Fix iostream.h include; incorrect variable names

[musteresel]

include <iostream.h> is wrong, it's include <iostream> as used in the other parts.

Variable names starting with an underscore followed by an uppercase letter are reserved for the implementation, same for variables containing two consecutive underscores. https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier

Note that the using namespace std; is actually frowned upon, but as it's so wildly in use (and also the default on ideone) I didn't change that. Also there's some minor terminology mistakes but I doubt that the book benefits from fixing those.