0 + 0 + 0... + 0 != 0

Question

I have a program that is finding paths in a graph and outputting the cumulative weight. All of the edges in the graph have an individual weight of 0 to 100 in the form of a float with at most 2 decimal places.

On Windows/Visual Studio 2010, for a particular path consisting of edges with 0 weight, it outputs the correct total weight of 0. However on Linux/GCC the program is saying the path has a weight of 2.35503e-38. I have had plenty of experiences with crazy bugs caused by floats, but when would 0 + 0 ever equal anything other than 0?

The only thing I can think of that is causing this is the program does treat some of the weights as integers and uses implicit coercion to add them to the total. But 0 + 0.0f still equals 0.0f! As a quick fix I reduce the total to 0 when less then 0.00001 and that is sufficient for my needs, for now. But what vodoo causes this?

NOTE: I am 100% confident that none of the weights in the graph exceed the range I mentioned and that all of the weights in this particular path are all 0.

EDIT: To elaborate, I have tried both reading the weights from a file and setting them in the code manually as equal to 0.0f No other operation is being performed on them other than adding them to the total.

Answer 1

Because it's an IEEE floating point number, and it's not exactly equal to zero.

http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm