For Loops

Welcome to one of the most important concepts in programming: the for loop. With it, we can ask the computer to do something many times, automatically

Why does it matter? Imagine we want to display all numbers from 1 to 10

public class Main {
    public static void main(String[] args) {
        System.out.println(1);
        System.out.println(2);
        System.out.println(3);
        // ... and so on, ten times
    }
}

Tedious. Now imagine 1 to 1000. Absolutely no chance we write 1000 printlns. This is where for saves us

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            System.out.println(i);
        }
    }
}

Run it. You'll see numbers from 1 to 10, one per line

The Java for loop has three parts inside the parentheses, separated by ;

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            System.out.println(i);
        }
    }
}

1. int i = 1: the starting point. We create a new variable i and set it to 1
2. i <= 10: the condition. As long as this is true, the loop keeps running
3. i++: what to do after each iteration. Here we increase i by 1

Reading it like a story: "start with i = 1. While i <= 10, run the body. After each run, do i++"

So i takes the values 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. When i becomes 11, the condition 11 <= 10 is false, and the loop ends

The three parts are entirely under your control. We can count by 2s, count down, do whatever we want

public class Main {
    public static void main(String[] args) {
        // counting by 2s
        for (int i = 0; i <= 10; i = i + 2) {
            System.out.println(i);
        }
    }
}

Output: 0, 2, 4, 6, 8, 10

public class Main {
    public static void main(String[] args) {
        // counting down
        for (int i = 10; i >= 1; i--) {
            System.out.println(i);
        }
    }
}

Output: 10, 9, 8, ..., 1. i-- means i = i - 1

A common mistake: forgetting to update i

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; ) {
            System.out.println(i);
        }
    }
}

This is an infinite loop. i stays 1 forever, the condition stays true forever, and the program prints 1 until you stop it. Always make sure something inside the loop moves the condition closer to becoming false

We can put anything in the loop body, including an if. Here we go through sectors 1 to 5 and mark sector 3 as the one to patrol

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            if (i == 3) {
                System.out.println("Patrol sector");
            } else {
                System.out.println(i);
            }
        }
    }
}

Output

1
2
Patrol sector
4
5

Mission: Garage Inspection

Tommy is inspecting the Vercetti mansion garage. The parking spots are numbered from 1 up to a total. One of the spots holds his Infernus. There, instead of the number, you want to print the car's name.

Store the total number of spots in an int named totalSpots and the spot where the Infernus sits in infernusSpot. Then use a for loop that goes through the spots from 1 to the total. For each spot:

• if it is the spot holding the Infernus → print Infernus
• otherwise → print the spot number

Example for 5 spots, with the Infernus on spot 3:

1
2
Infernus
4
5

Example for 5 spots, with the Infernus on spot 1 (the first):

Infernus
2
3
4
5

Example for 5 spots, with the Infernus on spot 5 (the last):

1
2
3
4
Infernus