C Round Function

CodeProject

It’s really weird that the C math library (math.h) doesn’t support the round function. It only includes the floor function which rounds down a float to an integer (can also be done by implicit or explicit casting) and the ceil function which rounds the value up.

For example:

int x;

x = floor(1.2);   //  x is set to 1
x = floor(1.8);   //  x is set to 1
x = (int)1.8;     //  x is set to 1 (Explicit Narrowing Conversion)
x = 1.8;          //  x is set to 1 (Implicit Narrowing Conversion)
x = ceil(1.2);    //  x is set to 2
x = ceil(1.8);    //  x is set to 2

The round function is supposed to round the float value to the nearest integer.

x = round(1.2);    //  x is set to 1
x = round(1.8);    //  x is set to 2

This can be done adding a 0.5 to the value and then truncating it.

x = (int)(1.2 + 0.5);  //  x is set to 1
x = (int)(1.8 + 0.5);  //  x is set to 2

We also have to take negative values into consideration by adding -0.5 and then truncating.

x = (int)(-1.2 - 0.5);  //  x is set to -1
x = (int)(-1.8 - 0.5);  //  x is set to -2

Thus, here is the resulting C function:

int round(double number)
{
    return (number >= 0) ? (int)(number + 0.5) : (int)(number - 0.5);
}

Note that you might want to use long rather than int to include support for larger numbers and avoid integer overflow.

That’s it, pretty much primitive, but fun!

Enjoy!
Ali B