Teacher Guide: Iteration (Loops)

Student page: Iteration IB Syllabus: B2.3.3 Estimated periods: 2–3 (90 min each) Prerequisites: Variables & Data Types, Selection

Contents

1. Lesson Plans
2. Differentiation
3. Answer Keys
4. Integration Notes
   1. Additional Resources

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Lesson Plans

Period 1: for and while loops

Phase	Time	Activity	Student Page Section
Warm-up	10 min	“Print 1 to 100 without a loop” — absurdity motivates loops	—
Teach	25 min	for loop anatomy (init, condition, update), while loop, loop choice guide	Key Concepts
Practice	40 min	Core #1-3 (Countdown, Sum of range, Even numbers)	Practice Exercises
Wrap-up	15 min	Quick Code Check MCQs	Quick Code Check

Teaching notes:

• Three questions that map to for loop: “What repeats? What changes? What stays the same?”
• Loop choice guide: for = known count, while = condition-based (may run 0 times), do-while = must run once
• Infinite loop deliberately: let students write one in controlled environment, then ask “What was missing?”

Additional activities:

• Scanner deep-dive: hasNextInt(), buffer/token/delimiter, nextInt()/nextLine() skip bug
• Guessing game with extensions (attempt counter, difficulty levels, limited attempts)

Period 2: do-while, nested loops, sentinel patterns

Phase	Time	Activity	Student Page Section
Warm-up	10 min	“A menu must display before the user makes a choice — which loop?”	—
Teach	20 min	do-while for input validation/menus, nested loops, sentinel value pattern	Worked Examples
Practice	45 min	Core #4, Extension #5-7	Practice Exercises
Wrap-up	15 min	Trace exercise from student page	Trace Exercise

Additional activities:

• Grade statistics program (max/min/average with sentinel value)
• Pair exchange: students swap solutions, give feedback on optimization

Period 3 (optional): Pattern programming and challenges

Phase	Time	Activity	Student Page Section
Warm-up	10 min	Draw a triangle of stars on paper — describe the pattern in words	—
Teach	15 min	Pattern analysis: rows vs columns, relationship between outer/inner loop	—
Practice	50 min	Extension #6 (right triangle), Challenge #8-9 (pyramid, menu)	Practice Exercises
Wrap-up	15 min	Pattern programming showcase — students present solutions	—

Additional pattern programming activities:

• Triangle of stars, chessboard (modulus), diagonal, cross, hollow square, number pyramid
• Each pattern reinforces the relationship between outer and inner loop variables

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Differentiation

Supporting weaker students:

Strategy	When to use	Example
Loop anatomy diagram	Can’t remember for loop parts	Label: for (init; condition; update) with arrows
While loop template	Confuses while and for	Provide skeleton: int i = 0; while (i < ???) { ... i++; }
Trace first, code second	Can’t write loop from scratch	Fill in trace table for sum 1-10, then translate to code
Reduce iteration count	Overwhelmed by debugging	“Print 1 to 5” before “1 to 100” — same logic, easier to verify

Extending stronger students:

Strategy	When to use	Example
Pattern programming	Finishes Core early	Chessboard (modulus), cross, hollow square
Guessing game	After Core #1	Random number guessing game with attempt counter
Challenge exercises	Extended learners	#8 Number pyramid, #9 Menu system
Big O preview	After nested loops	“How many times does the inner print run for n rows?” → n²
Fibonacci sequence	After sentinel pattern	Generate first n Fibonacci numbers using a loop

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Answer Keys

Core 1: Countdown timer

import java.util.Scanner;
public class Countdown {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Start from: ");
        int n = scanner.nextInt();
        while (n >= 1) {
            System.out.print(n + " ");
            n--;
        }
        System.out.println("Go!");
        scanner.close();
    }
}

Expected output (input 5): 5 4 3 2 1 Go!

Common mistakes:

• Using n > 0 (works but n >= 1 is clearer)
• Forgetting to decrement n (infinite loop)
• Printing “Go!” inside the loop

Marking notes: Accept for loop version. Key: must count DOWN, must print “Go!” after loop ends.

Core 2: Sum of range

public class SumOfRange {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        System.out.println("Sum: " + sum);
    }
}

Expected output: Sum: 5050

Common mistakes:

• Starting i at 0 instead of 1
• Using < 100 instead of <= 100
• Not initializing sum to 0

Marking notes: Gauss formula 100*101/2 is mathematically correct but misses the point (using a loop).

Core 3: Even number printer

public class EvenPrinter {
    public static void main(String[] args) {
        for (int i = 2; i <= 50; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();
    }
}

Expected output: 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Common mistakes:

• Using i++ and checking i % 2 == 0 inside (works but less efficient)
• Starting at 0

Marking notes: Accept both approaches (start at 2 step 2, or start at 1 step 1 with if). Both are correct.

Core 4: Input validation

import java.util.Scanner;
public class InputValidation {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int score;
        do {
            System.out.print("Enter a score (0-100): ");
            score = scanner.nextInt();
            if (score < 0 || score > 100) {
                System.out.println("Invalid. Must be 0-100.");
            }
        } while (score < 0 || score > 100);
        System.out.println("Valid score: " + score);
        scanner.close();
    }
}

Common mistakes:

• Using while instead of do-while (doesn’t prompt before first check)

• Wrong condition logic (using && instead of

  )

Marking notes: MUST use do-while as specified. The condition score < 0 || score > 100 means “keep looping while invalid.”

