for Loop

The for loop is the most common way to repeat code when you know how many times you want to run.

Basic Syntax

for (initialization; condition; update) {
    // body, runs while condition is true
}

1. Initialization runs once at the start
2. Condition is checked before each iteration; if false, the loop stops
3. Body runs if the condition is true
4. Update runs after the body, then goes back to step 2

for (int i = 0; i < 5; i = i + 1) {
    print(i);
}
// Output: 0 1 2 3 4

Step by step:

Iteration	i	i < 5	Action
Before	0		initialize
1	0	true	print(0), then i = 0 + 1
2	1	true	print(1), then i = 1 + 1
3	2	true	print(2), then i = 2 + 1
4	3	true	print(3), then i = 3 + 1
5	4	true	print(4), then i = 4 + 1
6	5	false	stop

Looping Through Arrays

Use for to access each element of an array by index:

int[] arr = [10, 20, 30, 40];
for (int i = 0; i < 4; i = i + 1) {
    print(arr[i]);
}
// Output: 10 20 30 40

Nested for Loops

A loop inside another loop, useful for working with tables or grids:

for (int i = 1; i <= 3; i = i + 1) {
    for (int j = 1; j <= 3; j = j + 1) {
        print(i * j);
    }
}
// Output: 1 2 3 2 4 6 3 6 9

Optional Parts

Each of the three parts in a for loop is optional. You can write an infinite loop by leaving them all empty:

for ( ; ; ) {
    // runs forever (until break or return)
}

Or skip the initialization if the variable already exists:

int i = 0;
for ( ; i < 5; i = i + 1) {
    print(i);
}

break and continue

break (exit the loop immediately)

for (int i = 1; i <= 5; i = i + 1) {
    if (i == 3) { break; }
    print(i);
}
// Output: 1 2

continue (skip to the next iteration)

continue skips the rest of the body but still runs the update step:

for (int i = 1; i <= 5; i = i + 1) {
    if (i == 2) { continue; }
    if (i == 4) { continue; }
    print(i);
}
// Output: 1 3 5  (2 and 4 skipped, but their i++ still ran)

Bubble Sort Example

int[] arr = [5, 2, 8, 1, 9];
int n = 5;

for (int i = 0; i < n - 1; i = i + 1) {
    for (int j = 0; j < n - i - 1; j = j + 1) {
        if (arr[j] > arr[j + 1]) {
            int tmp = arr[j];
            arr[j] = arr[j + 1];
            arr[j + 1] = tmp;
        }
    }
}

for (int k = 0; k < n; k = k + 1) {
    print(arr[k]);
}
// Output: 1 2 5 8 9

Advanced Patterns

Counting down

The update step can decrease the counter instead of increasing it:

for (int i = 3; i > 0; i = i - 1) {
    print(i);
}
// Output: 3 2 1

Stepping by more than one

for (int i = 0; i < 10; i = i + 2) {
    print(i);
}
// Output: 0 2 4 6 8

Iterating an array by its length

Instead of hard-coding the size, use .length() so the loop adapts if the array changes:

int[] arr = [10, 20, 30, 40];
for (int i = 0; i < arr.length(); i = i + 1) {
    print(arr[i]);
}

The loop variable is scoped to the loop

A variable declared in the initialization exists only inside the loop:

for (int i = 0; i < 3; i = i + 1) {
    print(i);
}
// print(i);    ERROR, 'i' does not exist out here

Declare it before the loop if you need the final value afterward:

int i = 0;
for ( ; i < 3; i = i + 1) { }
print(i);       // 3, 'i' survives because it was declared outside

Common Mistake: Infinite Loop

If the condition never becomes false, the loop never stops:

for (int i = 0; i >= 0; i = i + 1) {
    print(i);  // runs forever, i is always >= 0
}

What’s Next?

• Use while loops when you don’t know the number of iterations
• Try do-while loops when you need at least one run