📚 Unit Overview

Unit 2: Selection and Iteration is one of the most critical units in AP Computer Science A. This unit combines decision-making (selection) with repetition (iteration) to form the foundation of algorithmic problem-solving.

What You'll Learn

• How programs make decisions using boolean expressions and if statements
• How programs repeat code using while and for loops
• How to combine selection and iteration to implement algorithms
• How to analyze code efficiency through informal runtime analysis
• How to trace and debug code involving conditionals and loops

Unit 2 Topics at a Glance

🔢 Boolean Expressions & Relational Operators

A boolean expression is an expression that evaluates to either true or false. Boolean expressions are the foundation of all decision-making in Java programs.

The boolean Data Type

Java has a primitive data type called boolean that can hold exactly two values: true or false.

boolean isRaining = true;
boolean hasUmbrella = false;
boolean isSunny = !isRaining;  // false

// Boolean expressions evaluate to boolean values
int age = 18;
boolean isAdult = age >= 18;   // true

Relational Operators

Comparing Strings: equals() vs ==

One of the most common mistakes in Java is using == to compare Strings. Always use the equals() method to compare String content.

String s1 = "Hello";
String s2 = "Hello";
String s3 = new String("Hello");

// Using == (compares references) - WRONG!
System.out.println(s1 == s2);     // true (due to String pooling)
System.out.println(s1 == s3);     // false (different objects!)

// Using equals() (compares content) - CORRECT!
System.out.println(s1.equals(s2)); // true
System.out.println(s1.equals(s3)); // true

🔗 Logical Operators & Truth Tables

Logical operators allow you to combine multiple boolean expressions into more complex conditions.

The AND Operator (&&)

Returns true only if BOTH conditions are true.

int age = 25;
boolean hasLicense = true;

// Both conditions must be true
boolean canDrive = age >= 16 && hasLicense;  // true

// Practical example: checking a range
int temperature = 72;
boolean comfortable = temperature >= 65 && temperature <= 78;  // true

The OR Operator (||)

Returns true if AT LEAST ONE condition is true.

boolean isWeekend = true;
boolean isHoliday = false;

// At least one must be true
boolean dayOff = isWeekend || isHoliday;  // true

The NOT Operator (!)

Reverses a boolean value: true becomes false and vice versa.

boolean isRaining = true;
boolean notRaining = !isRaining;  // false

Truth Tables

Short-Circuit Evaluation

Java uses short-circuit evaluation for && and ||:

• For &&: If the first condition is false, the second is never evaluated
• For ||: If the first condition is true, the second is never evaluated

// Short-circuit prevents division by zero!
int x = 0;
if (x != 0 && 10 / x > 1) {
    System.out.println("Safe division");
}
// x != 0 is false, so 10/x is NEVER evaluated (no error!)

🔄 De Morgan's Law

De Morgan's Law provides rules for distributing a NOT operator over AND and OR expressions.

The Two Rules

Memory trick: When you distribute the NOT, the operator flips (AND becomes OR, OR becomes AND).

De Morgan's Law Reference Table

Practical Example

// Checking if x is NOT between 1 and 10 (inclusive)

// Method 1: Direct negation
if (!(x >= 1 && x <= 10)) {
    System.out.println("x is outside the range");
}

// Method 2: Applying De Morgan's Law (equivalent)
if (x < 1 || x > 10) {
    System.out.println("x is outside the range");
}

🔀 Selection with if Statements

Selection statements allow your program to make decisions and execute different code based on conditions.

Basic if Statement

if (condition) {
    // code executes only if condition is true
}

// Example
int number = 5;
if (number > 0) {
    System.out.println(number + " is positive");
}

if-else Statement

int score = 55;
if (score >= 60) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}
// Output: Fail

if-else-if Chain

Only the FIRST true condition's block executes—once a block runs, the rest are skipped.

int score = 85;
String grade;

if (score >= 90) {
    grade = "A";
} else if (score >= 80) {
    grade = "B";
} else if (score >= 70) {
    grade = "C";
} else if (score >= 60) {
    grade = "D";
} else {
    grade = "F";
}
System.out.println("Grade: " + grade);  // Output: Grade: B

Multiple Independent if Statements

Separate if statements (without else) are evaluated independently—multiple blocks can execute:

int x = 12;

// These are SEPARATE if statements - all can execute
if (x % 2 == 0) {
    System.out.println("x is even");
}
if (x % 3 == 0) {
    System.out.println("x is divisible by 3");
}
if (x > 10) {
    System.out.println("x is greater than 10");
}

// Output:
// x is even
// x is divisible by 3
// x is greater than 10

🔁 Iteration with while Loops

The while loop is the most fundamental form of iteration—it repeats as long as a condition remains true.

while Loop Syntax

while (condition) {
    // loop body - executes repeatedly while condition is true
    // MUST modify something to eventually make condition false!
}

// Example: Count from 1 to 5
int count = 1;
while (count <= 5) {
    System.out.println(count);
    count++;  // CRITICAL: update the loop control variable
}
// Output: 1 2 3 4 5

The Three Parts of a while Loop

Common while Loop Patterns

int n = 12345;
int sum = 0;
while (n > 0) {
    sum += n % 10;  // Add last digit to sum
    n /= 10;        // Remove last digit
}
System.out.println("Sum of digits: " + sum);  // Output: 15

int num = 12345;
int count = 0;
while (num > 0) {
    count++;       // Count digits
    num /= 10;     // Remove last digit
}
System.out.println("Digits: " + count);  // Output: 5

int original = 12345;
int reversed = 0;
int num = original;

while (num > 0) {
    int digit = num % 10;              // Get last digit
    reversed = reversed * 10 + digit;  // Append to result
    num /= 10;                         // Remove last digit
}
System.out.println("Reversed: " + reversed);  // Output: 54321

🔂 Iteration with for Loops

The for loop is ideal when you know in advance how many times you want to repeat. It combines initialization, condition, and update into a single, compact header line.

for Loop Syntax

for (initialization; condition; update) {
    // loop body
}

// Example: Print 0 through 4
for (int i = 0; i < 5; i++) {
    System.out.print(i + " ");
}
// Output: 0 1 2 3 4

// Example: Count by 2s
for (int i = 0; i <= 10; i += 2) {
    System.out.print(i + " ");
}
// Output: 0 2 4 6 8 10

for Loop Execution Order

1. Initialization executes ONCE at the very beginning
2. Condition is checked BEFORE each iteration
3. If condition is true, the loop body executes
4. Update executes AFTER each iteration of the body
5. Return to step 2 and repeat

for Loop vs while Loop Equivalence

// for loop version
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// Equivalent while loop version
int i = 0;           // Initialization moved before loop
while (i < 5) {      // Condition stays the same
    System.out.println(i);
    i++;             // Update moved to end of body
}

🔄 Nested Loops

A nested loop is a loop inside another loop. The inner loop completes all its iterations for each single iteration of the outer loop.

Basic Nested Loop

for (int outer = 1; outer <= 3; outer++) {
    for (int inner = 1; inner <= 4; inner++) {
        System.out.print("(" + outer + "," + inner + ") ");
    }
    System.out.println();
}

/* Output:
(1,1) (1,2) (1,3) (1,4)
(2,1) (2,2) (2,3) (2,4)
(3,1) (3,2) (3,3) (3,4)

Total iterations: 3 × 4 = 12
*/

Calculating Nested Loop Iterations

To find the total number of times a nested loop body executes, multiply the iteration counts:

// How many times does "X" print?
for (int i = 0; i < 5; i++) {        // 5 iterations
    for (int j = 0; j < 3; j++) {    // 3 iterations each
        System.out.print("X");
    }
}
// Answer: 5 × 3 = 15 times

Common Pattern: Triangle

// Print a right triangle
for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print("*");
    }
    System.out.println();
}
/* Output:
*
**
***
****
*****
*/

📝 String Algorithms

String processing is a major topic in AP CSA. You'll use loops to traverse strings character by character.

