Converting from Radians to Degrees

Question

I'm building a small Physics engine and I'm having trouble converting my Radian value to Degrees using atan, as I need an angle to output in Degrees only.

Firstly, I have an x and y value, and I need to find an angle using atan, so I divide y by x like so:

angleDivide = yN / xN;

Then, before putting this value into tan, I attempt to convert it to Degrees like this:

angleToDegrees = angleDivide * (3.14 / 180);

Then I place angleToDegrees into atan:

angle = atan(angleToDegrees);

But when I'm displaying angle, I'm, still getting radian values.

Please could you tell me what is wrong with my code and how to fix this?

That is a horrifically bad approximation to pi. **First**, use `M_PI` if it's defined, please, or else define it to `LDBL_DIG_10` digits precision. **Second**, you should be converting the _output_ of `atan()` from rads to degrees, not the input (which is just a slope). **Third**, you should use `atan2(y,x)` and not `atan(y/x)`. — Iwillnotexist Idonotexist, Nov 24 '16 at 01:25
Um... Firstly, your text says you want to put the value into `tan`, but the code then uses it in `atan`. So, is it for `tan` or for `atan`? Make up your mind. Secondly, if this is for `atan`, then why are you converting it? The argument of `atan` is not an angle. It is tangent value. Tangent is neither "radians" nor "degrees". Tangent is a unitless value. Converting "radians to degrees" for something that is not an angle makes no sense whatsoever. It is the *result* of `atan` that you might want to convert to degrees. The result, not the argument. — AnT stands with Russia, Nov 24 '16 at 01:35

score 14 · Accepted Answer · edited Jan 11 '18 at 18:05

14

You want to calculate radians=tan(y/x) first.

Then you can convert it to degrees:

radians = atan(y/x)
degrees = radians * (180.0/3.141592653589793238463)

See the reference here for atan:

On a side note, you also have to take into account what quadrant you are in to get the correct answer (since -y/x is the same number as y/-x)

edited Jan 11 '18 at 18:05

fivef

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answered Nov 24 '16 at 01:26

anthonybell

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About your side note: That's what `atan2(y,x)` exists for. – Iwillnotexist Idonotexist Nov 24 '16 at 01:29
This works perfectly, thank you! Just one question, my values come out as '38.7464', which of course is a degree value, but is there a way I can shorten the decimal places? – Lucy Loo Nov 24 '16 at 01:35
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@LucyLoo `char buf[64]; sprintf(buf, "%.1f", degrees); degrees = strtod(buf, 0);` – Iwillnotexist Idonotexist Nov 24 '16 at 01:38
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You really should change `3.14` to add some more decimals, – pingul Nov 24 '16 at 09:17
@LucyLoo If you're using C++ you should use its functionality to format the float. See http://stackoverflow.com/questions/14432043/c-float-formatting. – pingul Nov 24 '16 at 09:18
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I edited the answer to add more decimal places. I was wondering why my calculation was off by 0.04 degrees! – fivef Jan 11 '18 at 15:05

Converting from Radians to Degrees

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