floor is a function in the standard C++ headers that I think comes in through math.h, which comes in through the precompiled headers through stdafx.h

What tools are you using?

I used the 1998 version of Microsoft Visual C++

In other projects I have observed that the headers for the above C++ put many symbols in the global namespace that newer C++ header files put in other namespaces such as std. I don't know whether that is the problem in this case. Try
using namespace std;
See if that makes floor visible.
Otherwise, you need to find out where/how floor is declared in the header files for your C++ compiler.