In this unit, you’ll learn how programs:

• Decide what to do using boolean expressions and if statements
• Repeat code using while and for loops
• Combine decisions and repetition to implement algorithms

Main topics:

• Relational & logical operators (>, <, ==, !=, &&, ||, !)
• Boolean expressions and truth tables
• if, else-if, else, and nested conditionals
• De Morgan’s Law and simplifying conditions
• while loops, for loops, and nested loops
• Loop patterns: counting, summing, searching
• Off-by-one errors and infinite loops
• Tracing and debugging selection + iteration

A boolean expression is an expression that evaluates to either true or false. You’ll use these inside if-statements and loops.

Relational Operators

Examples

int x = 7;
boolean a = (x > 5);    // true
boolean b = (x == 7);   // true
boolean c = (x != 7);   // false

Logical operators combine or modify boolean values:

• && (AND) – true only if both conditions are true
• || (OR) – true if at least one condition is true
• ! (NOT) – reverses a boolean value

Truth Table

NOT Operator

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

Operator Precedence (Simplified)

• Relational: >, <, >=, <=, ==, !=
• Logical NOT: !
• Logical AND: &&
• Logical OR: ||

Short-Circuit Evaluation

Java uses short-circuiting:

• A && B → if A is false, B is never evaluated
• A || B → if A is true, B is never evaluated

if (x != 0 && 10 / x > 1) {
    // safe: 10/x only happens if x != 0
}

De Morgan’s Law lets you distribute a NOT over an AND/OR expression:

Example

Suppose we want to check that a value x is outside the range 1 to 10:

// x is NOT between 1 and 10 inclusive:
if (x < 1 || x > 10) {
    ...
}

// Equivalent to:
if (!(x >= 1 && x <= 10)) {
    ...
}

Selection lets your program choose between different paths of execution.

Basic if Statement

if (condition) {
    // runs only if condition is true
}

if-else

if (score >= 60) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

if-else-if Chain

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

Example – Two Separate if Statements

int x = 12;

if (x % 2 == 0)
    System.out.println("even");

if (x % 3 == 0)
    System.out.println("divisible by 3");

Output:

• even
• divisible by 3

Because these are two separate if statements, both can run.

You can place an if-statement inside another if or else block. This is called a nested conditional.

Nested Example

if (age >= 16) {
    if (hasPermit) {
        System.out.println("You can drive.");
    } else {
        System.out.println("Get your permit first.");
    }
} else {
    System.out.println("Too young to drive.");
}

Range Checking Pattern

if (score < 0 || score > 100) {
    System.out.println("Invalid score");
} else if (score >= 90) {
    grade = 'A';
} else if (score >= 80) {
    grade = 'B';
} else if (score >= 70) {
    grade = 'C';
} else {
    grade = 'D';
}

Multi-Condition Decisions

if (gpa >= 3.5 && communityServiceHours >= 50) {
    eligibleForScholarship = true;
}

Example 1

int x = 5;
if (x > 10)
    System.out.println("big");
else if (x > 3)
    System.out.println("medium");
else
    System.out.println("small");

Output: medium

Example 2

int a = 4;
int b = 7;

if (a > b || b % 2 == 1)
    System.out.println("A");
else if (a % 2 == 0 && b % 2 == 0)
    System.out.println("B");
else
    System.out.println("C");

Output: A (because b % 2 == 1 is true)

Example 3

boolean hot = true;
boolean sunny = false;

if (!hot && sunny)
    System.out.println("Cool day");
else if (hot && !sunny)
    System.out.println("Humid");
else
    System.out.println("Mixed");

Output: Humid

Iteration means repeating a block of code multiple times. In Java, the main loop constructs are:

• while loops – repeat while a condition is true
• for loops – repeat a specific number of times
• nested loops – loops inside loops

A while loop repeats as long as its condition evaluates to true.

while (condition) {
    // body runs repeatedly
}

Counting Example

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

Output: 0 1 2 3 4

Key Points

• The condition is checked before each iteration.
• The loop body may run zero times if the condition starts false.
• You must change something inside the loop so the condition eventually becomes false.

Common Mistake – Infinite Loop

int n = 10;
while (n > 0) {
    System.out.println(n);
    // n never changes → infinite loop
}

Before moving on to classes (Unit 3) and data collections (Unit 4), you should be fully comfortable with loops that operate on:

• integers (counting, summing, reversing)
• Strings (traversal, searching, building new strings)
• boolean conditions (flags, early exit with break)
• sentinel logic (looping until a condition becomes false)

These loop skills are essential for the AP CSA exam and appear heavily in MCQs and early FRQs—even before arrays show up.

Strings are perfect for practicing iteration. You can examine, count, or modify characters using length() and charAt(i).

