Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Combining STL containers

When using a thesaurus, you have a word and you want to know all the words that are similar. When you look up a word, then, you want a list of words as the result. Here, the “multi” containers ( multimap or multiset) are not appropriate. The solution is to combine containers, which is easily done using the STL. Here, we need a tool that turns out to be a powerful general concept, which is a map of vector:

//: C20:Thesaurus.cpp
// A map of vectors
#include <map>
#include <vector>
#include <string>
#include <iostream>
#include <algorithm>
#include <ctime>
using namespace std;

typedef map<string, vector<string> > Thesaurus;
typedef pair<string, vector<string> > TEntry;
typedef Thesaurus::iterator TIter;

ostream& operator<<(ostream& os,const TEntry& t){
  os << t.first << ": ";
  copy(t.second.begin(), t.second.end(),
    ostream_iterator<string>(os, " "));
  return os;
}

// A generator for thesaurus test entries:
class ThesaurusGen {
  static const string letters;
  static int count;
public:
  int maxSize() { return letters.size(); }
  ThesaurusGen() { srand(time(0)); }
  TEntry operator()() {
    TEntry result;
    if(count >= maxSize()) count = 0;
    result.first = letters[count++];
    int entries = (rand() % 5) + 2;
    for(int i = 0; i < entries; i++) {
      int choice = rand() % maxSize();
      char cbuf[2] = { 0 };
      cbuf[0] = letters[choice];
      result.second.push_back(cbuf);
    }
    return result;
  }
};

int ThesaurusGen::count = 0;
const string ThesaurusGen::letters("ABCDEFGHIJKL"
  "MNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz");

int main() {
  Thesaurus thesaurus;
  // Fill with 10 entries:
  generate_n(
    inserter(thesaurus, thesaurus.begin()), 
    10, ThesaurusGen());
  // Print everything:
  copy(thesaurus.begin(), thesaurus.end(),
    ostream_iterator<TEntry>(cout, "\n"));
  // Ask for a "word" to look up:
  while(true) {
    cout << "Select a \"word\", 0 to quit: ";
    for(TIter it = thesaurus.begin(); 
      it != thesaurus.end(); it++)
      cout << (*it).first << ' ';
    cout << endl;
    string reply;
    cin >> reply;
    if(reply.at(0) == '0') return 0; // Quit
    if(thesaurus.find(reply) == thesaurus.end())
      continue; // Not in list, try again
    vector<string>& v = thesaurus[reply];
    copy(v.begin(), v.end(), 
      ostream_iterator<string>(cout, " "));
    cout << endl;
  }
} ///:~

A Thesaurus maps a string (the word) to a vector<string> (the synonyms). A TEntry is a single entry in a Thesaurus. By creating an ostream operator<< for a TEntry, a single entry from the Thesaurus can easily be printed (and the whole Thesaurus can easily be printed with copy( )). The ThesaurusGen creates “words” (which are just single letters) and “synonyms” for those words (which are just other randomly-chosen single letters) to be used as thesaurus entries. It randomly chooses the number of synonym entries to make, but there must be at least two. All the letters are chosen by indexing into a static string that is part of ThesaurusGen.

In main( ), a Thesaurus is created, filled with 10 entries and printed using the copy( ) algorithm. Then the user is requested to choose a “word” to look up by typing the letter of that word. The find( ) member function is used to find whether the entry exists in the map (remember, you don’t want to use operator[ ] or it will automatically make a new entry if it doesn’t find a match!). If so, operator[ ] is used to fetch out the vector<string> which is displayed.

Because templates make the expression of powerful concepts easy, you can take this concept much further, creating a map of vectors containing maps, etc. For that matter, you can combine any of the STL containers this way.

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