Mistakes (FOR / WHILE / REPEAT UNTIL)

Task 1: Off-By-One In FOR Loop

Faulty Pseudocode

total ← 0
FOR i ← 1 TO 10
  INPUT num
  total ← total + num
NEXT i
OUTPUT total / 9

• Do:
  • Identify the mistake
  • Fix it
  • Give one test case where the wrong version fails

Test Data Suggestion

• Inputs: 9, 9, 9, 9, 9, 9, 9, 9, 9, 9

---

Task 2: Infinite WHILE Loop (Missing Update)

Faulty Pseudocode

i ← 1
WHILE i <= 5 DO
  OUTPUT i
ENDWHILE

• Do:
  • Identify why it never stops
  • Fix it

---

Task 3: Wrong STEP Direction In FOR Loop

Faulty Pseudocode

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

• Do:
  • Identify the issue
  • Fix it to count down properly

---

Task 4: REPEAT UNTIL Condition Reversed

Faulty Pseudocode

REPEAT
  INPUT mark
UNTIL mark < 0 OR mark > 100
OUTPUT mark

• Do:
  • Explain why this accepts invalid data
  • Rewrite the UNTIL condition correctly

---

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

Task 5: Validation Loop With Wrong Operator (Impossible Condition)

Faulty Pseudocode

INPUT age
WHILE age < 18 AND age > 60 DO
  INPUT age
ENDWHILE
OUTPUT age

• Do:
  • Identify why the loop may never run even for invalid age
  • Fix the condition

---

Task 6: Loop Runs One Time Too Many (Array Out Of Range)

Faulty Pseudocode

DECLARE arr : ARRAY[1..5] OF INTEGER
FOR i ← 1 TO 6
  INPUT arr[i]
NEXT i

• Do:
  • Identify the exact out-of-range access
  • Fix bounds

---

Sheet 2: IF / ELSE Logic Mistakes

Task 7: Pass/Fail Condition Reversed

Faulty Pseudocode

INPUT mark
IF mark < 50 THEN
  OUTPUT "Pass"
ELSE
  OUTPUT "Fail"
ENDIF

• Do:
  • Correct the condition
  • Give 2 test values: one pass, one fail

---

Task 8: ELSEIF Order Makes A Grade Unreachable

Faulty Pseudocode

INPUT score
IF score >= 50 THEN
  OUTPUT "C"
ELSEIF score >= 80 THEN
  OUTPUT "A"
ELSE
  OUTPUT "F"
ENDIF

• Do:
  • Explain why “A” never prints
  • Fix the ordering

---

Task 9: Condition Always TRUE

Faulty Pseudocode

INPUT x
IF x >= 0 OR x <= 100 THEN
  OUTPUT "Valid"
ELSE
  OUTPUT "Invalid"
ENDIF

• Do:
  • Explain why it always outputs “Valid”
  • Fix the logic

---

Task 10: Missing ELSE Causes Logic Gap

Faulty Pseudocode

INPUT temp
IF temp > 40 THEN
  OUTPUT "Hot"
ENDIF

• Do:
  • Explain what output happens when temp = 20
  • Rewrite with a complete decision structure

---

Sheet 3: Array Mistakes (1D + 2D)

Task 11: Wrong Index Used (Adds Same Element Repeatedly)

Faulty Pseudocode

sum ← 0
FOR i ← 1 TO n
  sum ← sum + arr[1]
NEXT i
OUTPUT sum

• Do:
  • Identify bug
  • Fix it

---

Task 12: Max Initialised Incorrectly (Fails For All Negative)

Faulty Pseudocode

max ← 0
FOR i ← 1 TO n
  IF arr[i] > max THEN
    max ← arr[i]
  ENDIF
NEXT i
OUTPUT max

• Do:
  • Explain failure case
  • Provide safe max algorithm pattern

---

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

Task 13: 2D Array Swapped Indices

Faulty Pseudocode

FOR row ← 1 TO r
  FOR col ← 1 TO c
    total ← total + table[col, row]
  NEXT col
NEXT row

• Do:
  • Explain what’s wrong
  • Fix the indexing

---

Task 14: Accumulator Not Reset For Each Row

Faulty Pseudocode

rowTotal ← 0
FOR row ← 1 TO r
  FOR col ← 1 TO c
    rowTotal ← rowTotal + table[row, col]
  NEXT col
  OUTPUT rowTotal
NEXT row

• Do:
  • Explain why totals keep increasing
  • Fix it so each row total is separate

---

Sheet 4: String Mistakes (Character-By-Character)

