🌱BeginnerLoops
for Loop
Repeat code a fixed number of times with Java's for loop - the most common loop for counting and iteration.
Overview
A loop runs a block of code repeatedly until a condition becomes false. The for loop is ideal when you know exactly how many times you want to repeat something - iterating over array indices, counting up or down, or running code n times.
A for loop has three parts in the header: initialisation (runs once at the start), condition (checked before each iteration), and update (runs after each iteration). All three are optional, but the semicolons are required.
Key Points
- •Syntax: for (initialisation; condition; update) { ... }
- •Initialisation runs once; condition is checked before each iteration; update runs after each body
- •Loop exits when the condition becomes false
- •Common pattern: for (int i = 0; i < n; i++) - iterates n times (i = 0, 1, ..., n-1)
- •You can count down: for (int i = 10; i >= 0; i--)
- •The loop variable (i) is local to the for loop and is not accessible after it
Examples
Basic for loop - print numbers 1 to 5
public class Main {
public static void main(String[] args) {
for (int i = 1; i <= 5; i++) {
System.out.println(i);
}
// prints: 1, 2, 3, 4, 5
}
}i starts at 1, the loop runs while i <= 5, and i increases by 1 after each iteration.
Counting down and using the loop variable
public class Main {
public static void main(String[] args) {
// Count down from 5 to 1
for (int i = 5; i >= 1; i--) {
System.out.print(i + " ");
}
System.out.println("Go!");
// Sum of 1 to 100
int sum = 0;
for (int i = 1; i <= 100; i++) {
sum += i;
}
System.out.println("Sum: " + sum); // 5050
}
}The update i-- decrements by 1. You can also use i -= 2 to skip by twos.
Nested for loops - multiplication table
public class Main {
public static void main(String[] args) {
for (int i = 1; i <= 3; i++) {
for (int j = 1; j <= 3; j++) {
System.out.print(i * j + " ");
}
System.out.println(); // new line after each row
}
// Output:
// 1 2 3
// 2 4 6
// 3 6 9
}
}The inner loop runs completely for each iteration of the outer loop.
Common Mistakes
- Off-by-one error: `for (int i = 1; i < 5; i++)` runs 4 times (i=1,2,3,4); `i <= 5` runs 5 times
- Accidentally modifying the loop counter inside the body - this makes loop behaviour unpredictable
- Infinite loop: forgetting the update or writing a condition that never becomes false: `for (int i = 0; i >= 0; i++)`
Note
Use zero-based indexing (i = 0; i < n) when iterating arrays, since array indices start at 0. Use one-based (i = 1; i <= n) when printing user-visible numbers.
Exercises
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