AP Computer Science A - Unit 4 Review: Lesson 4

Overview This lesson explores the three main types of loops in Java: for, while, and do-while. It also demonstrates practical applications such as reversing a string using a for loop and introduces the concept of nested loops, which are especially useful when working with multi-dimensional data structures.

1. For Loops

Purpose: Repeat a block of code a specific number of times.

Structure:

for (initialization; test; update) {
    // body of loop
}

Example: Printing 0 to 3

for (int i = 0; i < 4; i++) {
    System.out.println(i);
}

Execution Steps:

• Initialize i = 0
• Test i < 4: If true, execute body
• Print value of i
• Update i++
• Repeat until i < 4 is false

2. While Loops

Purpose: Repeat a block of code while a condition is true.

Structure:

while (condition) {
    // body of loop
}

Example: Countdown from 4

int i = 4;
while (i > 0) {
    System.out.println(i);
    i--;
}

Notes:

• Be cautious of infinite loops. If i is never changed inside the loop, it runs forever.
• A while loop may never run if the initial condition is false.

3. Do-While Loops

Purpose: Like a while loop, but guarantees the body executes at least once.

Structure:

do {
    // body of loop
} while (condition);

Example:

int i = -2;
do {
    System.out.println(i);
    i--;
} while (i > 0);

Note: Ends with a semicolon after the condition.

4. String Reversal Using For Loop

Goal: Reverse the characters in a string.

Example Code:

String original = "pupils";
String reverse = "";

for (int i = original.length() - 1; i >= 0; i--) {
    reverse += original.substring(i, i + 1);
}

System.out.println(reverse); // Output: slipup

Key Concepts:

• substring(i, i + 1) extracts one character at a time.
• Strings are 0-indexed; loop starts at length - 1.

5. Nested Loops

Definition: A loop inside another loop.

Example:

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        System.out.println("i = " + i + ", j = " + j);
    }
}

Execution:

• Outer loop runs twice (i = 0 and i = 1)
• For each outer loop run, inner loop runs three times (j = 0, 1, 2)
• Total iterations of inner loop: 2 * 3 = 6

Use Cases:

• Two-dimensional arrays
• Complex pattern generation

Summary Table

Key Terms:

• Counter Variable: Tracks iterations (e.g., i, j)
• Condition/Test Expression: Boolean controlling loop continuation
• Update Expression: Changes counter after each iteration
• Infinite Loop: A loop that never ends due to a non-false condition
• Substring Method: substring(start, end) retrieves part of a string (exclusive of end)

Next Steps:

• Practice writing loops with different conditions and structures
• Trace loop execution step-by-step to predict output
• Try modifying the string reversal algorithm to skip vowels
• Implement a nested loop to print a multiplication table

End of Lesson 4 Review