15.10. A proper distance matrix

Although this code works, it is not as well organized as it should be. Now that we have written a prototype, we are in a good position to evaluate the design and improve it.

What are some of the problems with the existing code?

1. We did not know ahead of time how big to make the distance matrix, so we chose an arbitrary large number (50) and made it a fixed size. It would be better to allow the distance matrix to expand in the same way a Set does. The matrix class has a function called resize that makes this possible.

2. The data in the distance matrix is not well-encapsulated. We have to pass the set of city names and the matrix itself as arguments to processLine, which is awkward. Also, use of the distance matrix is error prone because we have not provided accessor functions that perform error-checking. It might be a good idea to take the Set of city names and the matrix of distances, and combine them into a single object called a DistMatrix.

Here is a draft of what the header for a DistMatrix might look like:

class DistMatrix {
private:
  Set cities;
  matrix<int> distances;

public:
  DistMatrix (int rows);

  void add (const string& city1, const string& city2, int dist);
  int distance (int i, int j) const;
  int distance (const string& city1, const string& city2) const;
  string cityName (int i) const;
  int numCities () const;
  void print ();
};

Using this interface simplifies main:

#include <iostream>
#include <fstream>
using namespace std;


int main ()
{
  string line;
  ifstream infile ("distances");
  DistMatrix distances (2);

  while (true) {
    getline (infile, line);
    if (infile.eof()) break;
    processLine (line, distances);
  }

  distances.print ();
  return 0;
}

It also simplifies processLine:

void processLine (const string& line, DistMatrix& distances)
{
  char quote = '\"';
  vector<int> quoteIndex (4);
  quoteIndex[0] = line.find (quote);
  for (int i=1; i<4; i++) {
    quoteIndex[i] = find (line, quote, quoteIndex[i-1]+1);
  }

  // break the line up into substrings
  int len1 = quoteIndex[1] - quoteIndex[0] - 1;
  string city1 = line.substr (quoteIndex[0]+1, len1);
  int len2 = quoteIndex[3] - quoteIndex[2] - 1;
  string city2 = line.substr (quoteIndex[2]+1, len2);
  int len3 = line.length() - quoteIndex[2] - 1;
  string distString = line.substr (quoteIndex[3]+1, len3);
  int distance = convertToInt (distString);

  // add the new datum to the distances matrix
  distances.add (city1, city2, distance);
}

I will leave it as an exercise to you to implement the member functions of DistMatrix.

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