Task 15: Trying To Modify A String Directly

Faulty Pseudocode

INPUT text
text[1] ← "A"
OUTPUT text

• Do:
  • Explain why this is invalid logic
  • Fix by rebuilding the string

---

Task 16: Out-Of-Range Last Character Access

Faulty Pseudocode

INPUT word
last ← word[LENGTH(word) + 1]
OUTPUT last

• Do:
  • Identify the exact index error
  • Fix it

---

Task 17: Wrong Loop Bound For String Traversal

Faulty Pseudocode

INPUT text
FOR i ← 1 TO n
  OUTPUT text[i]
NEXT i

• Do:
  • Identify what should replace n
  • Fix it properly

---

Sheet 5: Searching / Sorting Mistakes

Task 18: Linear Search Never Moves Index (Infinite Loop)

Faulty Pseudocode

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

• Do:
  • Explain why it can loop forever
  • Fix it using safe search structure

---

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

Task 19: Sentinel Included In Total (Wrong Order)

Faulty Pseudocode

total ← 0
REPEAT
  INPUT num
  total ← total + num
UNTIL num = -1
OUTPUT total

• Do:
  • Explain why sentinel is included
  • Fix it so -1 is not added

---

Task 20: Bubble Sort Out Of Range

Faulty Pseudocode

FOR pass ← 1 TO n - 1
  FOR j ← 1 TO n
    IF arr[j] > arr[j + 1] THEN
      temp ← arr[j]
      arr[j] ← arr[j + 1]
      arr[j + 1] ← temp
    ENDIF
  NEXT j
NEXT pass

• Do:
  • Identify out-of-range access
  • Fix inner loop bounds correctly

---

Task 21: Bubble Sort Wrong Comparison For Descending

Faulty Pseudocode (intended descending)

FOR pass ← 1 TO n - 1
  FOR j ← 1 TO n - pass
    IF arr[j] > arr[j + 1] THEN
      temp ← arr[j]
      arr[j] ← arr[j + 1]
      arr[j + 1] ← temp
    ENDIF
  NEXT j
NEXT pass

• Do:
  • Explain why this sorts ascending
  • Fix the IF condition for descending

---

Sheet 6: Flags (State Mistakes)

Task 22: Flag Reset Inside Loop (Destroys Meaning)

Faulty Pseudocode

FOR i ← 1 TO n
  found ← FALSE
  IF arr[i] = target THEN
    found ← TRUE
  ENDIF
NEXT i

IF found = TRUE THEN
  OUTPUT "Found"
ELSE
  OUTPUT "Not Found"
ENDIF

• Do:
  • Explain why final result depends only on last element
  • Fix flag placement

---

Task 23: Swapped Flag Not Reset Each Pass (Bubble Sort Optimisation Bug)

Faulty Pseudocode

swapped ← TRUE
pass ← 1

WHILE pass <= n - 1 AND swapped = TRUE DO
  FOR j ← 1 TO n - pass
    IF arr[j] > arr[j + 1] THEN
      temp ← arr[j]
      arr[j] ← arr[j + 1]
      arr[j + 1] ← temp
      swapped ← TRUE
    ENDIF
  NEXT j
  pass ← pass + 1
ENDWHILE

• Do:
  • Explain why loop may never stop early
  • Fix by resetting swapped correctly

---

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

Trace Table Templates (Use For Any Task)

Trace Table: Loop + Variables

Step / Iteration	Key Variable(s)	Condition Result	Output

Trace Table: Array Traversal

i	arr[i]	Important Variables (total/max/count/found)	Output

Trace Table: String Traversal

i	text[i]	result (built string)	counters/flags	Output

---

ANSWER KEY (Fixes + Reasoning + Clean Correct Versions)

Task 1: Fix

• Error:
  • Dividing by 9 when 10 inputs were summed
• Fix:

OUTPUT total / 10

---

Task 2: Fix

• Error:
  • i never changes
• Fix:

i ← 1
WHILE i <= 5 DO
  OUTPUT i
  i ← i + 1
ENDWHILE

---

Task 3: Fix

• Error:
  • Counting down needs STEP -1
• Fix:

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

---

Task 4: Fix

• Error:
  • UNTIL condition is written for invalid data
• Fix:

REPEAT
  INPUT mark
UNTIL mark >= 0 AND mark <= 100
OUTPUT mark

---

Task 5: Fix

• Error:
  • age < 18 AND age > 60 is impossible
• Fix:

INPUT age
WHILE age < 18 OR age > 60 DO
  INPUT age
ENDWHILE
OUTPUT age

---

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

Task 6: Fix

• Error:
  • i = 6 causes arr[6] out of range
• Fix:

DECLARE arr : ARRAY[1..5] OF INTEGER
FOR i ← 1 TO 5
  INPUT arr[i]
NEXT i

---

Task 7: Fix

• Error:
  • Pass/fail reversed
• Fix:

