Teacher Guide: Selection (if/else)

Student page: Selection IB Syllabus: B2.3.2 Estimated periods: 2 (90 min each) Prerequisites: Variables & Data Types

Contents

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

---

Lesson Plans

Period 1: if, if-else, if-else if-else

Phase	Time	Activity	Student Page Section
Warm-up	10 min	“A student scored 75. What grade should they get?” — describe decision in English	—
Teach	25 min	if/else, else if chains, boolean operators, condition order	Key Concepts
Practice	40 min	Core #1-4 (Odd/even, Largest of three, Letter grade, Pos/neg/zero)	Practice Exercises
Wrap-up	15 min	Quick Code Check MCQs	Quick Code Check

Teaching notes:

• Plain English → flowchart → code progression
• Key point: Java evaluates top to bottom, takes FIRST true branch. Demo: check score >= 40 before >= 90, show 95 → wrong grade
• Reinforce .equals() for String comparison NOW

Additional HW exercises (selection only — no loops):

• Smallest of three, color by number, guess secret number, average/min/max without loops, range validation, withdrawal approval
• “Do not use a loop” constraint reinforces selection practice before loops are introduced
• Include pseudocode alongside Java — connects to computational thinking

Period 2: Nested conditions, boolean logic, switch

Phase	Time	Activity	Student Page Section
Warm-up	10 min	“Can you combine two conditions? How?” — introduce && and ||	—
Teach	15 min	Nested if, boolean operators, switch-case	Worked Examples
Practice	50 min	Extension #5-7, Challenge #8-9	Practice Exercises
Wrap-up	15 min	Trace exercise from student page + review	Trace Exercise

Additional activities:

• Flow of control overview: sequential → selection → iteration → subroutines
• Withdrawal approval exercise: combines amount > 0, amount <= balance, and confirmation — good compound condition practice

---

Differentiation

Supporting weaker students:

Strategy	When to use	Example
Flowchart first	Can’t translate problem to code	Draw decision tree for letter grade before coding
Condition builder worksheet	Can’t write boolean expressions	Fill-in: “Check if age is between 13 and 64” → age >= 13 && age <= 64
Reduce to Core #1-2	Overwhelmed	Odd/even and largest of three — simplest selection patterns
Pair programming	During Core exercises	Stronger student explains condition order

Extending stronger students:

Strategy	When to use	Example
Extra HW exercises	Finishes Core early	Smallest of three, guess secret number, withdrawal approval
Challenge exercises	Extended learners	#8 Triangle classifier, #9 Leap year
Short-circuit evaluation	After boolean operators	“Does Java evaluate the second condition if the first is false in &&?”
Pseudocode translation	After Challenge	Write pseudocode for triangle classifier, then implement

---

Answer Keys

Core 1: Odd or even

import java.util.Scanner;
public class OddOrEven {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = scanner.nextInt();
        if (num % 2 == 0) {
            System.out.println(num + " is even");
        } else {
            System.out.println(num + " is odd");
        }
        scanner.close();
    }
}

Expected output: 7 → 7 is odd, 4 → 4 is even

Common mistakes: Using num / 2 == 0 instead of num % 2 == 0

Marking notes: Accept num % 2 != 0 for odd check

Core 2: Largest of three

import java.util.Scanner;
public class LargestOfThree {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter three numbers: ");
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        int c = scanner.nextInt();
        int largest;
        if (a >= b && a >= c) {
            largest = a;
        } else if (b >= c) {
            largest = b;
        } else {
            largest = c;
        }
        System.out.println("Largest: " + largest);
        scanner.close();
    }
}

Common mistakes: Not handling equal values; using nested if instead of && compound condition

Marking notes: Accept Math.max approach if students discover it, but not required

Core 3: Letter grade

import java.util.Scanner;
public class LetterGrade {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter score (0-100): ");
        int score = scanner.nextInt();
        int grade;
        if (score >= 90) {
            grade = 7;
        } else if (score >= 80) {
            grade = 6;
        } else if (score >= 70) {
            grade = 5;
        } else if (score >= 60) {
            grade = 4;
        } else if (score >= 50) {
            grade = 3;
        } else if (score >= 40) {
            grade = 2;
        } else {
            grade = 1;
        }
        System.out.println("IB Grade: " + grade);
        scanner.close();
    }
}

Common mistakes: Wrong condition order (checking >= 40 first catches everything); using == instead of >=

Marking notes: Grade boundaries may vary — accept any reasonable set. Key skill is correct ordering of else-if chain.

