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tideman.c
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#include <cs50.h>
#include <stdio.h>
#include <string.h>
// Max number of candidates
#define MAX 9
// Preferences[i][j] is number of voters who prefer i over j
int preferences[MAX][MAX];
// Locked[i][j] means i is locked in over j
bool locked[MAX][MAX];
// Each pair has a winner, loser
typedef struct
{
int winner;
int loser;
}
pair;
// Array of candidates
string candidates[MAX];
pair pairs[MAX * (MAX - 1) / 2];
int pair_count;
int candidate_count;
// Function prototypes
bool vote(int rank, string name, int ranks[]);
void record_preferences(int ranks[]);
void add_pairs(void);
void sort_pairs(void);
void lock_pairs(void);
void print_winner(void);
bool has_cycle(int loser, int winner);
int main(int argc, string argv[])
{
// Check for invalid usage
if (argc < 2)
{
printf("Usage: tideman [candidate ...]\n");
return 1;
}
// Populate array of candidates
candidate_count = argc - 1;
if (candidate_count > MAX)
{
printf("Maximum number of candidates is %i\n", MAX);
return 2;
}
for (int i = 0; i < candidate_count; i++)
{
candidates[i] = argv[i + 1];
}
// Clear graph of locked in pairs
for (int i = 0; i < candidate_count; i++)
{
for (int j = 0; j < candidate_count; j++)
{
locked[i][j] = false;
}
}
pair_count = 0;
int voter_count = get_int("Number of voters: ");
// Query for votes
for (int i = 0; i < voter_count; i++)
{
// ranks[i] is voter's ith preference
int ranks[candidate_count];
// Query for each rank
for (int j = 0; j < candidate_count; j++)
{
string name = get_string("Rank %i: ", j + 1);
if (!vote(j, name, ranks))
{
printf("Invalid vote.\n");
return 3;
}
}
record_preferences(ranks);
printf("\n");
}
add_pairs();
sort_pairs();
lock_pairs();
print_winner();
return 0;
}
// Update ranks given a new vote
bool vote(int rank, string name, int ranks[])
{
for (int i = 0; i < candidate_count; i++)
{
if (strcmp(candidates[i], name) == 0)
{
ranks[rank] = i;
return true;
}
}
return false;
}
// Update preferences given one voter's ranks
void record_preferences(int ranks[])
{
for (int row = 0; row < candidate_count; row++)
{
for (int col = 0; col < candidate_count; col++)
{
if (row == col)
{
preferences[ranks[row]][ranks[col]] = 0;
}
else if (row < col)
{
preferences[ranks[row]][ranks[col]]++;
}
}
}
return;
}
// Record pairs of candidates where one is preferred over the other
void add_pairs(void)
{
/**
* Loop over the preferences array
* Compare the pairs, like 0 and 1
* If preferences[0][1] has a greater number than preferences[1][0]
* Then declare the winner and loser of the comparison
* Update the pairs array based on that
* Update the pairs count to be the number of pairs candidates
*
*/
pair_count = 0;
for (int row = 0; row < candidate_count; row++)
{
//loop through
for (int col = row + 1; col < candidate_count; col++)
{
if (row != col)
{
if (preferences[row][col] > preferences[col][row])
{
pairs[pair_count].winner = row;
pairs[pair_count].loser = col;
pair_count++;
}
else if (preferences[row][col] < preferences[col][row])
{
pairs[pair_count].winner = col;
pairs[pair_count].loser = row;
pair_count++;
}
}
}
}
}
// Sort pairs in decreasing order by strength of victory
void sort_pairs(void)
{
/**
* Loop through the pairs array
* If the preference of a candidate is higher bubble sort the pairs array
* Swap the pair to to the left
*/
for (int i = 0; i < pair_count - 1; i++)
{
for (int j = 0; j < pair_count - i - 1; j++)
{
if (preferences[pairs[j].winner][pairs[j].loser] < preferences[pairs[j + 1].winner][pairs[j + 1].loser])
{
pair temp = pairs[j];
pairs[j] = pairs[j + 1];
pairs[j + 1] = temp;
}
else if (preferences[pairs[i].winner][pairs[i].loser] == preferences[pairs[i].loser][pairs[i].winner])
{
return;
}
}
}
return;
}
bool has_cycle(int loser, int winner)
{
if (loser == winner)
{
return true;
}
for (int i = 0; i < candidate_count; i++)
{
if (locked[loser][i])
{
if (has_cycle(i, winner))
{
return true;
}
}
}
return false;
}
// Lock pairs into the candidate graph in order, without creating cycles
void lock_pairs(void)
{
/**
* Loop through the locked array
* Creating edges by decreasing of victory strengh
* If the winner of the first pair is the same as the loser of
* The last pair and they altenate between pairs,
* It is a sign of a cycle and that last arrow should be skipped
*/
for (int i = 0; i < pair_count; i++)
{
if (!has_cycle(pairs[i].loser, pairs[i].winner))
{
locked[pairs[i].winner][pairs[i].loser] = true;
}
}
return;
}
// Print the winner of the election
void print_winner(void)
{
for (int col = 0; col < candidate_count; col++)
{
int source_found = 0;
for (int row = 0; row < candidate_count; row++)
{
if (locked[row][col] == false)
{
source_found++;
if (source_found == candidate_count)
{
printf("%s\n", candidates[col]);
}
}
}
}
return;
}