Originally I posted this explanation about nested loops on 16.02.2020 at Hyperykill. From what I've learned in the theory part I was unable to understand the concept of a nested loops. So I came up with this guide, after I did research the topic online.

Ok guys, so I don't know how you think about the nested for loops, but the example here on Hyperskill doesn't explain anything. I simply don't understand why the code in §3. does what it does.

As an example, the following code prints the multiplication table of numbers from 1 to 9 (inclusive).

for (int i = 1; i < 10; i++) {
    for (int j = 1; j < 10; j++) {
        System.out.print(i * j + "\t");
    }
    System.out.println();
}

It outputs:

1   2   3   4   5   6   7   8   9  
2   4   6   8   10  12  14  16  18  
3   6   9   12  15  18  21  24  27  
4   8   12  16  20  24  28  32  36  
5   10  15  20  25  30  35  40  45  
6   12  18  24  30  36  42  48  54  
7   14  21  28  35  42  49  56  63  
8   16  24  32  40  48  56  64  72  
9   18  27  36  45  54  63  72  81 

Because there is no explanation here, I did some research and came up with a really simple and great explanation.

We have two for loops:

Loop 1 - Step 1: ----> for (int i = 1; i < 10; i++) {
Loop 2 - Step 1: ----> for (int j = 1; j < 10; j++) {
Loop 2 - Step 2: ----> System.out.print(i * j + "\t");
    }
   Loop 1 - Step 2: ----> System.out.println();
}

What happens here during execution?

Loop 1 - Step 1 is started, the variable i is set to the value 1. Loop 2 - Step 1 is started, the variable j is set to the value 1. Now Loop 2 - Step 2 is started, i = 1 is multiplied by j = 1 and a TAB is inserted.

ATTENTION: Loop 1 - Step 2 won't start yet!! Nope, now Loop 2 - Step 1 starts again, which means the code checks if j is still smaller than 10 - yes (j < 10; true;). Therefore j++ will be now executed, so we now have j = 2. Next again Loop 2 - Step 2, so i (still with value 1) is multiplied by j = 2 and a TAB is inserted.

Loop 2 - Step 2 will be executed until j has reached the value 10 (j < 10; false;). Only NOW Loop 1 - Step 2 will be executed! A new line is inserted and the code starts again at Loop 1 - Step 1.

You can also imagine the whole thing figuratively like a clock. Loop 1 is the hour hand. Loop 2 is the minute hand. Only after Loop 2 has been executed 60 times on a clock (60 minutes = 1 hour ;-) ), the hour hand moves forward and the process is starting all over again.

I hope by this could explain to you how nested for loops work :-)