Week 2: Pseudocode Basics

BSCS1001 — IIT Madras BS Degree Prerequisite: Week 1 (Flowcharts, Iteration) Cross-links: BSCS1002-Python (Week 2 — Control Flow), BSCS2002-PDSA (Week 1 — Algorithms)

1. Motivation: From Pictures to Text

💡 Key Insight: Think of pseudocode as "programming in English" — precise enough for a computer, but readable by a human.

The Evolution

2. What is Pseudocode?

• Standard phrases like while, if, else
• Mathematical notation for operations
• Indentation (or braces {}) to show blocks
• Dot notation (.) for record field access

Example: Counting Cards

sql
Count = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    Count = Count + 1
}

Notice: The pseudocode is shorter, more precise, and easier to modify than the flowchart. But both describe the same algorithm.

Pseudocode Conventions in This Course

3. Core Pseudocode Constructs

4. Assignment Statement

Syntax

pseudo
VariableName = Expression

How It Works

1. Evaluate the expression on the right side using current variable values
2. Store the result in the variable on the left side

Examples

Tracing Assignment

⚠️ Important: The = sign is not mathematical equality. Sum = Sum + 1 does NOT mean "Sum equals Sum plus 1" (which is impossible in math). It means "increase Sum by 1."

5. Conditional Execution

If Statement

pseudo
if (condition) {
    Statement 1
    Statement 2
    ...
}

If-Else Statement

pseudo
if (condition) {
    Statements for True case
}
else {
    Statements for False case
}

Flowchart of If-Else

Worked Example: Separate Boy/Girl Sums

sql
BoySum = 0
GirlSum = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X.Gender == "M") {
        BoySum = BoySum + X.Maths
    }
    else {
        GirlSum = GirlSum + X.Maths
    }
}

Nested Conditionals

pseudo
if (X.Gender == "M") {
    if (X.Maths > 80) {
        Count = Count + 1
    }
}

6. Iteration with while

Syntax

pseudo
while (condition) {
    // Block of statements to repeat
}

How It Works

1. Check the condition
2. If True → execute the block, then go back to step 1
3. If False → exit the loop and continue after }

Flowchart of While Loop

The Standard Dataset Iteration Pattern

sql
Initialize variables
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    // Update variables based on X
}

Why "Pile 1 has more cards"?

• Starts as True (if the dataset is non-empty)
• Becomes False after the last card is moved
• Guarantees every card is processed exactly once

7. Equality vs Assignment

How to Read Them Aloud

Why Two Different Symbols?

pseudo
// WRONG - this is assignment, not comparison!
if (X.Gender = "M") {

Memory Aid

8. Complete Pseudocode Examples

Example 1: Sum of All Maths Marks

sql
Sum = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    Sum = Sum + X.Maths
}

Example 2: Finding Maximum with Card ID

sql
MaxM = 0
MaxCard = -1
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X.Maths > MaxM) {
        MaxM = X.Maths
        MaxCard = X.Id
    }
}

Example 3: Below Average Students

sql
// First pass: calculate average
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.Maths
    Count = Count + 1
}
Average = Sum / Count
// Second pass: find below-average students
BelowCount = 0
while (Pile 2 has more cards) {   // Note: cards are now in Pile 2!
    Pick a card X from Pile 2
    Move X to Pile 1              // Moving back to Pile 1
    if (X.Maths < Average) {
        BelowCount = BelowCount + 1
    }
}

9. Flowchart to Pseudocode Translation

Translation Guide

Worked Translation

sql
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.Maths
    Count = Count + 1
}
Average = Sum / Count

💡 Tip: The loop back arrow in the flowchart maps directly to the while keyword. Everything inside the loop body is what's inside the rectangle and diamond shapes in the loop path.

10. Practice Questions

Basic Questions

pseudo
x = 10
x = x + 5
x = x * 2

Show Answer

Show Answer

= is assignment (store a value). == is equality comparison (check if two values are the same, returns True/False). Q3. Trace this pseudocode for input values [4, 8, 2, 6]:

sql
Sum = 0
Count = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X > 5) {
        Sum = Sum + X
    }
    Count = Count + 1
}

Show Answer

Final: Sum = 14, Count = 4 Q4. Write pseudocode to find the minimum value in a dataset. Show Answer

sql
Min = INFINITY   // Very large number
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X < Min) {
        Min = X
    }
}

Alternatively, initialize Min to the first card's value.

Intermediate Questions

Show Answer

sql
Total = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X.Gender == "F" AND X.Maths > 70) {
        Total = Total + X.Maths
    }
}

Show Answer

If you don't move the card, the same card will be picked again in the next iteration! The condition Pile 1 has more cards will remain True forever, creating an infinite loop — the algorithm never ends.

This is why Move X to Pile 2 is a critical step in every iteration. Q7. Write pseudocode that counts the number of students who scored between 60 and 80 (inclusive) in Maths. Show Answer

sql
Count = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X.Maths >= 60 AND X.Maths <= 80) {
        Count = Count + 1
    }
}

sql
Sum = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Sum = Sum + X.Maths
}

Show Answer

The card X is never moved to Pile 2. The same card will be picked repeatedly, creating an infinite loop. The fix is to add Move X to Pile 2 after picking the card.

Advanced Questions

Show Answer

sql
Max = -INFINITY
Min = INFINITY
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X > Max) {
        Max = X
    }
    if (X < Min) {
        Min = X
    }
}

This requires only one pass (one iteration) to find both. Q10. Trace the execution: What does this pseudocode output for dataset [2, 5, 1, 8, 3]?

sql
Result = 0
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    if (X > Result) {
        Result = X
    }
    else {
        Result = Result - X
    }
}

Show Answer

Final Result = 5 Q11. What is the "standard dataset iteration pattern"? Why is every piece necessary? Show Answer

The standard pattern is:

sql
Initialize variables
while (Pile 1 has more cards) {
    Pick a card X from Pile 1
    Move X to Pile 2
    // Process X
}

Each piece is necessary:

• Initialize: Without this, variables start with garbage values
• Check condition: Ensures we stop when done
• Pick a card: Gets the next item
• Move to Pile 2: Ensures progress (prevents infinite loop)
• Process: Does the actual work Q12. Compare and contrast: flowchart vs pseudocode. When would you use each? Show Answer

Use flowcharts when explaining to beginners or debugging logic visually. Use pseudocode when writing real algorithms, collaborating, or preparing to code.

📚 Cross-References

Next Topic: 04 — Iteration & Filtering

Quiz Tip: Quiz 1 heavily tests your ability to trace pseudocode. Practice with the tracing tables! Join Discord PreviousData Types & RepresentationNextIteration & Filtering