how to update global variable in if else statement (C++)

I am a newbie in C++ and it is a very easy task but all tutorials that I looked use cout some string in if-else conditions.

I want to update a global variable in if-else statement.

Here is the code;

        double theta_radian{};
        if (l2 >= l1)
        {
            double theta_radian = atan2(e2.y(), e2.x());
            std::cout << "theta_local" << theta_radian << std::endl;
            
        }
        else
        {
            double theta_radian = atan2(e1.y(), e1.x());
            
        }
        std::cout << "theta_radian" << theta_radian << std::endl;

I want to update theta_radian variable based on the if-else condition.

I don't want to create a vector and push_back to it since it is a simple practice and I want to learn how to do it properly.

Thanks in advance!