C++ Primer Plus

    const int N = 6;

    string s1[N] = {"buffoon", "thinkers", "for", "heavy", "can", "for"};

    string s2[N] = {"metal", "any", "food", "elegant", "deliver","for"};

    set A(s1, s1 + N);

    set B(s2, s2 + N);

    ostream_iterator out(cout, " ");

    cout << "Set A: ";

    copy(A.begin(), A.end(), out);

    cout << endl;

    cout << "Set B: ";

    copy(B.begin(), B.end(), out);

    cout << endl;

    cout << "Union of A and B:\n";

    set_union(A.begin(), A.end(), B.begin(), B.end(), out);

    cout << endl;

    cout << "Intersection of A and B:\n";

    set_intersection(A.begin(), A.end(), B.begin(), B.end(), out);

    cout << endl;

    cout << "Difference of A and B:\n";

    set_difference(A.begin(), A.end(), B.begin(), B.end(), out);

    cout << endl;

    set C;

    cout << "Set C:\n";

    set_union(A.begin(), A.end(), B.begin(), B.end(),

        insert_iterator >(C, C.begin()));

    copy(C.begin(), C.end(), out);

    cout << endl;

    string s3("grungy");

    C.insert(s3);

    cout << "Set C after insertion:\n";

    copy(C.begin(), C.end(),out);

    cout << endl;

    cout << "Showing a range:\n";

    copy(C.lower_bound("ghost"),C.upper_bound("spook"), out);

    cout << endl;

    return 0;

}

Here is the output of the program in Listing 16.13:

Set A: buffoon can for heavy thinkers

Set B: any deliver elegant food for metal

Union of A and B:

any buffoon can deliver elegant food for heavy metal thinkers

Intersection of A and B:

for

Difference of A and B:

buffoon can heavy thinkers

Set C:

any buffoon can deliver elegant food for heavy metal thinkers

Set C after insertion:

any buffoon can deliver elegant food for grungy heavy metal thinkers

Showing a range:

grungy heavy metal

Like most of the examples in this chapter, the code in Listing 16.13 takes the lazy route for handling the std namespace:

using namespace std;

It does so in order to simplify the presentation. The examples use so many elements of the std namespace that using directives or the scope-resolution operators would tend to make the code look a bit fussy:

std::set B(s2, s2 + N);

std::ostream_iterator out(std::cout, " ");

std::cout << "Set A: ";

std::copy(A.begin(), A.end(), out);

A multimap Example

Like set, multimap is a reversible, sorted, associative container. However, with multimap, the key type is different from the value type, and a multimap object can have more than one value associated with a particular key.

The basic multimap declaration specifies the key type and the type of value, stored as template arguments. For example, the following declaration creates a multimap object that uses int as the key type and string as the type of value stored:

multimap codes;

An optional third template argument can be used to indicate a comparison function or an object to be used to order the key. By default, the less<> template (discussed later) is used with the key type as its parameter. Older C++ implementations may require this template parameter explicitly.

To keep information together, the actual value type combines the key type and the data type into a single pair. To do this, the STL uses a pair template class for storing two kinds of values in a single object. If keytype is the key type and datatype is the type of the stored data, the value type is pair. For example, the value type for the codes object declared earlier is pair.

Suppose that you want to store city names, using the area code as a key. This happens to fit the codes declaration, which uses an int for a key and a string as a data type. One approach is to create a pair and then insert it into the multimap object:

pair item(213, "Los Angeles");

codes.insert(item);

Or you can create an anonymous pair object and insert it in a single statement:

codes.insert(pair (213, "Los Angeles"));

Because items are sorted by key, there’s no need to identify an insertion location.

Given a pair object, you can access the two components by using the first and second members:

pair item(213, "Los Angeles");

cout << item.first << ' ' << item.second << endl;

What about getting information about a multimap object? The count() member function takes a key as its argument and returns the number of items that have that key. The lower_bound() and upper_bound() member functions take a key and work as they do for set. Also the equal_range() member function takes a key as its argument and returns iterators representing the range matching that key. In order to return two values, the method packages them into a pair object, this time with both template arguments being the iterator type. For example, the following would print a list of cities in the codes object with area code 718:

pair::iterator,

     multimap::iterator> range

                         = codes.equal_range(718);

cout << "Cities with area code 718:\n";