C++ Could somebody help me explain a map/for_each code [EDIT]

Question

[Thank you for the answers, and also sorry for not really understanding all, I guess I'm trying to run yet know know how to walk] If somebody could explain me what happens here, I would really appriciate it! (Just learning C++ and I couldn't really understand by myself/Mr.Google)

string luck;
int choice;
std::map< int, std::string > cookies {
{ 1, "Tomorrow is a day" },
{ 2, "Tomorrow can be cool" },
{ 3, "Summer is coming" }
};
while( cookies.size() ) {
cout << "What cookie do you want? [";
for_each( cookies.begin(), cookies.end(), []( std::map< int, string >::value_type & v ) { cout << v.first << ','; } );
cout << ']';
cin >> choice;

map< int, string >::iterator iter( cookies.find( choice ) );
if( iter == cookies.end() )
    cout << "Sorry, that cookie has been taken" << endl;
else {
    cout << iter->second << endl;
    cookies.erase( iter );
}

If possible, please try to explain it to me like you're explaining how to walk to a child, I really only know the basics of the basic.

Thank you

It sounds like you could use a good primer to C++, from a book perhaps. There are even some free ones. Try C++ Annotations. — Yam Marcovic, Jan 23 '16 at 18:56
Here is some good books list : http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — Angelus Mortis, Jan 23 '16 at 18:56
Too many unrelated questions. Break them up and be more specific. — ThomasMcLeod, Jan 23 '16 at 19:02

PeterSW · Answer 1 · 2016-01-24T20:46:27.853

2

The #include <ctime> is unneeded in the code you have posted. Removing it would have no effect.

The #include <algorithm> is needed for the std::for_each. The std:: prefix is not needed as some of the arguments are also in the std namespace.

std::for_each is a function that calls the functor given as the third argument on each element between the given begin and end iterators.

[]( std::map< int, string >::value_type & v ) { cout << v.first << ','; } is a lambda function used as the above mentioned functor.

The for_each block is maybe easier to understand written as a range based for loop:

for(auto v: cookies)
{
    cout << v.first << ',';
}

edited Jan 24 '16 at 20:46

answered Jan 23 '16 at 19:07

PeterSW

4,921
1
24
35

It is interesting to note that this ugly `using namespace std;` is not required for `for_each`, thanks to ADL. – Quentin Jan 24 '16 at 02:32
Good point, thanks @Quentin – PeterSW Jan 24 '16 at 20:47

score 1 · Answer 2 · edited Jan 27 '16 at 20:56

#include <map> you need for using standart stl map container more info here
the same for header <algorithm> it's allow you to use algorithm for_each more info here
Atually I don't know for what header <ctime> needed here but you can read about it here
fore_each

template <class InputIterator, class Function> Function for_each (InputIterator first, InputIterator last, Function fn);

Applies function fn to each of the elements in the range [first,last).

And also more info you also can find in different books

C++ Could somebody help me explain a map/for_each code [EDIT]

2 Answers2