Essential String Methods

String Traversal

// String traversal using substring()
String word = "Java";
for (int i = 0; i < word.length(); i++) {
    String letter = word.substring(i, i + 1);
    System.out.print(letter + " ");
}
// Output: J a v a

Common String Algorithms

public int countVowels(String str) {
    String vowels = "aeiouAEIOU";
    int count = 0;
    
    for (int i = 0; i < str.length(); i++) {
        String letter = str.substring(i, i + 1);
        if (vowels.indexOf(letter) != -1) {
            count++;
        }
    }
    return count;
}

public String reverse(String str) {
    String reversed = "";
    for (int i = str.length() - 1; i >= 0; i--) {
        reversed += str.substring(i, i + 1);
    }
    return reversed;
}

// Usage
String result = reverse("hello");  // Returns "olleh"

public boolean isPalindrome(String str) {
    String reversed = "";
    for (int i = str.length() - 1; i >= 0; i--) {
        reversed += str.substring(i, i + 1);
    }
    return str.equals(reversed);
}

// Usage
isPalindrome("racecar");  // Returns true
isPalindrome("hello");    // Returns false

public String removeChar(String str, String toRemove) {
    String result = "";
    for (int i = 0; i < str.length(); i++) {
        String current = str.substring(i, i + 1);
        if (!current.equals(toRemove)) {
            result += current;
        }
    }
    return result;
}

// Usage
removeChar("Mississippi", "s");  // Returns "Miiippi"

⚠️ Common Errors and How to Avoid Them

1. Infinite Loops

// WRONG: n never changes
int n = 10;
while (n > 0) {
    System.out.println(n);
    // Missing: n--;
}

// CORRECT
int n = 10;
while (n > 0) {
    System.out.println(n);
    n--;  // Update the variable!
}

2. Off-By-One Errors

// WRONG: Prints 1 through 4 (one too few)
for (int i = 1; i < 5; i++) {
    System.out.print(i + " ");  // Output: 1 2 3 4
}

// CORRECT: Prints 1 through 5
for (int i = 1; i <= 5; i++) {
    System.out.print(i + " ");  // Output: 1 2 3 4 5
}

3. Using == with Strings

// WRONG
if (s1 == s2) { ... }

// CORRECT
if (s1.equals(s2)) { ... }

4. Misplaced Semicolons

// WRONG: Semicolon after if
if (x > 5);  // Empty statement!
{
    System.out.println("x is large");  // ALWAYS runs
}

// CORRECT
if (x > 5) {
    System.out.println("x is large");
}

📝 Practice Questions

int x = 7;
if (x > 5 && x < 10) {
    System.out.println("A");
} else {
    System.out.println("B");
}

int count = 0;
for (int i = 1; i <= 10; i++) {
    if (i % 3 == 0) {
        count++;
    }
}

int x = 0;
if (x != 0 && 10 / x > 1) {
    System.out.println("Yes");
} else {
    System.out.println("No");
}

for (int i = 0; i < 4; i++) {
    for (int j = 0; j < 3; j++) {
        System.out.print("X");
    }
}

int sum = 0;
for (int i = 1; i <= 5; i++) {
    sum += i;
}
System.out.println(sum);

📋 Quick Reference

Relational Operators

Logical Operators

De Morgan's Law

Common Loop Patterns

// Counter
int count = 0;
for (...) { if (condition) count++; }

// Accumulator
int sum = 0;
for (...) { sum += value; }

// String traversal
for (int i = 0; i < str.length(); i++) {
    String ch = str.substring(i, i + 1);
}

// Digit extraction
while (n > 0) {
    int digit = n % 10;
    n /= 10;
}