Unit 2 - Selection and Iteration¶

AP Exam Weighting: 25–35%

2.1 Algorithms with Selection and Repetition¶

Goals

Understand how algorithms can use selection (if/else) and repetition (loops) to solve problems.
Trace through algorithms to predict their output.

An algorithm is a step-by-step set of instructions to solve a problem. In Java, algorithms use:

Selection: Choosing different paths based on conditions (if, if/else, switch)
Repetition: Repeating steps until a condition is met (while, for, do-while)

2.2 Boolean Expressions¶

Goals

Understand Boolean expressions.
Use relational operators to compute the value of a Boolean expression.
- I will be able to write a program that utilizes a Boolean expression

George Boole formulated the basic rules of Boolean algebra back in 1847. Below are the relational operators that you will need to know for the AP exam.

Relational Operator	Description	Boolean Value
a < b	a less than b	Equates to ‘true’ if a is less than b,’false’ otherwise
a <= b	a less than or equal to b	Equates to ‘true’ if a is less than or equal to b, ‘false’ otherwise.
a > b	a greater than b	Equates to ‘true’ if a is greater than b, ‘false’ otherwise.
a >= b	a greater than or equal to b	Equates to ‘true’ if a is greater than or equal to b, ‘false’ otherwise.
a == b	a equals b*	Equates to ‘true’ if a has the same value as b.
a != b	a does not equal b	Equates to ‘true’ if a does not have the same value as b, ‘false’ otherwise.

Logical Operators

Logical operators are used to combine Boolean expressions:

AND Operator ( && ) – if( a && b ) [if true execute else don't]
OR Operator ( || ) – if( a || b) [if one of them is true execute else don't]
NOT Operator ( ! ) – !(a<b) [returns false if a is smaller than b]

Short-Circuit Evaluation: When using &&, if the first expression is false, the second is never evaluated. When using ||, if the first expression is true, the second is never evaluated.

A	!A
false	true
true	false

Activity 2.2.1¶

Fill in the truth table for a && (b || c):

a	b	c
T	T	T
T	T	F
T	F	T
T	F	F
F	T	T
F	T	F
F	F	T
F	F	F

Solution

a	b	c	b \|\| c	a && (b \|\| c)
T	T	T	T	T
T	T	F	T	T
T	F	T	T	T
T	F	F	F	F
F	T	T	T	F
F	T	F	T	F
F	F	T	T	F
F	F	F	F	F

Activity 2.2.2 - Boolean Expression¶

Activity 2.2.3 - Comparing Objects with .equals()¶

IMPORTANT: Object Equality

== compares object references (memory locations). To compare the values of two objects, use .equals():

String s1 = new String("HELLO");
String s2 = new String("HELLO");

System.out.println(s1 == s2);      // false – different memory locations
System.out.println(s1.equals(s2)); // true  – same content

2.3 if Statements¶

Goals

Write if statements to control the flow of a program.
I will be able to write a program that uses if/else statements.

if (condition) {
    // code executed when condition is true
}

Activity 2.3.1 - guessChecker Pt 1¶

Activity 2.3.2 - Sevens Game¶

2.4 Nested if Statements¶

Goals

Write nested if/else if/else statements for multi-branch logic.

if (score >= 90) {
    grade = "A";
} else if (score >= 80) {
    grade = "B";
} else if (score >= 70) {
    grade = "C";
} else {
    grade = "F";
}

Activity 2.4.1 - guessChecker Pt 2¶

Activity 2.4.2 - Activity Director¶

Activity 2.4.3 - Smallest Divisible Number¶

2.4.1 Switch Statements — Oracle Foundations 1Z0-811¶

Oracle Exam Note: switch statements and the ternary operator are tested on the Oracle Java Foundations exam (1Z0-811).

A switch statement is an alternative to long if/else if chains when testing a single variable against multiple constant values.

int day = 3;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    case 3:
        System.out.println("Wednesday");
        break;
    default:
        System.out.println("Other day");
}

Important rules:

Each case ends with break to prevent fall-through (executing subsequent cases).
default is optional and runs if no case matches — similar to a final else.
switch works with int, char, String, and enum types.

Fall-Through Example:

int month = 4;
switch (month) {
    case 4:
    case 6:
    case 9:
    case 11:
        System.out.println("30 days");
        break;
    case 2:
        System.out.println("28 or 29 days");
        break;
    default:
        System.out.println("31 days");
}

Ternary Operator — Oracle Foundations 1Z0-811¶

The ternary operator ? : is a compact one-line if/else:

// Syntax: condition ? valueIfTrue : valueIfFalse
int age = 17;
String status = (age >= 18) ? "adult" : "minor";
System.out.println(status);  // minor

int a = 10, b = 20;
int max = (a > b) ? a : b;   // max = 20

Use the ternary operator for simple conditional assignments; prefer if/else for more complex logic.

Activity 2.4.1.1 - Text-Based Calculator with Mode Selection¶

Write a Java program that:

Asks the user to choose a mode: basic, scientific, or stats
Uses a switch statement to route to the correct mode
Within each mode, uses ternary operators for simple comparisons (e.g., “result is positive/negative”)
Includes a default case for invalid input

2.5 Compound Boolean Expressions¶

Goals

Combine multiple conditions using &&, ||, and !.

