Question

I’m having some some difficulty predicting how my C code will truncate results. Refer to the following:

float fa,fb,fc;
short ia,ib;

fa=160
fb=0.9;
fc=fa*fb;
ia=(short)fc;
ib=(short)(fa*fb);

The results are ia=144, ib=143.

I can understand the reasoning for either result, but I don’t understand why the two calculations are treated differently. Can anyone refer me to where this behaviour is defined or explain the difference?

Edit: the results are compiled with MS Visual C++ Express 2010 on Intel core i3-330m. I get the same results on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) under Virtual Box on the same machine.

Answer 1

The compiler is allowed to use more precision for a subexpression like fa*fb than it uses when assigning to a float variable like fc. So it’s the fc= part which is very slightly changing the result (and happening to then make a difference in the integer truncation).