Module 06 — Loops

1. What Are Loops?

A loop repeats a block of code. Instead of writing the same lines over and over, you write them once and tell Java how many times — or under what conditions — to keep running them.

Here is the difference without and with a loop. Imagine printing "Hello!" five times:

// Without a loop — tedious, doesn't scale
System.out.println("Hello!");
System.out.println("Hello!");
System.out.println("Hello!");
System.out.println("Hello!");
System.out.println("Hello!");

// With a loop — one statement, any number of times
for (int i = 0; i < 5; i++) {
    System.out.println("Hello!");
}

Want to print it a thousand times? Change one number. The code stays the same size.

Why Loops Matter in Real Programs

• Processing lists — read every item in a shopping cart, every student in a grade book
• Input validation — keep prompting until the user enters something valid
• Reading files — process each line until you reach the end
• Calculations — sum values, count occurrences, find a minimum or maximum
• Retrying operations — try a network request again if it fails

Java's Three Loop Statements

2. The while Loop

The while loop is the fundamental loop in Java. It checks a condition before each iteration. If the condition is true, the body runs. Then it checks again. This continues until the condition is false.

Syntax

while (condition) {
    // body — runs as long as condition is true
}

Because the condition is checked before the body runs, a while loop is called a pre-test loop. If the condition is false on the very first check, the body never executes at all.

A Counter-Controlled while Loop

When you use a counter variable to control a while loop, you need four things: initialize, check, do work, update.

int count = 1;               // 1. Initialize before the loop

while (count <= 5) {         // 2. Check the condition
    System.out.println(count); // 3. Do the work
    count++;                   // 4. Update — or you get an infinite loop!
}

Trace it mentally: count starts at 1, prints 1, becomes 2. Prints 2, becomes 3. … Prints 5, becomes 6. Condition 6 <= 5 is false — loop ends.

A Sentinel-Controlled while Loop

When you don't know in advance how many times to loop, use a sentinel value — a special input that signals "I'm done." The loop keeps running until it sees that value.

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
System.out.print("Enter a number (-1 to quit): ");
int number = sc.nextInt();

while (number != -1) {                      // -1 is the sentinel
    System.out.println("You entered: " + number);
    System.out.print("Enter a number (-1 to quit): ");
    number = sc.nextInt();
}
System.out.println("Goodbye!");

Notice that the prompt and read appear twice: once before the loop (to get the first value) and once at the end of the loop body (to get the next value). This is the standard pattern for sentinel loops.

int x = 1;
while (x < 10) {
    System.out.println(x);
    // BUG: x never changes, so x < 10 is always true — infinite loop!
}

3. The do-while Loop

The do-while loop is almost identical to while, with one important difference: it runs the body first, then checks the condition. That makes it a post-test loop — the body always executes at least once.

Syntax

do {
    // body — runs at least once, guaranteed
} while (condition);   // <-- note the semicolon here!

The semicolon after the closing parenthesis is required. Java will give you a compile error if you leave it out.

Classic Use Case: Input Validation

You need to show the user a prompt at least once before you can check what they typed. do-while handles this perfectly:

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
int age;

do {
    System.out.print("Enter your age (0-120): ");
    age = sc.nextInt();
} while (age < 0 || age > 120);   // keep asking if invalid

System.out.println("Age recorded: " + age);

With a regular while loop, you would have to read the first value before the loop and read again inside the loop — duplicating the prompt and read statements. do-while keeps it in one place.

do-while vs. while: Side by Side

4. The for Loop

When you know exactly how many times to loop, the for loop is the cleanest choice. It bundles the initialization, condition check, and update all into one header line.

Syntax

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

• Initialization — runs exactly once, before the loop starts. Usually declares and sets a counter variable.
• Condition — checked before every iteration. Loop runs while it is true.
• Update — runs after every iteration. Usually increments or decrements the counter.

Execution Order

Counting Up

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

Counting Down

for (int i = 5; i >= 1; i--) {
    System.out.println(i);
}
System.out.println("Done!");

Counting by 2s

for (int i = 0; i <= 10; i += 2) {
    System.out.print(i + " ");
}
System.out.println();

When to Use for vs. while

Use for when the number of iterations is known before the loop starts. Use while when the loop should continue based on something you discover while running. In practice: counting, iterating over arrays → for; reading input until valid, processing until a sentinel → while.

// BAD — skips every other number
for (int i = 0; i < 10; i++) {
    System.out.println(i);
    i++;   // oops — double increment, skips 1, 3, 5, 7, 9
}

5. Loop Control: break and continue

Sometimes you need to exit a loop before the condition naturally becomes false, or skip the rest of one iteration and jump straight to the next. break and continue handle those cases.

break — Exit the Loop Immediately

When Java hits break, it exits the loop entirely. Execution jumps to the first statement after the closing brace.

for (int i = 1; i <= 10; i++) {
    if (i == 7) {
        System.out.println("Found 7! Stopping early.");
        break;
    }
    System.out.println(i);
}
System.out.println("After the loop.");

A common use: searching an array for a value. Once you find it, break so you don't waste time checking the rest.

continue — Skip to the Next Iteration

continue skips the remaining statements in the current iteration. In a for loop, it jumps to the update step. In a while loop, it jumps back to the condition check.