// Compound condition: between 18 and 65
if (age >= 18 && age <= 65) {
    System.out.println("Working age");
}

// Compound condition: invalid input
if (score < 0 || score > 100) {
    System.out.println("Invalid score");
}

Activity 2.5.1¶

Activity 2.5.2¶

Activity 2.5.3 - guessChecker Pt 3¶

2.6 Comparing Boolean Expressions¶

Goals

Understand equivalent Boolean expressions.
Apply De Morgan’s Laws.

De Morgan’s Laws:

!(a && b)  ==  (!a || !b)    // NOT (A AND B) equals (NOT A) OR (NOT B)
!(a || b)  ==  (!a && !b)    // NOT (A OR B) equals (NOT A) AND (NOT B)

Activity 2.6.1¶

Activity 2.6.2¶

2.7 while Loops¶

Goals

Write while loops to repeat code while a condition is true.
I will be able to write a program that uses while loops.

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}
// Prints 1 through 5

Important: Make sure the loop condition eventually becomes false; otherwise you create an infinite loop.

Activity 2.7.1¶

Activity 2.7.2¶

2.7a do-while Loops — Oracle Foundations 1Z0-811¶

Oracle Exam Note: do-while loops are tested on the Oracle Java Foundations exam (1Z0-811) but are not assessed on the AP CSA exam.

A do-while loop executes the code block at least once before checking the condition — the opposite of a while loop.

//Do While Example
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner posNum = new Scanner(System.in);
        int number;

        do {
            System.out.print("Enter a positive number: ");
            number = posNum.nextInt();
            if (number <= 0) {
                System.out.println("Please enter a positive number.");
            }
        } while (number <= 0);  // checks AFTER the block executes

        System.out.println("You entered a positive number: " + number);
    }
}

When to use do-while: Use it when you need to guarantee the loop body runs at least once, such as menu-driven programs or input validation.

Activity 2.7a.1 - Guessing Game¶

Write a java program that asks the user to pick a number between 1 - 100. Use a do-while loop to compare a randomly generated number with the user’s guess. Tell the user how close they are if incorrect. End with a congratulatory message when correct. Include the Java Time API and test cases.

Activity 2.7a.2 - Convert program¶

Activity 2.7a.3 - Age/Name¶

Activity 2.7a.4 - `char` — Oracle¶

Break and Continue Keywords — Oracle Foundations 1Z0-811¶

break immediately exits a loop. continue skips the rest of the current iteration and moves to the next.

// break example: stop at first even number
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        System.out.println("First even: " + i);
        break;
    }
}

// continue example: skip odd numbers
for (int i = 1; i <= 10; i++) {
    if (i % 2 != 0) continue;
    System.out.println(i);  // prints 2 4 6 8 10
}

Activity 2.7a.5 - Break & Continue¶

2.8 for Loops¶

Goals

Write for loops for counter-controlled repetition.

// Standard for loop
for (int i = 0; i < 5; i++) {
    System.out.println(i);   // 0, 1, 2, 3, 4
}

IMPORTANT: off-by-one error¶

The most common loop mistake is iterating one too many or too few times:

// WRONG: runs 6 times (0-5) when you want 5 iterations (0-4)
for (int i = 0; i <= 5; i++) { ... }

// CORRECT: runs 5 times (0-4)
for (int i = 0; i < 5; i++) { ... }

Activity 2.8.1¶

Activity 2.8.2 - Factorial X¶

Activity 2.8.3¶

2.9 Implementing Selection and Iteration Algorithms¶

Goals

Combine if/else and loops to implement common algorithms.

Project Mad Libs¶

Apply all you’ve learned about conditionals and loops to create an interactive Mad Libs program.

2.10 Implementing String Algorithms¶

Goals

Use String methods combined with loops to process text.

// Count vowels in a string
String word = "programming";
int vowelCount = 0;
for (int i = 0; i < word.length(); i++) {
    char c = word.charAt(i);
    if ("aeiouAEIOU".indexOf(c) >= 0) {
        vowelCount++;
    }
}
System.out.println("Vowels: " + vowelCount);

Activity 2.10.1 - EveryOther, vowels, dbpq¶

Activity 2.10.2 - wordCount/vowelCount/Hexadecimal¶

2.11 Nested Iteration¶

Goals

Write nested loops (a loop inside another loop).
Determine how many times nested loops execute.

// Nested loop: print multiplication table
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.printf("%4d", i * j);
    }
    System.out.println();
}

Activity 2.11.1 - Java Patterns¶

2.12 Informal Run-Time Analysis¶

Goal

Determine the number of times a code segment will execute.

What is Informal Code Analysis?

Informal code analysis involves reviewing and reasoning about code without executing it — also known as code tracing. Successful code tracing requires you to pay attention to the logic, notating how many times code executes, all variable values, and what output is produced.