Extension 5: FizzBuzz

public class FizzBuzz {
    public static void main(String[] args) {
        for (int i = 1; i <= 30; i++) {
            if (i % 3 == 0 && i % 5 == 0) {
                System.out.println("FizzBuzz");
            } else if (i % 3 == 0) {
                System.out.println("Fizz");
            } else if (i % 5 == 0) {
                System.out.println("Buzz");
            } else {
                System.out.println(i);
            }
        }
    }
}

Common mistakes:

• Checking % 3 and % 5 separately BEFORE the combined check — prints “Fizz” for 15 instead of “FizzBuzz”. Order matters: combined check MUST come first.

Marking notes: The order of conditions is the key teaching point. Accept i % 15 == 0 as equivalent to i % 3 == 0 && i % 5 == 0.

Extension 6: Right triangle

import java.util.Scanner;
public class RightTriangle {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter height: ");
        int height = scanner.nextInt();
        for (int i = 1; i <= height; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print("* ");
            }
            System.out.println();
        }
        scanner.close();
    }
}

Expected output (height 4):

*
* *
* * *
* * * *

Common mistakes:

• Inner loop condition wrong (using j <= height instead of j <= i)
• Forgetting println() after inner loop

Marking notes: Key insight: inner loop runs i times, not height times. The connection between outer and inner loop variable IS the pattern.

Extension 7: Average calculator (sentinel)

import java.util.Scanner;
public class AverageCalc {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int count = 0;
        int sum = 0;
        System.out.print("Enter a number (-1 to stop): ");
        int num = scanner.nextInt();
        while (num != -1) {
            sum += num;
            count++;
            System.out.print("Enter a number (-1 to stop): ");
            num = scanner.nextInt();
        }
        if (count > 0) {
            double avg = (double) sum / count;
            System.out.println("Count: " + count);
            System.out.println("Sum: " + sum);
            System.out.println("Average: " + avg);
        } else {
            System.out.println("No numbers entered.");
        }
        scanner.close();
    }
}

Common mistakes:

• Including -1 in the sum/count
• Division by zero when no numbers entered
• Integer division for average

Marking notes: MUST handle the edge case of no numbers entered (count == 0). Must NOT include sentinel in calculations. Must use double division for average.

Challenge 8: Number pyramid

import java.util.Scanner;
public class NumberPyramid {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter number of rows: ");
        int rows = scanner.nextInt();
        int num = 1;
        for (int i = 1; i <= rows; i++) {
            // Print leading spaces
            for (int s = 1; s <= rows - i; s++) {
                System.out.print(" ");
            }
            // Print numbers for this row
            for (int j = 1; j <= i; j++) {
                System.out.print(num + " ");
                num++;
            }
            System.out.println();
        }
        scanner.close();
    }
}

Expected output (4 rows):

   1
  2 3
 4 5 6
7 8 9 10

Common mistakes:

• Resetting num each row
• Wrong number of leading spaces
• Not incrementing num correctly

Marking notes: Three nested patterns: spaces, numbers, newline. The counter num is NOT reset per row — it carries across rows. This is the key insight.

Challenge 9: Simple menu system

import java.util.Scanner;
public class MenuCalc {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int choice;
        do {
            System.out.println("\n--- Calculator Menu ---");
            System.out.println("1. Add");
            System.out.println("2. Subtract");
            System.out.println("3. Multiply");
            System.out.println("4. Quit");
            System.out.print("Choice: ");
            choice = scanner.nextInt();
            if (choice >= 1 && choice <= 3) {
                System.out.print("Enter first number: ");
                double a = scanner.nextDouble();
                System.out.print("Enter second number: ");
                double b = scanner.nextDouble();
                if (choice == 1) {
                    System.out.println("Result: " + (a + b));
                } else if (choice == 2) {
                    System.out.println("Result: " + (a - b));
                } else {
                    System.out.println("Result: " + (a * b));
                }
            } else if (choice != 4) {
                System.out.println("Invalid choice.");
            }
        } while (choice != 4);
        System.out.println("Goodbye!");
        scanner.close();
    }
}

Common mistakes:

• Using while instead of do-while (menu must show before first choice)
• Not handling invalid choices
• Forgetting the quit condition

Marking notes: MUST use do-while. Must loop back to menu after each operation. Must handle invalid choice gracefully. Accept switch-case alternative.

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Integration Notes

Item	In Class	Homework	Link
Worked Examples	Walk through together	—	Worked Examples
Quick Code Check	End of Period 1	—	Quick Code Check
Core #1-3	Period 1 practice	—	Practice Exercises
Core #4, Extension #5-7	Period 2 practice	Complete for homework	Practice Exercises
Challenge #8-9	Period 3 (optional)	Optional extension	Practice Exercises
Trace Exercise	Period 1 or 2 wrap-up	—	Trace Exercise
GitHub Classroom	—	Due end of week	GitHub Classroom

Additional Resources

• Pattern programming sheet (6 patterns: triangle, chessboard, diagonal, cross, hollow square, number pyramid)
• Guessing game starter code (enrichment activity)
• Scanner buffer clearing reference sheet (nextInt()/nextLine() skip bug)