Week 3: Procedures & Parameters

BSCS1001 — IIT Madras BS Degree Prerequisite: Week 2 (Pseudocode, Iteration) Cross-links: BSCS1002-Python (Week 4 — Functions), BSCS2002-PDSA (Week 3 — Modularity)

1. Motivation: Why Procedures?

sql
// Max Maths
MaxMaths = 0
while (Pile 1 has more cards) { Pick X; if X.Maths > MaxMaths → update }
// Max Physics (copy-paste, change field name)
MaxPhysics = 0
while (Pile 1 has more cards) { Pick X; if X.Physics > MaxPhysics → update }
// Max Chemistry (copy-paste again)
MaxChemistry = 0
while (Pile 1 has more cards) { Pick X; if X.Chemistry > MaxChemistry → update }

• 4 times the code
• If you find a bug, you must fix it in 4 places
• If you want to improve the algorithm, you change it 4 times
• Boring to write, boring to read Procedures solve this: write the logic once, use it anywhere.

Real-world analogy: A procedure is like a power drill. You buy one drill (write one procedure) and attach different bits (pass different parameters) for different tasks.

2. What is a Procedure?

Benefits of Procedures

3. Procedure Syntax

Defining a Procedure

pseudo
Procedure ProcedureName(parameter1, parameter2, ...) {
    // Body of the procedure
    // Statements using parameters
    return(value)
End ProcedureName

Example: Sum of Maths Marks for a Gender

sql
Procedure SumMaths(gen) {
    Sum = 0
    while (Pile 1 has more cards) {
        Pick a card X from Pile 1
        Move X to Pile 2
        if (X.Gender == gen) {
            Sum = Sum + X.Maths
        }
    }
    return(Sum)
End SumMaths

Breaking Down the Syntax

4. Parameters: Making Procedures Flexible

How Parameters Work

Example: Parameter as Field Name

sql
Procedure SumMarks(gen, fld) {
    Sum = 0
    while (Pile 1 has more cards) {
        Pick a card X from Pile 1
        Move X to Pile 2
        if (X.Gender == gen) {
            Sum = Sum + X.fld    // fld determines which field!
        }
    }
    return(Sum)
End SumMarks

Calls to This Procedure

Multiple Parameters

sql
// Finding maximum in any subject for any gender
Procedure MaxMarks(gen, subject) {
    MaxVal = 0
    MaxId = -1
    while (Pile 1 has more cards) {
        Pick a card X from Pile 1
        Move X to Pile 2
        if (X.Gender == gen AND X.subject > MaxVal) {
            MaxVal = X.subject
            MaxId = X.Id
        }
    }
    return(MaxId, MaxVal)     // Return multiple values
End MaxMarks

5. Calling Procedures

Procedure Call as Expression

python
Run Inline
GirlChemSum = SumMarks(F, Chemistry)
BoyChemSum = SumMarks(M, Chemistry)
if (GirlChemSum > BoyChemSum) {
    print("Congratulate the girls!")
}
else {
    print("Congratulate the boys!")
}

Procedure Call as Statement

sql
Procedure UpdateMarks(cardId, subject, newMarks) {
    // Find card with matching Id and update the marks
    // No return value needed
End UpdateMarks
// Call it:
UpdateMarks(17, Physics, 88)

How Arguments Match Parameters

pseudo
Procedure Example(a, b, c) {
    // a gets first argument, b gets second, c gets third
}
Call:   Example(10, 20, 30)
               ↑    ↑    ↑
               a ← 10, b ← 20, c ← 30

Complete Example: Top Student Analysis

sql
Procedure MaxMarks(fld) {
    MaxVal = 0
    while (Pile 1 has more cards) {
        Pick a card X from Pile 1
        Move X to Pile 2
        if (X.fld > MaxVal) {
            MaxVal = X.fld
        }
    }
    return(MaxVal)
End MaxMarks
// Now use it:
MaxMaths = MaxMarks(Maths)
MaxPhysics = MaxMarks(Physics)
MaxChem = MaxMarks(Chemistry)
MaxTotal = MaxMarks(Total)
SubjTotal = MaxMaths + MaxPhysics + MaxChem
if (MaxTotal == SubjTotal) {
    SingleTopper = True
}
else {
    SingleTopper = False
}