Key Concepts:

Tracing Code: Follow the flow of logic and variable values.
Identifying Errors: Spot compile-time errors (syntax) and runtime errors (NullPointerException, array bounds violations).
Understanding Behavior: Predict what a program will output for given inputs.
Checking for Logic Errors: Find faults in logic that produce incorrect results.

Hand-Tracing Tools:

https://pythontutor.com/render.html#mode=display

https://cscircles.cemc.uwaterloo.ca/java_visualize/

Summary of Informal Analysis Process:

Understand the Problem: Read the prompt carefully.
Simulate Execution: Walk through the code logically.
Check for Errors: Syntax, runtime, logic.
Test Edge Cases: null, empty arrays, extreme values.

Project 2 — Choose Your Own Adventure¶

Goals

Apply all you’ve learned about conditionals to create a “Choose Your Own Adventure” game.
Apply the development process to create a project.
Each student will create their own project

Requirements:

At least three Boolean expressions that change the path in the program
At least three nested if/else-if/else clauses
Print statements depending on current and previous choices

You Must Include:

Digital flowchart (appropriate shapes and lines)
Digital timeline chart
Main Block Method comment header

Deliverables: Flowchart, timeline, quality screenshots of output, and completed .java file submitted as one PDF.

Java Time API — Oracle Foundations 1Z0-811¶

Oracle Exam Note: The java.time package (Calendar data) is tested on the Oracle Java Foundations exam (1Z0-811).

Java 8 introduced the java.time package to fix issues with the older Date and Calendar classes.

Concept	Class	Example	Description
Representing a date	`LocalDate`	`LocalDate.of(2024, 8, 26)`	Date (no time zone)
Representing a time	`LocalTime`	`LocalTime.now()`	Time (no date)
Representing both	`LocalDateTime`	`LocalDateTime.now()`	Date and time combined
Formatting	`DateTimeFormatter`	`DateTimeFormatter.ofPattern("MM-dd-yyyy")`	Format into readable string
Date differences	`Period`	`Period.between(d1, d2)`	Years, months, days between dates

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class DateExample {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        System.out.println("Today's date: " + today);

        LocalDate firstDay = LocalDate.of(2024, 8, 26);
        System.out.println("First day of school: " + firstDay);

        // Format a date
        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("MMMM d, yyyy");
        System.out.println(today.format(fmt));

        // Measure elapsed time
        LocalDateTime start = LocalDateTime.now();
        // ... do something ...
        LocalDateTime end = LocalDateTime.now();
        System.out.println("Elapsed: " + start.until(end, java.time.temporal.ChronoUnit.MILLIS) + " ms");
    }
}

Unit 2 - Selection and Iteration¶

2.1 Algorithms with Selection and Repetition¶

2.2 Boolean Expressions¶

Activity 2.2.1¶

Activity 2.2.2 - Boolean Expression¶

Activity 2.2.3 - Comparing Objects with .equals()¶

2.3 if Statements¶

Activity 2.3.1 - guessChecker Pt 1¶

Activity 2.3.2 - Sevens Game¶

2.4 Nested if Statements¶

Activity 2.4.1 - guessChecker Pt 2¶

Activity 2.4.2 - Activity Director¶

Activity 2.4.3 - Smallest Divisible Number¶

2.4.1 Switch Statements — Oracle Foundations 1Z0-811¶

Ternary Operator — Oracle Foundations 1Z0-811¶

Activity 2.4.1.1 - Text-Based Calculator with Mode Selection¶

2.5 Compound Boolean Expressions¶

Activity 2.5.1¶

Activity 2.5.2¶

Activity 2.5.3 - guessChecker Pt 3¶

2.6 Comparing Boolean Expressions¶

Activity 2.6.1¶

Activity 2.6.2¶

2.7 while Loops¶

Activity 2.7.1¶

Activity 2.7.2¶

2.7a do-while Loops — Oracle Foundations 1Z0-811¶

Activity 2.7a.1 - Guessing Game¶

Activity 2.7a.2 - Convert program¶

Activity 2.7a.3 - Age/Name¶

Activity 2.7a.4 - char — Oracle¶

Break and Continue Keywords — Oracle Foundations 1Z0-811¶

Activity 2.7a.5 - Break & Continue¶

2.8 for Loops¶

IMPORTANT: off-by-one error¶

Activity 2.8.1¶

Activity 2.8.2 - Factorial X¶

Activity 2.8.3¶

2.9 Implementing Selection and Iteration Algorithms¶

Project Mad Libs¶

2.10 Implementing String Algorithms¶

Activity 2.10.1 - EveryOther, vowels, dbpq¶

Activity 2.10.2 - wordCount/vowelCount/Hexadecimal¶

2.11 Nested Iteration¶

Activity 2.11.1 - Java Patterns¶

2.12 Informal Run-Time Analysis¶

Project 2 — Choose Your Own Adventure¶

Java Time API — Oracle Foundations 1Z0-811¶

Activity — Birthday Countdown¶

Activity — Date Formatting Challenge¶

Activity — Event Timer¶

Activity 2.7a.4 - `char` — Oracle¶