Question

I’m writing a program that is using the functions sin() and cos() from the math.h library. However, I noticed I was getting funky results. After searching around and checking my math multiple times, I decided to do a simple check with this:

int main()
{
    cout << "sin(45.0) = " << sin(45) << endl;
    cout << "cos(45.0) = " << cos(45) << endl;

    return 0;
}

And I get this output:

 sin(45) = 0.850904
 cos(45) = 0.525322

These should be equal right? Is there something special about the math.h library? Am I doing something wrong?

Here are the equations in WolframAlpha:

sin(45)

cos(45)

Answer 1

You should use cmath in C++, rather than the old C header.

std::sin() and std::cos() both take a floating point value representing an angle in radian.

The GCC version of this file includes a handy constant for π, (which does not exist in the C++ standard but) which will make it easier for you to convert degrees to radian:

#include <cmath>
#include <iostream>

double degrees_to_radian(double deg)
{
    return deg * M_PI / 180.0;
}

int main()
{
    std::cout << "sin(45.0) = " << std::sin(degrees_to_radian(45)) << std::endl;
    std::cout << "cos(45.0) = " << std::cos(degrees_to_radian(45)) << std::endl;
}

See it run!