Core 4: Positive, negative, or zero

import java.util.Scanner;
public class PosNegZero {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = scanner.nextInt();
        if (num > 0) {
            System.out.println("Positive");
        } else if (num < 0) {
            System.out.println("Negative");
        } else {
            System.out.println("Zero");
        }
        scanner.close();
    }
}

Common mistakes: Using num == 0 first and missing the else-if pattern

Extension 5: Ticket pricing

import java.util.Scanner;
public class TicketPricing {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter age: ");
        int age = scanner.nextInt();
        if (age < 0) {
            System.out.println("Invalid age");
        } else if (age < 5) {
            System.out.println("Free");
        } else if (age <= 12) {
            System.out.println("$8");
        } else if (age <= 64) {
            System.out.println("$15");
        } else {
            System.out.println("$10");
        }
        scanner.close();
    }
}

Common mistakes: Missing the negative age validation; using > instead of >= for boundary ages

Marking notes: Must validate negative age. Accept any reasonable output format for prices.

Extension 6: Quadrant finder

import java.util.Scanner;
public class QuadrantFinder {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter x: ");
        int x = scanner.nextInt();
        System.out.print("Enter y: ");
        int y = scanner.nextInt();
        if (x == 0 || y == 0) {
            System.out.println("On an axis");
        } else if (x > 0 && y > 0) {
            System.out.println("Quadrant I");
        } else if (x < 0 && y > 0) {
            System.out.println("Quadrant II");
        } else if (x < 0 && y < 0) {
            System.out.println("Quadrant III");
        } else {
            System.out.println("Quadrant IV");
        }
        scanner.close();
    }
}

Common mistakes: Not checking for axis case (x==0 or y==0) — must check this first; confusing quadrant numbering

Extension 7: Day of the week

import java.util.Scanner;
public class DayOfWeek {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter day number (1-7): ");
        int day = scanner.nextInt();
        String name;
        if (day == 1) {
            name = "Monday";
        } else if (day == 2) {
            name = "Tuesday";
        } else if (day == 3) {
            name = "Wednesday";
        } else if (day == 4) {
            name = "Thursday";
        } else if (day == 5) {
            name = "Friday";
        } else if (day == 6) {
            name = "Saturday";
        } else if (day == 7) {
            name = "Sunday";
        } else {
            name = "Invalid";
        }
        System.out.println(name);
        scanner.close();
    }
}

Common mistakes: Forgetting the “Invalid” case for numbers outside 1-7

Marking notes: Accept switch-case as alternative; starting from Sunday or Monday both acceptable

Challenge 8: Triangle classifier

import java.util.Scanner;
public class TriangleClassifier {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter side a: ");
        int a = scanner.nextInt();
        System.out.print("Enter side b: ");
        int b = scanner.nextInt();
        System.out.print("Enter side c: ");
        int c = scanner.nextInt();
        // Triangle inequality: sum of any two sides must be greater than the third
        if (a + b > c && a + c > b && b + c > a) {
            if (a == b && b == c) {
                System.out.println("Equilateral");
            } else if (a == b || a == c || b == c) {
                System.out.println("Isosceles");
            } else {
                System.out.println("Scalene");
            }
        } else {
            System.out.println("Not a valid triangle");
        }
        scanner.close();
    }
}

Common mistakes: Forgetting triangle inequality check; not checking all three pairs for isosceles

Marking notes: Must validate triangle inequality BEFORE classifying. All three inequality conditions required.

Challenge 9: Leap year checker

import java.util.Scanner;
public class LeapYear {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a year: ");
        int year = scanner.nextInt();
        boolean isLeap;
        if (year % 400 == 0) {
            isLeap = true;
        } else if (year % 100 == 0) {
            isLeap = false;
        } else if (year % 4 == 0) {
            isLeap = true;
        } else {
            isLeap = false;
        }
        if (isLeap) {
            System.out.println(year + " is a leap year");
        } else {
            System.out.println(year + " is not a leap year");
        }
        scanner.close();
    }
}

Common mistakes: Wrong order of checks (must check % 400 before % 100); not understanding the century exception

Marking notes: Order matters — % 400 → % 100 → % 4. Accept one-line boolean: (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0). Key test cases: 2000 (leap), 1900 (not leap), 2024 (leap), 2023 (not leap).

---

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 #1-4	Period 1 practice	—	Practice Exercises
Extension #5-7	Start in Period 2	Complete for homework	Practice Exercises
Challenge #8-9	—	Optional extension	Practice Exercises
Trace Exercise	Period 2 wrap-up	—	Trace Exercise
GitHub Classroom	—	Due end of week	GitHub Classroom

Additional Resources

• Extra HW exercises (selection only, no loops): smallest of three, color by number, guess secret number, avg/min/max without loops, range validation, withdrawal approval
• Flowchart templates (blank decision diamonds for students to fill in)
• Pseudocode ↔ Java translation worksheet