Why this program shows error?

Question

#include <iostream>
int counter=50;

int main() {
register int counter;
for(counter=1;counter<10;counter++){
cout<<endl<<::counter/counter;
}
return 0;
}

main.cpp:15:5: error: ‘cout’ was not declared in this scope
cout<<endl<<::counter/counter;
^~~~
main.cpp:15:5: note: suggested alternative:
In file included from main.cpp:9:0:
/usr/include/c++/6/iostream:61:18: note: ‘std::cout’
extern ostream cout; /// Linked to standard output
^~~~
main.cpp:15:11: error: ‘endl’ was not declared in this scope
cout<<endl<<::counter/counter;
^~~~
main.cpp:15:11: note: suggested alternative:
In file included from /usr/include/c++/6/iostream:39:0,
from main.cpp:9:
/usr/include/c++/6/ostream:590:5: note: ‘std::endl’
endl(basic_ostream<_CharT, _Traits>& __os)
^~~~

Answer 1

The error message says it

" 'cout was not declrared....'"
"suggested alternative: ..... 'std::cout' "

'cout' and 'endl' are defined in the std namespace and you need to access them from there, like 'std::cout' and 'std::endl'.

Alternatively, just bring the members of the std namespace into scope by putting this line before main() or as the first line of main()
`using mamespace std;`

Answer 2

You can also define reference of `std` namespace for any of its specific member

using std::cout, std::endl;

There's another warning there after the above steps, stating C++17 support for `register` storage specifier, so you might wanna remove that `register` and just define <counter> as follows

int counter {};

Or let <counter> be only inside the for..loop scope, since local <counter> is only used within loop body.

for( int counter {1}; counter < 10; counter++ )
{
// loop body
}