Notes

Outline

Problem-Solving Case Study – City Council Election Problem: In a city council election, citizens in four voting precincts have cast their ballots for four candidates. We want to know how many votes each candidate received in each precinct, how many total votes each candidate received, and how many total votes were cast in each precinct. Input: An arbitrary number of votes in a file voteFile, with each vote represented as a pair of numbers: a precinct number and a candidate number. The candidate names from keyboard.

Problem-Solving Case Study – City Council Election Output: A tabular report showing how many votes each candidate received in each precinct, the total number of votes for each candidate, and the total number of votes in each precinct, all written to reportFile. Discussion: If I were doing this by hand, I would probably write down the candidate names as the row labels of a table and use the precinct numbers as the column labels and put a tally mark in the appropriate place as I read each vote.

Problem-Solving Case Study – City Council Election More Discussion: A table is easily represented by a two-dimensional array. We can save the names of the candidates in a one-dimensional array. The book example uses precincts for row headings and candidate numbers for column headings. Data Structures: A two-dimensional array names votes, where the rows represent precincts and the columns represent candidates. A one-dimensional array of strings containing the names of the candidates.

Main Open voteFile for input (and verify success) Open reportFile for output (and verify success) Get candidate names Set votes array to 0 Read precinct, candidate from voteFile WHILE NOT EOF on voteFile   Increment votes[precinct-1][candidate-1] Read precinct, candidate from voteFile Write report to reportFile Write totals per candidate to reportFile Write totals per precinct to reportFile

Write Totals per Precinct (In: votes; Inout: reportFile) FOR each precinct Set total = 0 // Compute row sum FOR each candidate Add votes[precinct][candidate] to total Write “Total votes for precinct”, precinct+1, ‘:’, total to reportFile

Implementation //*************************************************************** // Election program // This program reads votes represented by precinct number and // ballot position from a data file, calculates the sums per // precinct and per candidate, and writes all totals to an // output file //*************************************************************** #include … using namespace std; const int NUM_PRECINCTS = 4; const int NUM_CANDIDATES = 4; typedef int VoteArray[NUM_PRECINCTS][NUM_CANDIDATES];                                  // 2-dimensional array type                                  //   for votes

Implementation void GetNames( string[] ); void OpenForInput( ifstream& ); void OpenForOutput( ofstream& ); void WritePerCandidate(const VoteArray, const string[],                         ofstream& ); void WritePerPrecinct( const VoteArray, ofstream& ); void WriteReport( const VoteArray, const string[], ofstream& ); void ZeroVotes( VoteArray ); int main() {     string    name[NUM_CANDIDATES];  // Array of candidate names     VoteArray votes;        // Totals for precincts vs. candidates     int       candidate;   // Candidate number input from voteFile     int       precinct;    // Precinct number input from voteFile     ifstream  voteFile;     // Input file of precincts, candidates     ofstream  reportFile;    // Output file receiving summaries

Implementation     OpenForInput(voteFile);     if ( !voteFile )         return 1;     OpenForOutput(reportFile);     if ( !reportFile )         return 1;     GetNames(name);     ZeroVotes(votes);     // Read and tally votes     voteFile >> precinct >> candidate;     while (voteFile)     {         votes[precinct-1][candidate-1]++;         voteFile >> precinct >> candidate;     }

Implementation     // Write results to report file     WriteReport(votes, name, reportFile);     WritePerCandidate(votes, name, reportFile);     WritePerPrecinct(votes, reportFile);     return 0; }

Implementation void OpenForInput( /*inout*/ ifstream& someFile )  // File to be                                                      // opened // Prompts the user for the name of an input file // and attempts to open the file {     string fileName;    // User-specified file name     cout << "Input file name: ";     cin >> fileName;     someFile.open(fileName.c_str());     if ( !someFile )         cout << "** Can't open " << fileName << " **" << endl; }

Implementation void OpenForOutput( /*inout*/ ofstream& someFile )  // File to be                                                     // opened {     string fileName;    // User-specified file name     cout << "Output file name: ";     cin >> fileName;     someFile.open(fileName.c_str());     if ( !someFile )         cout << "** Can't open " << fileName << " **" << endl; }

Implementation void GetNames( /*out*/ string name[] )    // Array of candidate                                           //   names // Reads the candidate names from standard input {     string inputStr;      // An input string     int    candidate;     // Loop counter     cout << "Enter the names of the candidates, one per line,"          << endl << "in the order they appear on the ballot."          << endl;     for (candidate = 0; candidate < NUM_CANDIDATES; candidate++)     {         cin >> inputStr;         name[candidate] = inputStr.substr(0, 10);     } }

Implementation void ZeroVotes( /*out*/ VoteArray votes ) // Array of vote totals // Zeroes out the votes array` {    int precinct;       // Loop counter    int candidate;      // Loop counter    for (precinct = 0; precinct < NUM_PRECINCTS; precinct++)      for (candidate = 0; candidate < NUM_CANDIDATES; candidate++)         votes[precinct][candidate] = 0; }

Implementation void WriteReport(        /*in*/    const VoteArray votes,      // Total votes        /*in*/    const string    name[],     // Candidate names        /*inout*/       ofstream& reportFile )   // Output file // Writes the vote totals in tabular form to the report file {     int precinct;       // Loop counter     int candidate;      // Loop counter     // Set up headings     reportFile << "            ";     for (candidate = 0; candidate < NUM_CANDIDATES; candidate++)         reportFile << setw(12) << name[candidate];     reportFile << endl;

Implementation    // Print array by row    for (precinct = 0; precinct < NUM_PRECINCTS; precinct++)    {      reportFile << "Precinct" << setw(4) << precinct + 1;      for (candidate = 0; candidate < NUM_CANDIDATES; candidate++)          reportFile << setw(12) << votes[precinct][candidate];      reportFile << endl;    }    reportFile << endl; }

Implementation void WritePerCandidate(        /* in */    const VoteArray votes,       // Total votes        /* in */    const string    name[],      // Candidate names        /* inout */       ofstream& reportFile ) // Output file // Sums the votes per person and writes the totals to the // report file {     int precinct;       // Loop counter     int candidate;      // Loop counter     int total;          // Total votes for a candidate     for (candidate = 0; candidate < NUM_CANDIDATES; candidate++)     {         total = 0;

Implementation         // Compute column sum         for (precinct = 0; precinct < NUM_PRECINCTS; precinct++)             total = total + votes[precinct][candidate];         reportFile << "Total votes for"                    << setw(10) << name[candidate] << ":"                    << setw(3) << total << endl;     }     reportFile << endl; }

Implementation void WritePerPrecinct(            /*in*/    const VoteArray votes,       // Total votes            /*inout*/       ofstream& reportFile ) // Output file // Sums the votes per precinct and writes the totals to the // report file {     int precinct;       // Loop counter     int candidate;      // Loop counter     int total;          // Total votes for a precinct     for (precinct = 0; precinct < NUM_PRECINCTS; precinct++)     {         total = 0;

Implementation      // Compute row sum      for (candidate = 0; candidate < NUM_CANDIDATES; candidate++)          total = total + votes[precinct][candidate];      reportFile << "Total votes for precinct"                   << setw(3) << precinct + 1 << ':'                   << setw(3) << total << endl;   } }