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;     // skip the rest of this iteration for even numbers
    }
    System.out.print(i + " ");
}
System.out.println();

Use Both Sparingly

break and continue are legitimate tools, but overusing them makes loops hard to read and reason about. If you're reaching for them frequently, your condition logic probably needs a rewrite instead.

6. Nested Loops

A nested loop is a loop inside another loop. The outer loop drives the rows; the inner loop drives the columns. Every time the outer loop runs once, the inner loop runs all the way through.

How Iteration Counts Multiply

Outer runs 3 times × inner runs 4 times = 12 total executions of the inner body. This multiplying effect matters a lot when you're dealing with large data sets — but for classroom-sized examples it's perfectly fine.

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

Example: Multiplication Table

for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= 5; col++) {
        System.out.printf("%4d", row * col);
    }
    System.out.println();   // move to next row after each row is complete
}

Example: Triangle of Stars

The inner loop runs a different number of times on each row — exactly row times — because the inner loop's limit is the outer loop variable:

for (int row = 1; row <= 5; row++) {
    for (int star = 1; star <= row; star++) {
        System.out.print("*");
    }
    System.out.println();
}

7. Common Loop Patterns

These patterns appear constantly in real programs. Learn to recognize and write them without thinking.

Running Total (Accumulator)

Initialize a variable to zero before the loop. Add each new value to it inside the loop.

int sum = 0;   // accumulator — starts at 0

for (int i = 1; i <= 10; i++) {
    sum += i;
}

System.out.println("Sum 1 to 10: " + sum);

Counting Occurrences

Same idea as an accumulator, but you only add 1 when a specific condition is met.

int evenCount = 0;

for (int i = 1; i <= 20; i++) {
    if (i % 2 == 0) {
        evenCount++;
    }
}

System.out.println("Even numbers from 1-20: " + evenCount);

Finding the Maximum

Initialize max to the first value (or the smallest possible integer). Update it whenever you find something bigger.

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
System.out.print("How many numbers? ");
int n = sc.nextInt();

System.out.print("Enter number 1: ");
int max = sc.nextInt();   // seed max with the first value

for (int i = 2; i <= n; i++) {
    System.out.print("Enter number " + i + ": ");
    int val = sc.nextInt();
    if (val > max) {
        max = val;
    }
}

System.out.println("Maximum: " + max);

Input Validation Loop

Keep asking until the user provides acceptable input. This is the do-while pattern from Section 3, but worth calling out explicitly as its own pattern:

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
int choice;

do {
    System.out.println("Choose an option:");
    System.out.println("  1. Start");
    System.out.println("  2. Settings");
    System.out.println("  3. Quit");
    System.out.print("Enter 1-3: ");
    choice = sc.nextInt();
} while (choice < 1 || choice > 3);

System.out.println("You chose: " + choice);

8. Common Loop Mistakes

These are the bugs that show up on nearly every intro Java exam. Know what they look like.

Off-by-One Error

Looping one too many or one too few times. Usually a wrong comparison operator (< vs. <=).

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

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

Always ask: "Should the loop run when i equals the boundary value?" If yes, use <=. If no, use <.

Infinite Loop (Forgot to Update Counter)

int i = 0;
while (i < 5) {
    System.out.println(i);
    // BUG: i never changes — loops forever!
}

// Fix: add i++ inside the body
int i = 0;
while (i < 5) {
    System.out.println(i);
    i++;   // now the condition eventually becomes false
}

Wrong Comparison Operator

// BUG: condition is false immediately — loop never runs
for (int i = 10; i < 1; i++) {
    System.out.println(i);
}

// Fix: count down if starting high
for (int i = 10; i >= 1; i--) {
    System.out.println(i);
}

Modifying the Loop Variable Inside a for Loop Body

// BUG: i is incremented by the header AND by the body
// Skips 1, 3, 5, 7, 9 — prints only 0, 2, 4, 6, 8
for (int i = 0; i < 10; i++) {
    System.out.println(i);
    i++;   // accidental double increment
}

9. The Enhanced for Loop (for-each)

Java has a fourth loop syntax designed specifically for iterating over arrays and collections. It is called the enhanced for loop, or informally the for-each loop. You will use it extensively once we cover arrays in a later module — here's a quick preview.

Syntax

for (dataType variableName : arrayOrCollection) {
    // use variableName here
}

Read the colon as "in". So for (int score : scores) means "for each int named score in the scores array."

Example with an Array

int[] scores = {85, 92, 78, 96, 88};

for (int score : scores) {
    System.out.println("Score: " + score);
}

Accumulator with for-each

int[] scores = {85, 92, 78, 96, 88};
int total = 0;

for (int score : scores) {
    total += score;
}

double average = (double) total / scores.length;
System.out.printf("Average: %.1f%n", average);

When to Use for-each vs. Regular for

Vocabulary Review

Module 06 Quiz

Name: _________________________________    Date: _____________    Score: _____ / 10

int x = 10;
while (x > 7) {
    System.out.print(x + " ");
    x--;
}

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

for (int i = 1; i <= 6; i++) {
    if (i % 3 == 0) {
        continue;
    }
    System.out.print(i + " ");
}

int total = 0;
for (int i = 1; i <= 4; i++) {
    total += i;
}
System.out.println(total);

int count = 0;
while (count < 5) {
    System.out.println("Count: " + count);
}

✅ Answer Key