cast(int) getting an unexpected number

[Steven Schveighoffer]

On Wed, 04 Nov 2009 06:14:36 -0500, Joel Christensen <joelcnz at gmail.com>  
wrote:

> Input:
> import std.stdio;
> void main() {
> 	float f=0.01;
> 	writefln("%0.2f->%d",f,cast(int)(f*100f));
> }
>
> Output:
> 0.01->0
>
> Comment:
> What!?

hehe.  .01 is not exactly represented by float, because it's stored as a  
binary value, not a decimal value.  Think about how there is no way to  
represent 1/3 in decimal.

If you imagined that the computer stored things in decimal, see how the  
1/3 example would work

a = 1.0/3; // truncated to 0.3333333
a *= 3; // 0.9999999
auto i = cast(int)a; // 0

This is analagous to what you are asking the compiler to do.

To be safe, whenever converting to int, always add a small epsilon.  I  
think you can use float.epsilon, but I don't have any experience with  
whether that is always reasonable.

-Steve