is it necessary to include using namespace std

Answer 1

No, but if you dont, every single object that is inside the std namespace need to have std:: before it.
Example:
#include <iostream>
using namespace std;
...
cout<<1; // Fine
...
----------------
#include <iostream>
...
std::cout<<1; // Fine, but needs to have std:: at the beginning
If you have a lot of cout statements, using the std namespace is good.

Answer 2

thank you

Answer 3

yes it is necessary

Answer 4

well said learned friends

Answer 5

yaa

Answer 6

can we use #include <iostream.h>