# Expecting calculation to be 0

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If I pass in amount=25 rotation=90 to the method below, the result is a weird -1.5308086E-15 instead of (what I thought) should be 0. Why is this?

/** * @param amount The distance to move. Will always be positive. * @param rotation * @return The distance resulting from moving <code>amount</code> in the direction of <code>rotation</code>. */ public static float getYDistanceMovedBy(float amount, float rotation) { return Math.abs(amount) * (float) -Math.cos(Math.toRadians(rotation)); }

public void testGetYDistanceMovedBy2() { float amount = 25.0f; float rotation = 90.0f; float distance = Geometry.getYDistanceMovedBy(amount, rotation); assertEquals(0.0f, distance); }

The same code for the x axis works as expected, with the result being 0:
/** * @param amount The distance to move. Will always be positive. * @param rotation * @return The distance resulting from moving <code>amount</code> in the direction of <code>rotation</code>. */ public static float getXDistanceMovedBy(float amount, float rotation) { return Math.abs(amount) * (float) Math.sin(Math.toRadians(rotation)); }

public void testGetXDistanceMovedBy2() { float amount = 25.0f; float rotation = 0.0f; float distance = Geometry.getXDistanceMovedBy(amount, rotation); assertEquals(0.0f, distance); }

Cheers.

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Without looking over much of your code, it's because floats can easily have rounding errors, you will notice your incorrect value is really close to 0 anyway (Hopefully close enough):

http://www.cprogramm...ting_point.html

Yeah I thought it might be something to do with that, but I don't know how to account for it without affecting legitimate values.

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To account for this, you use "epsilon comparison". That is, you say that if the value is 0 +/- some epsilon, it is close enough to zero to be good enough.

This is required reading: What Every Computer Scientist Should Know About Floating-Point Arithmetic

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It is also worth noting that the vast majority of the time, you can get away with doing only "less than" or "more than" comparisons, in such a way that in borderline cases it doesn't matter much whether those test as true or false. Algorithms with this feature tend to be robust.

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I'm roughly familiar with epsilon comparison, but I was under the impression that it was a far smaller threshold than -1.5 (the value in question)? I'm also a bit confused as to how comparison relates to my problem when there are none taking place. If it was a smaller number (ie < 1), I'd understand...

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If I pass in amount=25 rotation=90 to the method below, the result is a weird -1.5308086E-15 instead of (what I thought) should be 0. Why is this?
...

I'm roughly familiar with epsilon comparison, but I was under the impression that it was a far smaller threshold than -1.5 (the value in question)?

The value in question seems me -1.5*10[sup]-15[/sup] (notice the E-15 in -1.5308086E-15, see e.g. this in wikipedia), what is -0.0000000000000015 (if I've counted correctly).

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I'm roughly familiar with epsilon comparison, but I was under the impression that it was a far smaller threshold than -1.5 (the value in question)? I'm also a bit confused as to how comparison relates to my problem when there are none taking place. If it was a smaller number (ie < 1), I'd understand...

The value [color=#1C2837][size=2]-1.5308086E-15 actually represents something like -0.0000000000000153 - which is rather close to 0.

[color="#1C2837"]

You can think of the "E-15" as a shift of the decimal point. Its a short hand notation for extremely large or small numbers.

[color="#1C2837"]

The previous posters have linked some nice resources, especially rip-off's post.

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[quote name='Mybowlcut' timestamp='1320564695' post='4881003']
If I pass in amount=25 rotation=90 to the method below, the result is a weird -1.5308086E-15 instead of (what I thought) should be 0. Why is this?
...

I'm roughly familiar with epsilon comparison, but I was under the impression that it was a far smaller threshold than -1.5 (the value in question)?

The value in question seems me -1.5*10[sup]-15[/sup], what is -0.0000000000000015 (if I've counted correctly). That is somewhat close to 0, and definitely less than 1 ;)
[/quote]
Why is it that the one time someone responds quickly on GameDev, I make a really stupid post? I tried deleting it, but you're just too quick. Haha.

Ok, so basically clamp to 0 if it's an acceptable margin of error?

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