Question

In my code, I am using float to do currency calculation but the rounding has yielded undesired results so I am trying to convert it all to int. With as little change to the infrastructure as possible, in my init functions, I did this:

-(id)initWithPrice:(float)p;
{
[self setPrice:(int)(p*100)];
}

I multiply by 100 b/c in the main section, the values are given as .xx to 2 decimals. I abnormally I notice is that for float 1.18, the int rounds it to 117. Does anyone know it does that? The float leaves it as 1.18. I expect it to be 118 for the int equiv in cents.

Thanks.

Answer 1

Floating point is always a little imprecise. With IEEE floating point encoding, powers of two can be represented exactly (like 4,2,1,0.5,0.25,0.125,0.0625,…) , but numbers like 0.1 are always an approximation (just try representing it as a sum of powers of 2).

Your (int) cast will truncate whatever comes in, so if p*100 is resolving to 117.9999995 due to this imprecision , that will become 1.17 instead of 1.18.

Better solution is to use something like roundf on p*100. Even better would be if you can go upstream and fully convert to fixed-point math using integers in the entire program.