Head First Java exercise problem

[Louis Merz]

Hello everyone,

Lately I've been trying to learn Java with the Head First Java book. It seems like a good book so far except it's frustrating because the book doesn't explain how the code works in the exercises at the end of the chapters. They give the solutions to the exercises but no explanation.

Any help understanding how this code gets the following output would be great, thanks!

class Test {
  public static void main(String [] args) {
    int x = 0;
    int y = 0; 
    while ( x < 5 ) {
    y= x - y;
  
     System.out.print(x + "" + y +" ");
      x = x + 1;
 }
  }
}

Program Output: 00 11 21 32 42

[Jeff Verdegan]

What part don't you understand? What output would you expect?

[Ian Paul Budiman]

Hi Louise,

I am a newbie also, but the logic solution for that particular is:

You will print out value X, Value Y from X=0, X=1, X=2, X=3, X=4

Which the format would be: "System.out.print(x + "" + y +" "); "

As you know the given parameter is, x = x + 1 and y = x - y

So, it will give you result:

when x=0
0,0(0-0)


when x=1
1,1(1-0)


when x=2
2,1(2-1)

when x=3
3,2(3-1)

when x=4
4,2(4-2)

Which the bold part is "Y"

Hope it helps.

[Matthew Brown]

And welcome to The Ranch, Louis!

[Louis Merz]

Thanks Ian, that makes a lot more sense.

Also I'm not sure what the "" in the "System.out.print(x + "" + y + " "); " means. Does it simply mean the program will print two numbers at a time?

And welcome to The Ranch, Louis!

Thanks Matthew, I'm glad to be here!

[fred rosenberger]

Louis Merz wrote:Also I'm not sure what the "" in the "System.out.print(x + "" + y + " "); " means. Does it simply mean the program will print two numbers at a time?

Try taking it out, and see what you get!!!

System.out.print(x + y + " ");

[Ricci Sithole]

when x=0
0,0(0-0)


when x=1
1,1(1-0)


when x=2
2,1(2-1)

when x=3
3,2(3-1)

when x=4
4,2(4-2)

Could you kindly expand on these equations - I cannot seem to find how you get your y-axis values - what's the formula that you are using?

[Anayonkar Shivalkar]

Ricci Sithole wrote:I cannot seem to find how you get your y-axis values - what's the formula that you are using?

You might find it yourself after doing paperwork.
New value of y = current value of x - current value of y.

[Ricci Sithole]

Anayonkar Shivalkar wrote:
New value of y = current value of x - current value of y.

Thank you so much Anayonkar - it makes sense now.

The second part of this question has this piece of code in place of (y = x - y)

code:
x = x + 1
y = y + x
with outcome expected to be: 11 34 59

Am I correct in saying that, to work out the y-axis, you'd first need to add +1 to the value of x, before you can plug in that value into (y = y + x) to get your corresponding y-axis value?

Rgds
Ricci

[Campbell Ritchie]

Welcome to the Ranch

I don’t think those values are axes, but simply x and y. I think the bit about "" previously might be a misprint and should have read " ", which means you get 1 1 instead of 11. I don’t have my copy of the book to hand at present.