Thread: Vector Math in Java

I have in other threads posted problems and have received a great deal of help and good feed back from the old timers here. I want to thank you all for that and I have just one remaining issue.

I was working on translating and old QBasic program to Java I finally got my code to compile AND run. I then went back to the original QB code for comparison trying to get the vector math to work correctly because it did not work as expected. I did notice some typos in my java algorithms, and now the math looks exactly the way it did in the QB code, but the output still does not look right.

My question is, is java math the same as QB math? namley, does java.sqrt() work out the square root the same way that sqrt() did in QB?

here is the math:

I need to move an object(star) at x,y on a line from the center of the screen to the edge of the screen. To do this my data uses a 0,0 @ center grid.

So to get the angle I do:

angle = java.sqrt((x1*x1)+(y1*y1))

This is the same as:

angle = java.hypot(x1,y1)

the new position is relative to the old one, so:

x2 = x1 / angle
y2 = y1 / angle

since this new position is relative, I have to add it to the old position to get the real new position, so:

x1 = x1 + x2
y1 = y1 + y2

Then I translate it all to a 0,0 @ top left grid by adding half the grid width to both x and y (newX = x + (width / 2)) only when I need to draw the star.

This works real perfect in QBasic but the stars do not move correctly in java. If someone here knows vector math, I would really like some help. I did not post the code because the code works fine. I will continue to look for errors in my vector math, but if java does not calculate things in the same order as other languages or something else equally as silly please let me know.

The only thing I can think of that might be an issue is precision. Remember that binary is not good at representing floating point values. So the square root of 8 might be 2.8284 or it might be 2.8283999998.

Try doing a comparison between the 2 languages. Set up a simple loop that displays the sqrt of the numbers 1 - 20 (or some other reasonable value).

Thank you that makes so much sense....

Point to note: if I drop the object class and methods and make it a straight forward main loop; it works fine...I believe it's something to do with the order things are executed in, my moveStar method includes an "if (isValid == true) then moveStar else restart star"... this should probably be moved to a separate method. This would clean things up a bit and make it easier to read; as well a straightening out the order of things. I think isValid is always false for some reason.

Currently it's "if star inside valid locations the moveStar else restart star". If I make it separate then it would be "if star outside valid locations then restart star else doNothing" followed by a call to moveStar.

angle = java.sqrt((x1*x1)+(y1*y1))

Perhaps I've forgotten my math or just misunderstand the problem, but I'm confused...how does calculating the length of the hypotenuse (the right side of the equation above) equal the angle?

I forgot to say if (angle == 0) then angle = 1

Let's say a star is currently at 3,3. the integer of the Hypotenuse would be 4. int(3/4) = 0, angle defaults to 1. then new location is one pixel away, 3+1,3+1 = 4,4.

Let's say a star is at 11,15. Then integer of the hypotenuse is 18. int(11/18) && int(15/18) both = 0, so angle again defaults to 1 and the new location is 12,16.

this works for -x,-y locations as well.

oops I guess your right the starAngle variable is miss-named. it should be starVector