INPUT mark
IF mark >= 50 THEN
  OUTPUT "Pass"
ELSE
  OUTPUT "Fail"
ENDIF

---

Task 8: Fix

• Error:
  • score >= 50 catches everything including 80+
• Fix:

INPUT score
IF score >= 80 THEN
  OUTPUT "A"
ELSEIF score >= 50 THEN
  OUTPUT "C"
ELSE
  OUTPUT "F"
ENDIF

---

Task 9: Fix

• Error:
  • x >= 0 OR x <= 100 always TRUE
• Fix:

INPUT x
IF x >= 0 AND x <= 100 THEN
  OUTPUT "Valid"
ELSE
  OUTPUT "Invalid"
ENDIF

---

Task 10: Fix

• Logic gap:
  • No output when temp <= 40
• Fix:

INPUT temp
IF temp > 40 THEN
  OUTPUT "Hot"
ELSE
  OUTPUT "Not Hot"
ENDIF

---

Task 11: Fix

• Error:
  • Adds arr[1] repeatedly
• Fix:

sum ← 0
FOR i ← 1 TO n
  sum ← sum + arr[i]
NEXT i
OUTPUT sum

---

Task 12: Fix (Safe Max)

• Error:
  • max ← 0 fails if all values negative
• Fix:

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

---

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

Task 13: Fix

• Error:
  • Using table[col, row] swaps meaning
• Fix:

FOR row ← 1 TO r
  FOR col ← 1 TO c
    total ← total + table[row, col]
  NEXT col
NEXT row

---

Task 14: Fix

• Error:
  • rowTotal must reset each row
• Fix:

FOR row ← 1 TO r
  rowTotal ← 0
  FOR col ← 1 TO c
    rowTotal ← rowTotal + table[row, col]
  NEXT col
  OUTPUT rowTotal
NEXT row

---

Task 15: Fix (Rebuild String)

• Error:
  • Direct character assignment to string
• Fix:

INPUT text
newText ← ""

FOR i ← 1 TO LENGTH(text)
  IF i = 1 THEN
    newText ← newText & "A"
  ELSE
    newText ← newText & text[i]
  ENDIF
NEXT i

text ← newText
OUTPUT text

---

Task 16: Fix

• Error:
  • last index is LENGTH(word) not + 1
• Fix:

INPUT word
last ← word[LENGTH(word)]
OUTPUT last

---

Task 17: Fix

• Error:
  • Wrong loop bound variable
• Fix:

INPUT text
FOR i ← 1 TO LENGTH(text)
  OUTPUT text[i]
NEXT i

---

Task 18: Fix (Safe Linear Search)

• Error:
  • index never changes when not found
• Fix:

found ← FALSE
index ← 1

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

---

Task 19: Fix (Exclude Sentinel)

• Error:
  • Adds -1 before stopping
• Fix:

total ← 0

REPEAT
  INPUT num
  IF num <> -1 THEN
    total ← total + num
  ENDIF
UNTIL num = -1

OUTPUT total

---

Task 20: Fix (Bubble Sort Bounds)

• Error:
  • j = n causes arr[n+1]
• Fix:

FOR pass ← 1 TO n - 1
  FOR j ← 1 TO n - pass
    IF arr[j] > arr[j + 1] THEN
      temp ← arr[j]
      arr[j] ← arr[j + 1]
      arr[j + 1] ← temp
    ENDIF
  NEXT j
NEXT pass

---

Task 21: Fix (Descending)

• Error:
  • Uses ascending comparison
• Fix:

FOR pass ← 1 TO n - 1
  FOR j ← 1 TO n - pass
    IF arr[j] < arr[j + 1] THEN
      temp ← arr[j]
      arr[j] ← arr[j + 1]
      arr[j + 1] ← temp
    ENDIF
  NEXT j
NEXT pass

---

Task 22: Fix (Flag Outside Loop)

• Error:
  • Resetting flag each iteration
• Fix:

found ← FALSE
FOR i ← 1 TO n
  IF arr[i] = target THEN
    found ← TRUE
  ENDIF
NEXT i

IF found = TRUE THEN
  OUTPUT "Found"
ELSE
  OUTPUT "Not Found"
ENDIF

---

Task 23: Fix (Reset swapped Each Pass)

• Error:
  • swapped never reset to FALSE at start of pass
• Fix:

swapped ← TRUE
pass ← 1

WHILE pass <= n - 1 AND swapped = TRUE DO
  swapped ← FALSE

  FOR j ← 1 TO n - pass
    IF arr[j] > arr[j + 1] THEN
      temp ← arr[j]
      arr[j] ← arr[j + 1]
      arr[j + 1] ← temp
      swapped ← TRUE
    ENDIF
  NEXT j

  pass ← pass + 1
ENDWHILE