arithmetic operations in QT

I am new to QT, learning through some small programs. I am trying to do arithmetic operations, it is not happening. I am having multiple files. All variables are declared and defined in main.cpp file. In other files I using it with the keyword extern. Even simple arithmetic operations like a = a+1 or a++ is not working. I noticed this issue with a floating point variable first. Now no arithmetic operations are working. I tried by declaring these variables as local to the function and global to the file. still no change. How to tackle this issue? what is the reason for this?

You need to go back to your C++ books and learn about variables and scoping. Your problem has nothing to do with Qt and everything to do with basic C++.