Example: Count the vowels in a String

String word = "computer science";
int count = 0;

for (int i = 0; i < word.length(); i++) {
    char ch = word.charAt(i);
    if ("aeiouAEIOU".indexOf(ch) != -1) {
        count++;
    }
}

System.out.println("Vowels: " + count);
    

Example: Reverse a String

String original = "hello";
String reversed = "";

for (int i = original.length() - 1; i >= 0; i--) {
    reversed += original.charAt(i);
}

System.out.println(reversed); // "olleh"
    

1. Counter Loop

Counts occurrences of something:

int count = 0;
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        count++;
    }
}
System.out.println(count); // 5 even numbers
    

2. Accumulator Loop

Adds values together:

int sum = 0;
for (int i = 1; i <= 100; i++) {
    sum += i;
}
System.out.println(sum); // 5050
    

3. Flag Loop

Stops early when something is found:

String s = "aP cSa test";
boolean found = false;

for (int i = 0; i < s.length(); i++) {
    if (Character.isUpperCase(s.charAt(i))) {
        found = true;
        break;
    }
}

System.out.println(found); // true
    

4. Sentinel Loop (While Loop)

Scanner input = new Scanner(System.in);
System.out.print("Enter a positive number (-1 to stop): ");
int num = input.nextInt();

while (num != -1) {
    System.out.println("You entered: " + num);
    num = input.nextInt();
}
    

Example: Count the digits in a number

int n = 483902;
int digits = 0;

while (n > 0) {
    n /= 10;   // remove last digit
    digits++;
}

System.out.println(digits); // 6
    

Example: Check if a number is prime

int num = 37;
boolean isPrime = true;

for (int i = 2; i < num; i++) {
    if (num % i == 0) {
        isPrime = false;
        break;
    }
}

System.out.println(isPrime);
    

Example: Sum digits of a number

int n2 = 762;
int sumDigits = 0;

while (n2 > 0) {
    sumDigits += n2 % 10;
    n2 /= 10;
}

System.out.println(sumDigits); // 15
    

Triangle Pattern

for (int row = 1; row <= 4; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print("*");
    }
    System.out.println();
}
    

Grid Pattern

for (int r = 1; r <= 3; r++) {
    for (int c = 1; c <= 5; c++) {
        System.out.print("#");
    }
    System.out.println();
}
    

Try these practice problems to test your understanding of Unit 2 loops before you move on:

• Write a loop that counts how many characters in a String are digits (hint: Character.isDigit()).
• Given a String, determine whether it contains three consecutive vowels.
• Use a while loop to reverse an integer (for example, 12345 becomes 54321).
• Write a for loop that prints every other character in a String starting at index 0.
• Write a nested loop that prints a 5x5 grid of numbers from 1 to 25 in row-major order.

Question 1

What is printed by the following code?

int x = 7;

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

• A. A
• B. B
• C. Nothing
• D. Causes a runtime error

Answer: A
Both conditions (x > 5 and x < 10) are true, so the if block runs.

Question 2

What is the value of count after this code runs?

int count = 0;

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

• A. 2
• B. 3
• C. 4
• D. 5

Answer: B
Numbers divisible by 3 between 1 and 10 are 3, 6, and 9 → 3 total.

Question 3

What is printed?

int x = 0;

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

• A. Yes
• B. No
• C. Runtime error
• D. Nothing

Answer: B
Because of short-circuit evaluation, 10 / x is never evaluated when x != 0 is false.

Question 4

What is printed?

int n = 5;

while (n > 0) {
    n -= 2;
    System.out.print(n + " ");
}

• A. 3 1 -1
• B. 3 1
• C. 5 3 1
• D. Infinite loop

Answer: A
Values printed after subtraction: 3, 1, -1. Loop stops once n is no longer > 0.

Question 5

Which boolean expression correctly checks if x is between 10 and 20 inclusive?

• A. x >= 10 || x <= 20
• B. x > 10 && x < 20
• C. x >= 10 && x <= 20
• D. !(x < 10 && x > 20)

Answer: C
Inclusive range requires both bounds using &&.

Question 6

What is printed?

String s = "java";
int i = 0;

while (i < s.length()) {
    if (s.charAt(i) == 'a') {
        System.out.print(i + " ");
    }
    i++;
}

• A. 0 2
• B. 1 3
• C. 2 4
• D. 1 2

Answer: B
Indices: j=0, a=1, v=2, a=3 → 'a' occurs at indices 1 and 3.

Question 7

Which loop correctly prints the numbers 1 through 5?

• A.

  for (int i = 1; i < 5; i++)

• B.

  for (int i = 0; i <= 5; i++)

• C.

  for (int i = 1; i <= 5; i++)

• D.

  for (int i = 5; i >= 1; i++)

Answer: C
Starts at 1 and includes 5 → classic counting loop.

Question 8

What is printed?

int x = 10;

if (x > 5)
    if (x < 8)
        System.out.println("A");
    else
        System.out.println("B");

• A. A
• B. B
• C. Nothing
• D. Compile-time error

Answer: B
The else pairs with the nearest unmatched if.

Question 9

What is the output?

int sum = 0;

for (int i = 1; i <= 4; i++) {
    sum += i;
}

System.out.println(sum);

• A. 6
• B. 10
• C. 12
• D. 16

Answer: B
Accumulator pattern: 1 + 2 + 3 + 4 = 10.

Question 10

Which statement about loops is TRUE?

• A. A while loop always runs at least once.
• B. A for loop cannot be infinite.
• C. Loop variables must be declared outside the loop.
• D. A loop must eventually make its condition false.

Answer: D
Every correct loop must change state so its condition eventually becomes false.

Once you're comfortable with loops, boolean logic, and String traversal, you're ready for Unit 3: Class Creation, where you'll learn how to design your own classes, write constructors, and define methods.

Continue your AP CSA path here:
Continue to AP CSA Unit 3: Class Creation →