6. Return Values

Procedures That Return a Value

pseudo
Procedure SumList(L) {
    total = 0
    foreach x in L {
        total = total + x
    }
    return(total)
End SumList
result = SumList([10, 20, 30])   // result = 60

Procedures That Don't Return a Value

python
Run Inline
Procedure PrintAll(L) {
    foreach x in L {
        print(x)
    }
End PrintAll
PrintAll([10, 20, 30])   // Prints 10, 20, 30 (no return value)

The return Statement

7. The Three Prizes Problem

Problem Statement

Solution Approach

Procedure: TopThreeMarks

pseudo
Procedure TopThreeMarks(Subj) {
    max = 0
    secondmax = 0
    thirdmax = 0
    while (Table 1 has more rows) {
        Read the first row X in Table 1
        if (X.Subj > max) {
            thirdmax = secondmax
            secondmax = max
            max = X.Subj
        }
        if (max > X.Subj AND X.Subj > secondmax) {
            thirdmax = secondmax
            secondmax = X.Subj
        }
        if (secondmax > X.Subj AND X.Subj > thirdmax) {
            thirdmax = X.Subj
        }
        Move X to Table 2
    }
    return(thirdmax)
End TopThreeMarks

Building Lists of Top Students

pseudo
cutoffMaths = TopThreeMarks(Maths)
cutoffPhys = TopThreeMarks(Physics)
cutoffChem = TopThreeMarks(Chemistry)
mathsList = []
physList = []
chemList = []
while (Table 1 has more rows) {
    Read the first row X in Table 1
    if (X.Mathematics >= cutoffMaths) {
        mathsList = mathsList ++ [X.SeqNo]
    }
    if (X.Physics >= cutoffPhys) {
        physList = physList ++ [X.SeqNo]
    }
    if (X.Chemistry >= cutoffChem) {
        chemList = chemList ++ [X.SeqNo]
    }
    Move X to Table 2
}

Finding Students in All Three Lists

pseudo
// Students in both Maths and Physics top-3
mathsPhysList = []
foreach x in mathsList {
    foreach y in physList {
        if (x == y) {
            mathsPhysList = mathsPhysList ++ [x]
        }
    }
}
// Students in (Maths+Physics) AND Chemistry top-3
mathsPhysChemList = []
foreach x in mathsPhysList {
    foreach y in chemList {
        if (x == y) {
            mathsPhysChemList = mathsPhysChemList ++ [x]
        }
    }
}

8. Procedures vs Plain Code

Comparison Table

When NOT to Use a Procedure

• For very simple operations (a single line)
• When the operation is used only once (sometimes)
• When the overhead of calling is not justified

9. Practice Questions

Basic Questions

Show Answer

A procedure is a named block of pseudocode that performs a specific task.

Benefits (any two):

1. Reusability — write once, use many times
2. Modularity — break complex problems into smaller pieces
3. Readability — procedure name describes what it does
4. Maintainability — fix one place, all callers benefit Q2. What is the difference between a parameter and an argument? Show Answer

• Parameter: The variable listed in the procedure definition (e.g., gen in Procedure SumMaths(gen))
• Argument: The actual value passed when calling (e.g., "F" in SumMaths(F))

The parameter receives the argument's value. Q3. Write a procedure called AverageMarks that takes a field name and returns the average of that field. Show Answer

sql
Procedure AverageMarks(fld) {
    Sum = 0
    Count = 0
    while (Pile 1 has more cards) {
        Pick a card X from Pile 1
        Move X to Pile 2
        Sum = Sum + X.fld
        Count = Count + 1
    }
    return(Sum / Count)
End AverageMarks

Show Answer

It returns 0 because Sum starts at 0 and the condition X.Gender == "F" never matches, so Sum stays 0.

