Java nested for while loops

Question

I am having troubles with nested while loops in a for loop. I understand the nested for loop:

    for (int i = 0; i<5;i++)
    {
        for (int j=i;j<5;j++)    
        {
           System.out.print("*"); 
        }
        System.out.println();
    }

When it comes to a while loop inside a for loop I'm kinda lost, can someone explain it to me?

Expected output:
*****
****
***
**
*

Answer 1

In terms of the equivalences between for loops and while loops, it's basically this:

for (INIT; CONDITION; POSTOP) {     INIT;
    BODY;                           while (CONDITION) {
}                                       BODY;
                                        POSTOP;
                                    }

(with variations for scope and other such things that we don't need to go into here).

Hence, to solve your problem with a for/while solution, you could use something like:

for (int i = 0; i < 5; i++) {
    int j = i;
    while (j < 5) {
        System.out.print("*");
        j++;
    }
    System.out.println();
}

---

It's sometimes helpful to run through the code in your head, with a pen and a bit of paper to maintain variables, such as:

 i   j   output
--- ---  ------

If you just "execute" each line of the code (either your original or my for/while variant) in your head for a few iterations, you should see what's happening. And, if you do them side-by-side, you'll see the equivalency between both variants.

Basically, the outer loop counts (iterates) from 0 to 4 inclusive, running the inner loop then outputting a newline character.

For each of those iterations, the inner loop counts from i to 4 inclusive, outputting a * each time (with no newline character).

So, in the first outer loop iteration, the inner loop runs from 0 to 4 , outputting five stars.

In the second outer loop iteration, the inner loop runs from 1 to 4 , outputting four stars.

And so on, to the final outer loop iteration where i is 4 , so the inner loop runs from 4 to 4 , outputting one star.

In terms of the pen-and-paper method, you would get something along the following lines:

 i   j   output
--- ---  ------
 0   0     *
 0   1     *
 0   2     *
 0   3     *
 0   4     *
           \n
 1   1     *
 1   2     *
 1   3     *
 1   4     *
           \n
 2   2     *
 2   3     *
 2   4     *
           \n
 3   3     *
 3   4     *
           \n
 4   4     *
           \n

Answer 2

Well, first your for loop(s) could be written like

for (int i = 0; i < 5; i++) {
    for (int j = i; j < 5; j++) {
        System.out.print("*");
    }
    System.out.println();
}

Next, let's look at the three parts of a for loop (from the Wikipedia link)

for(INITIALIZATION; CONDITION; INCREMENT/DECREMENT){
    // Code for the for loop's body
    // goes here.
}

We can move that to a while loop like

INITIALIZATION;
while (CONDITION) {
    // Code for the while loop's body
    // goes here.
    INCREMENT/DECREMENT;
}

As a single practical example,

int j = i;
while (j < 5) {
    System.out.print("*");
    j++;
}

Answer 3

A while loop inside a for loop can be made to behave like a nested for loop if you simply update the counter variable within the body of the while loop. Alternatively, think about what a for loop is really saying per iteration.

for (int i = 0; i<5;i++)
{
    int j = i;
    // for (int j=i;j<5;j++)    
    while(j < 5)
    {
       System.out.print("*");
       j++;
    }
    System.out.println();
}