Why C is still in use especially in area of numerical optimization (instead of C++) ? C and C++ aren't fully compatible so mayby you know some differances that make the difference ?
C is not used for any particular reason in numerical optimizations other than for legacy reasons. However, there are areas where C is preferred over C++ though even C is not the preferred language of choice. To mind comes programming FPGAs. Though VHDL and Verilog are by far the standards. But "behavioral synthesis" allows to utilize C or C relatives such as SystemC to transfer logic to HDL. In that C is the preferred choice over C++. (Though having started working with FPGAs I start to get the sense that VHDL and Verilog are so popular for good reasons; even though the learning curve is a lot steeper they are much closer to the core of the particular hardware logic (Gate Arrays have a vastly different structure than a CPU).
So, C has its applications and is the preferred choice of weapon in some areas but more often than not people prefer using what they are best at. At least in numerical optimization I do not see a single reason why C would be preferred over C++ and actually I doubt such claim.
who told you that ? I am used to create new trade systems in C++ to make the customers requirements feasible.
CERN used C++ to prove higgs boson particle. I see people using C to program embedded like microwaves or fridges :D
but it is just my opnion, I would like to hear others.
I don't know how widely used C is over C++ when it comes to doing numerical optimization; however, if there is a preference towards C it likely comes from the fact that C++ name mangling is not standard making C++ libraries very hard to integrate into other languages and environments. With C you get a well known calling convention making it straightforward to integrate which is why you see C (and Fortran) libraries that are used in Python, R, Ruby, Java, C#, etc.