Intermediate Questions

pseudo
Procedure Mystery(L) {
    result = 0
    foreach x in L {
        if (x > result) {
            result = x
        }
    }
    return(result)
End Mystery
Call: answer = Mystery([2, 5, 1, 8])

Show Answer

answer = 8 (the procedure finds maximum) Q6. Write a procedure CountAboveThreshold that takes a field name and a threshold value, and counts how many students exceed that threshold in that field. Show Answer

sql
Procedure CountAboveThreshold(fld, threshold) {
    Count = 0
    while (Pile 1 has more cards) {
        Pick a card X from Pile 1
        Move X to Pile 2
        if (X.fld > threshold) {
            Count = Count + 1
        }
    }
    return(Count)
End CountAboveThreshold

// Example: CountAboveThreshold(Maths, 80)

Show Answer

When MaxMarks(Maths) is called, the parameter fld gets the value "Maths". Inside the loop, X.fld is interpreted as X.Maths — accessing the Maths field of card X.

When MaxMarks(Physics) is called, fld = "Physics", and X.fld accesses X.Physics.

This is a form of dynamic field access — the field name is treated as data. Q8. What would happen if we called SumMarks(M, Physics) but our procedure SumMarks only had one parameter? Show Answer

This would cause an error because the procedure expects 2 arguments but only received 1. The number of arguments must match the number of parameters.

Advanced Questions

Show Answer

pseudo
Procedure IsSubjectTopper(X, math3, phys3, chem3) {
    if (X.Mathematics >= math3) {
        return(True)
    }
    if (X.Physics >= phys3) {
        return(True)
    }
    if (X.Chemistry >= chem3) {
        return(True)
    }
    return(False)
End IsSubjectTopper

This procedure is used in the Three Prizes problem to check if a student qualifies. Q10. Design a procedure GradeAllocator that assigns grades A, B, C, D based on marks thresholds. Thresholds are passed as parameters. Show Answer

pseudo
Procedure GradeAllocator(marks, aThreshold, bThreshold, cThreshold) {
    if (marks >= aThreshold) {
        return("A")
    }
    if (marks >= bThreshold) {
        return("B")
    }
    if (marks >= cThreshold) {
        return("C")
    }
    return("D")
End GradeAllocator

// Usage:
// GradeAllocator(85, 90, 75, 60) returns "B"

Show Answer

Yes! This is called composition.

sql
Procedure Average(fld) {
    Sum = 0
    Count = 0
    while (Pile 1 has more cards) {
        Pick a card X from Pile 1
        Move X to Pile 2
        Sum = Sum + X.fld
        Count = Count + 1
    }
    return(Sum / Count)
End Average

Procedure CompareBoysGirls(subject) {
    boyAvg = Average(M, subject)    // Calling Average with gender filter?
    girlAvg = Average(F, subject)
    // (Note: Would need to modify Average to accept gender parameter)
}

Actually, to really compose, we'd need Average to accept a gender parameter too. Q12. In the Three Prizes problem, why do we compute the 3rd highest mark (not the 1st or 2nd) as the cutoff? Show Answer

If we used the 1st highest mark as the cutoff, only the single top student in each subject would qualify. But we want top 3 students per subject.

The 3rd highest mark tells us: "anyone at or above this mark is in the top 3." Students with marks equal to the 3rd highest are included, which gives us at least 3 students (possibly more if there are ties).

Using the 3rd highest mark as the cutoff ensures we capture all students who belong in the top 3 (including those who tied for 3rd place).

📚 Cross-References

Next Topic: 06 — Side Effects of Procedures

Quiz Tip: Procedure questions often ask you to trace the return value or identify parameter-argument matching errors! Join Discord PreviousIteration & FilteringNextSide Effects of Procedures