The Standard Library also defines a helper function that facilitates the creation of a pair type:
is an associative container that uses a pair of types, the first of which is the index and the second the associated value.
We can use a map like this:
#include map addresses; To add an item to a map, use the subscript operator.
addresses["Paul W."]="[email protected] ";Here, the string Paul W. serves as the index or key, and [email protected]
addresses["Paul W."]= "[email protected] "; //overrides "[email protected] "Step 2: Searching find() member function. find() has two overloaded versions:
iterator find(const key_type& k);const_iterator find(const key_type& k) const;As usual, a typedef can make the code less cumbersome:
typedef map ::const_iterator CIT;CIT cit=addresses.find("Paul W.");if (cit==addresses.end()) cout << "sorry, no such key" << endl;else cout << cit->first << ' ' << cit->second << endl; The expressions cit->first and cit->second return the key and its associated value, respectively.
There is another way to test whether a certain pair is present without changing its associated value, namely, using the insert() member function:
// calls pair map::insert(const value_type& x);// the bool part reports whether the insertion succeededif (addresses.insert(make_pair("Paul W.", "[email protected] ")).second== false){ // pair not inserted because the key already has a value // original value left unchanged} Step 3: Traversal
Bob 35Bob 90Jane 80.25Sue 100Jane 65.5Your application has to sum up all these bonuses and print the total for each agent. First, create a map and read the data from the file:
map bonuses;string agent;double bonus=0; ifstream bonusfile("bonuses.dat");if(!bonusfile){ //report the error and terminate}while (bonusfile >> agent >> bonus){ bonuses[agent]+=bonus;//accumulate bonuses per agent} Believe it or not, that's all! Let's analyze this loop. After opening the data file, the while-loop reads each pair into the objects agent and bonus . Next, it inserts agent and bonus into the map. The trick here is that if the key (agent) already exists, the += operator adds the newly-read bonus to the current bonus stored in the map. This is possible because the expression
map[key] returns the value associated with key . This value is then added to the newly-read bonus when the overloaded operator += is invoked. Finally, the sum of these two values overrides the existing associated value in the map.
Luckily, when the map doesn't contain the sought-after key, the expression
map[key] returns a default-initialized T where T is double in our example. A default-initialized double equals 0.
At this stage, the map contains pairs of agents and their combined bonus value. The following loop displays these pairs:
for(CIT p=bonuses.begin(); p!=bonuses.end(); ++p){ cout << p->first <<' ' << p->second <
Figure 1 .
The output is shown in Figure 1 .
Hashed Associative Containers hashed associative container yet. Such containers can improve performance significantly as they use a hashing algorithm to derive a short key from the original string index. However, many IDEs offer hashed containers as a non-standard extension. Fortunately, the new C++0X standard remedies this omission by adding a set of hashed containers and related algorithms to C++.
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elational databases, scientific apps, and Web-based systems often need a vector-like container whose index can be of any type, not necessarily int. Such containers are known as associative containers or maps. For instance, a directory service application can store private names as indexes and phone numbers as their associated values:
