Standard Algorithm & Logic Patterns: Sentinel Values In Algorithms (Copy)

Sentinel Values In Algorithms

What A Sentinel Value Is

• A sentinel value is a special value used to:
  • Signal the end of input
  • Stop a loop without knowing the number of inputs in advance
• The sentinel:
  • Is not processed
  • Is checked before or during loop termination
• Common examples:
  • -1, 0, "END", "X" (only if clearly stated)

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Core Examiner Rule (High-Frequency)

• Sentinel value:
  • Must be tested correctly
  • Must not be included in calculations
• Using sentinel incorrectly = logic error + mark loss

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When Sentinel Values Are Used

• Unknown number of inputs
• Continuous user input
• File-style or stream-style data
• Repeated input until “stop” condition occurs

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Sentinel Value vs Fixed Loop Count

Feature	Sentinel-Controlled	Fixed Loop
Number of inputs known	No	Yes
Loop ends when	Sentinel entered	Count reached
Flexibility	High	Limited
Risk	Forgetting to exclude sentinel	Wrong loop bounds

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Basic Sentinel-Controlled Loop (REPEAT UNTIL)

Standard Pattern (Exam-Preferred)

REPEAT
  INPUT value
UNTIL value = sentinel

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Important Rule

• This loop accepts sentinel once
• Processing must occur only if value is not sentinel

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Sentinel With Processing (Correct Pattern)

Example: Sum Numbers Until -1

total ← 0

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

OUTPUT total

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Why This Is Correct

• Sentinel -1:
  • Stops loop
  • Is not added to total

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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

Sentinel Using WHILE Loop (Pre-Test)

When To Use

• Input already exists
• Sentinel checked before loop runs

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Example: Count Inputs Until 0

count ← 0
INPUT num

WHILE num <> 0 DO
  count ← count + 1
  INPUT num
ENDWHILE

OUTPUT count

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Examiner Note

• Initial input before loop
• Re-input inside loop
• Forgetting either causes errors

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Sentinel With Counting And Accumulation (Very Common)

Example: Average Until -1

total ← 0
count ← 0

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

IF count > 0 THEN
  average ← total / count
  OUTPUT average
ENDIF

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Key Safety Check

• count > 0 prevents division by zero

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Sentinel With Validation Combined

Example: Accept Positive Numbers Until -1

REPEAT
  INPUT num
  IF num > 0 THEN
    OUTPUT num
  ENDIF
UNTIL num = -1

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Examiner Insight

• Sentinel check comes after processing condition
• Sentinel still terminates loop correctly

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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

Sentinel In Array Filling

Example: Store Values Until -1 Or Array Full

i ← 1

REPEAT
  INPUT num
  IF num <> -1 AND i <= 10 THEN
    arr[i] ← num
    i ← i + 1
  ENDIF
UNTIL num = -1 OR i > 10

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Why This Is Important

• Prevents array overflow
• Handles unknown input size safely

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Sentinel With Strings

Example: Input Words Until “END”

count ← 0

REPEAT
  INPUT word
  IF word <> "END" THEN
    count ← count + 1
  ENDIF
UNTIL word = "END"

OUTPUT count

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Sentinel In Nested Logic

Example: Stop Searching On Sentinel Or Found

found ← FALSE

REPEAT
  INPUT num
  IF num = target THEN
    found ← TRUE
  ENDIF
UNTIL num = -1 OR found = TRUE

IF found = TRUE THEN
  OUTPUT "Found"
ELSE
  OUTPUT "Not Found"
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 AS Level Computer Science Full Scale Course

Common Sentinel Errors (Very High Frequency)

Error 1: Including Sentinel In Calculation

total ← total + num
UNTIL num = -1

• Sentinel added before stop

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Error 2: Wrong Loop Condition

UNTIL num <> -1

• Loop stops immediately
• Condition reversed

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Error 3: Forgetting Re-Input

WHILE num <> -1 DO
  total ← total + num
ENDWHILE

• Infinite loop

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Error 4: Processing Before Sentinel Check

• Sentinel processed accidentally

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Sentinel Value Design Rules (Exam Language)

• “The sentinel value signals termination”
• “The sentinel is excluded from processing”
• “The loop continues until the sentinel is entered”
• “A sentinel allows unknown input size”

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Sentinel vs Flag (Quick Comparison)

Feature	Sentinel	Flag
Purpose	Stop input	Track condition
Value type	Data value	Boolean
Stops loop	Yes	Indirectly
Included in data	No	N/A

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Exam-Ready Sentinel Templates

REPEAT UNTIL Sentinel

REPEAT
  INPUT value
  IF value <> sentinel THEN
    PROCESS value
  ENDIF
UNTIL value = sentinel

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WHILE Sentinel

INPUT value
WHILE value <> sentinel DO
  PROCESS value
  INPUT value
ENDWHILE

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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 Rules To Memorise

• Sentinel ends input, not data
• Never process sentinel
• Re-input inside WHILE loops
• REPEAT UNTIL is safest
• Sentinel logic must be crystal clear