Using an Unordered Container

Aside from operations that manage the hashing, the unordered containers provide the same operations (find, insert, and so on) as the ordered containers. That means that the operations we’ve used on map and set apply to unordered_map and unordered_set as well. Similarly for the unordered versions of the containers that allow multiple keys.

As a result, we can usually use an unordered container in place of the corresponding ordered container, and vice versa. However, because the elements are not stored in order, the output of a program that uses an unordered container will (ordinarily) differ from the same program using an ordered container.

For example, we can rewrite our original word-counting program from § 11.1 (p. 421) to use an unordered_map:

// count occurrences, but the words won't be in alphabetical order
unordered_map<string, size_t> word_count;
string word;
while (cin >> word)
    ++word_count[word]; // fetch and increment the counter for word
for (const auto &w : word_count) // for each element in the map
     // print the results
     cout <<  w.first << " occurs " << w.second
          << ((w.second > 1) ? " times" : " time") << endl;

The type of word_count is the only difference between this program and our original. If we run this version on the same input as our original program,

containers. occurs 1 time
use occurs 1 time
can occurs 1 time
examples occurs 1 time
...

we’ll obtain the same count for each word in the input. However, the output is unlikely to be in alphabetical order.