Answer from Function is incorrect

Question

The answer of this function is returned into the main function of the program, and that works fine. The issue is that any value where the cosine should be 0, it turns out to give a weird irrational number (something like 1.30431912*10^-13). So, 90, 450, and so on, all turn out irrational answers. What's the matter?

  float cosineDegrees() {
        string i;
        double iDouble;

        cout << "Give me a number to find the value of degrees in. ";
        getline(cin, i);

        iDouble = stod(i);

        double PI = 3.14159265359;

        float answer = cos((PI/180)*iDouble);

        return answer;


    }

1.30431912*10^-13 is not irrational. It's perfectly rational, but very close to zero. It's as close to zero as you're likely to get. See also [Is floating point math broken?](http://stackoverflow.com/questions/588004/is-floating-point-math-broken). — molbdnilo, Mar 08 '16 at 08:41

score 3 · Accepted Answer · answered Mar 08 '16 at 08:29

3

Floating-point math has finite precision. Your (PI/180)*iDouble value is about 1.30E-13 radian away from 90 degrees.

answered Mar 08 '16 at 08:29

MSalters

173,980
10
155
350

So that's as about as precise I can really get things, then? – Rocky Mar 08 '16 at 08:33
I think it would help the OP to also present a solution, or an explanation of how to craft one. – stijn Mar 08 '16 at 08:33
@stijn: The question appears to ask for an explanation, not a solution, and thast's non-trivial anyway. cos(45) is not going to return √2. – MSalters Mar 08 '16 at 09:10

score 1 · Answer 2 · answered Mar 08 '16 at 08:32

1

If you lock Pi to a limited precision you can't expect your 'answer' to be more precise. 10^-13 is basically zero in fact, and it's not 'irrational.' Everything is rational in a computer.

answered Mar 08 '16 at 08:32

involution

11
2

Well, `std::complex` is not rational ;) – MSalters Mar 08 '16 at 09:11

Answer from Function is incorrect

2 Answers2