Getting different output from seemingly identical calculations

Question

Can anyone tell me why the calculations on lines 9 and 11, which seem to be identical, produce two different outputs. I know the difference isn\'t that great, but I am using the

Answer 1

The real answer is somewhere around 409.9999999.

This is outputting a double and rounding to 410 because the math is all inlined:

std::cout <<  400+(sin((90*3.14159)/180)*10) << std::endl;

since ypos is declared as an int, the double value is being truncated to 409 (which is the defined behavior when casting from double to int):

ypos=ypos+(sin((90*3.14159)/180)*10);

/// Output: 409.
std::cout << ypos << std::endl;

Note that you could also increase the accuracy by using a better constant for PI:

const double PI = 3.141592653589793238463;

std::cout <<  400+(sin((90*PI)/180)*10) << std::endl;

but I would still store the result in a double instead of an int to avoid truncating. If you need an integer result then I would round first:

ypos += round(sin((90*PI)/180)*10);