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# Maths problem

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Hi I am programming using xna and c# and I have a maths problem that has been bugging me for some time.

I have a location position vector and a rotation angle in radians. I want to be able to move the position vector by 1 unit in the rotation direction. So for instance if the rotation is PI/2 and the position is (0,0) the new position is now (0,-1) as the rotation is facing downwards so it moves 1 unit down.

At the moment I have tried using a rotation matrix to get the correct new location:

Vector2 t = new Vector2(1,0);
t.X = (float)(t.X * Math.Cos(rotation) - t.Y * Math.Sin(rotation));
t.Y = (float)(t.X * Math.Sin(rotation) + t.Y * Math.Cos(rotation));
location += t;

I am seeing some really odd behaviour though. It seems it works with some directions but fails with others.

[Edited by - jkh13 on July 12, 2010 6:21:08 PM]

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You've made the classic error here: you're overwriting t.X in the first line, and then using the *new* value in the second line (when it should be the original value).

Anyway, since you're just rotating the vector (1,0), you can reduce this to:
Vector2 t(Math.Cos(rotation), Math.Sin(rotation));



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Doh! Silly mistake :P thats done the job, works perfectly now, thanks!

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I've made that mistake myself at least five times over the years. In my defense, it did take me less and less time to realize what the problem was. :)

With the coding style I have develop over the last few years, this type of mistake is virtually impossible, so I'll explain one rule that might help you:
"Whenever possible, declare a variable as late as possible, give it a descriptive name, assign its value when its constructed, and never change it."

I sometimes make these variables `const', but perhaps this ends up getting in the way of readability, so I am not going to advocate doing it.

Of course there are variables whose value changes in each iteration of a loop, or cases where following this rule would result in unclear code, so use common sense.

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