How to securely convert this double into an int?

Question

I have an double which is a time interval in seconds. Valid values are 0.0, 0.5, 1.0, 1.5, etc.

Now I need to convert this value into an int. But I cannot do this:

int converted = (int)theDouble;

Because what may happen is, that my double due to floating point errors is 0.999999999 rather than 1.000000000. It would just cut off the tail, and we'd end up with 0 instead of 1. Also, when my double is 0.9, I want an int that is 1. When it is 1.1, I want the int to be 1. When it is 1.8, I want the int to be 2.

There's a round() function but Xcode doesn't show a documentation for this. The header only tells it returns a double. So not what I need.

What's the safest way to get a close int representation of that double? Although it is a double I'll never need higher precision than 0.5 or one fragmental digit (I'm no math genius and don't know the exact scientific terms).

Answer 1

You're probably looking for the lround(double function). The signature looks like this.

long int lround(double).

Other options are

• long int lrint(double)
• long long int llrint(double)
• long long int llround(double)

Answer 2

int converted = (int)round(theDouble);