Java for Loops

Master the most fundamental iteration construct in Java: control flow with initialization, condition, and update in a single, powerful statement.

AP Computer Science A

What Is a Loop?

A loop is a control structure that repeats a block of code multiple times. Instead of writing the same statement over and over, a loop lets you automate repetition with just a few lines.

Why Use a for Loop?

A for loop is ideal when you know in advance how many times the code should repeat. It bundles three parts into one clean header: initialization, condition, and update.

Counting

Iterate from \( 0 \) to \( n - 1 \) to process each element.

Accumulating

Sum values, build strings, or aggregate results step by step.

Searching

Scan through data to find a target value or compute a maximum.

Anatomy of a for Loop

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

Initialization

int i = 0 — runs once before the loop starts.

Condition

i < 5 — checked before each iteration. Loop continues while true.

Update

i++ — executes after each iteration of the body.

Execution Flow

The Three Parts in Detail

1

Initialization: declares and sets the loop variable. Runs exactly once. The variable's scope is limited to the loop.

2

Boolean condition: evaluated before each iteration. Must be a boolean expression. When false, the loop terminates.

3

Update: modifies the loop variable after each pass. Common forms: i++, i--, i += 2.

for vs while

	for Loop	while Loop
Best when	Number of iterations is known	Number of iterations is unknown
Structure	Init, condition, update in one line	Condition only; init/update are separate
Variable scope	Loop variable is local to the loop	Variable must be declared outside
Risk of infinite loop	Lower: update is built-in	Higher: easy to forget update
Example use	Traverse an array by index	Read input until sentinel value

How to Write a for Loop

1

Identify the range: What is the starting value? What is the stopping value? How do you step through?

2

Write the header: for (int i = start; i < stop; i += step)

3

Write the body: Place the repeated action inside braces { }.

4

Trace the values: Mentally step through \( i = 0, 1, 2, \ldots \) to verify the output.

5

Check boundaries: Does the loop run the correct number of times? Watch for off-by-one errors.

Example 1: Print Numbers

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

Example 2: Sum from 1 to \( n \)

int n = 5;
int sum = 0;
for (int i = 1; i <= n; i++) {
    sum += i;
}
System.out.println(sum); // 15

Example 3: Counting Backwards

for (int i = 5; i >= 1; i--) {
    System.out.print(i + " ");
}
// Output: 5 4 3 2 1

Nested for Loops

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 4; col++) {
        System.out.print("* ");
    }
    System.out.println();
}

How Many Iterations?

For a standard loop for (int i = a; i < b; i++), the number of iterations is:

Loop Header	Iterations	Values of i
i = 0; i < 5	5	0, 1, 2, 3, 4
i = 1; i <= 5	5	1, 2, 3, 4, 5
i = 3; i < 10	7	3, 4, 5, 6, 7, 8, 9
i = 0; i < n	\( n \)	0, 1, ..., \( n - 1 \)

Traversing an Array

int[] scores = {85, 92, 78, 95, 88};
for (int i = 0; i < scores.length; i++) {
    System.out.println("Score " + i + ": " + scores[i]);
}

Common Mistakes

Off-by-One Error

Using <= when you mean < (or vice versa), causing one too many or too few iterations.

Infinite Loop

Forgetting to change the loop variable, or updating it in the wrong direction (e.g., i++ when counting down).

Out-of-Bounds Access

Using i <= arr.length instead of i < arr.length, which accesses an index that does not exist.

Summary Table

Concept	Key Point
Structure	for (init; condition; update) { body }
Initialization	Runs once; declares the loop variable
Condition	Checked before each iteration; must be boolean
Update	Runs after each iteration (e.g., i++)
Iteration count	\( b - a \) for i = a; i < b
Array traversal	Use i < array.length
Nested loops	Inner loop completes fully per outer iteration

Question 1 of 5

What is the output of the following code?

for (int i = 1; i <= 3; i++) {
    System.out.print(i * 2 + " ");
}

The loop runs for i = 1, 2, 3. Each iteration prints \( i \times 2 \), giving 2, 4, 6.

Question 2 of 5

How many times does the following loop execute its body?

for (int k = 0; k < 10; k += 3) {
    System.out.println(k);
}

The variable k takes values 0, 3, 6, 9. When k becomes 12, the condition \( k < 10 \) is false. That is 4 iterations.

Question 3 of 5

What is the value of result after the loop?

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

This computes \( 4! = 1 \times 2 \times 3 \times 4 = 24 \). The loop multiplies result by each value of i.

Question 4 of 5

Which loop correctly prints the values 10, 8, 6, 4, 2?

We need to start at 10, go down by 2, and include 2. Option B: i = 10; i >= 2; i -= 2 produces 10, 8, 6, 4, 2. Option D misses the value 2 because i > 2 excludes it.

Question 5 of 5

What happens when this code runs?

int[] arr = {3, 7, 1};
for (int i = 0; i <= arr.length; i++) {
    System.out.println(arr[i]);
}

The array has length 3, so valid indices are 0, 1, 2. The condition i <= arr.length allows i = 3, which is out of bounds. The fix is to use i < arr.length.