Simple nested for loop example

Question

Currently I am studying for my Java test. Whist studying I've come across a small problem.

In this for loop:

for ( int i=1; i <= 3 ; i++ ) {
    for (int j=1; j <= 3 ; j++ ) {
        System.out.println( i + " " + j );
    }
}

The output is:

1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3

My problem is, I don't understand it. When I read this code I keep thinking it should look like this:

1 1
2 2
3 3

Why is this not the case?

Answer 1

Each iteration of i, you're starting a completely new iteration of j.

So, you start with i==1, then j==1,2,3 in a loop. Then i==2, then j==1,2,3 in a loop, etc.

Step through it one step at a time, and it will make sense.

Answer 2

What you have is one loop inside of another. Think of it like the minute hand and the hour hand on a clock. The minute hand is going to go around (1,2,3,4...59) while the hour hand is still on 1. So, if we treat hours as i, and minutes as j, we have:

1 1
1 2
1 3
...
1 60

And then the hours change to 2, and the minute hand keeps going around:
2 1
2 2
2 3
...
2 60

And we finish once we get to

12 1
12 2
12 3
...
12 60

which is where the loop ends. Your example is much like this.

(For the pedantic, I know it's zero-based, but in order to illustrate, this may be easier to understand)