for Loops

When you know how many times to repeat — or you're walking through a range of numbers or the items of a collection — the for loop is the natural choice. It keeps the counter's start, condition and update together in one line, and the enhanced for-each loop makes iterating over arrays and lists effortless.

The counting for loop

JAVA

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

Output

Step 1
Step 2
Step 3
Step 4
Step 5

The header has three parts separated by semicolons:

Initialisation — int i = 1, runs once before the loop.
Condition — i <= 5, checked before each repetition.
Update — i++, runs after each repetition.

The variable i exists only inside the loop.

Other step sizes

JAVA

void main() {
    for (int i = 10; i > 0; i -= 3) {
        System.out.print(i + " ");
    }
    System.out.println();
    for (int i = 0; i <= 100; i += 25) {
        System.out.print(i + " ");
    }
    System.out.println();
}

Output

10 7 4 1
0 25 50 75 100

Accumulating a result

JAVA

void main() {
    int sum = 0;
    for (int i = 1; i <= 100; i++) {
        sum += i;
    }
    System.out.println("1 + 2 + … + 100 = " + sum);

    long factorial = 1;
    for (int i = 2; i <= 15; i++) {
        factorial *= i;
    }
    System.out.println("15! = " + factorial);
}

Output

1 + 2 + … + 100 = 5050
15! = 1307674368000

The for-each loop

JAVA

void main() {
    String[] colours = {"red", "green", "blue"};
    for (String colour : colours) {
        System.out.println(colour.toUpperCase());
    }

    int total = 0;
    for (int n : new int[]{4, 8, 15, 16, 23, 42}) {
        total += n;
    }
    System.out.println(total);
}

Output

RED
GREEN
BLUE
108

Read for (String colour : colours) as "for each colour in colours". It works with arrays and every collection (lists, sets…), and avoids index mistakes. Use the counting loop when you need the index or want to skip or modify elements by position.

Off-by-one errors

JAVA

void main() {
    int[] numbers = {5, 6, 7};
    for (int i = 0; i <= numbers.length; i++) {
        System.out.println(numbers[i]);
    }
}

Output

5
6
7
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
    at Main.main(Main.java:4)

Indexes run from 0 to length - 1, so the condition must be i < numbers.length. Using <= is the classic off-by-one mistake.

Multiple variables

JAVA

void main() {
    String word = "level";
    boolean palindrome = true;
    for (int i = 0, j = word.length() - 1; i < j; i++, j--) {
        if (word.charAt(i) != word.charAt(j)) {
            palindrome = false;
        }
    }
    System.out.println(word + " is a palindrome: " + palindrome);
}

Output

level is a palindrome: true

A for header can declare and update several variables, separated by commas — here two indexes walking towards each other.

Try it yourself

Print a multiplication table for 7 with aligned columns.

Output

 1 x 7 =   7
 2 x 7 =  14
 3 x 7 =  21
 4 x 7 =  28
 5 x 7 =  35
 6 x 7 =  42
 7 x 7 =  49
 8 x 7 =  56
 9 x 7 =  63
10 x 7 =  70

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