Mastering CS50 Tideman: A Complete Guide to the Ranked-Choice Voting Algorithm
bool creates_cycle(int winner, int loser)
for (int i = 0; i < candidate_count; i++) bool is_winner = true; for (int j = 0; j < candidate_count; j++) if (locked[j][i]) is_winner = false; break;
Some students only check if preferences[i][j] > preferences[j][i] , forgetting that ties should be skipped entirely.
This function is identical to the one in plurality. It should record the voter’s rank for each candidate. Cs50 Tideman Solution
max_pair_wins = max(pair_wins) winners = [candidates[i] for i, pair_wins_i in enumerate(pair_wins) if pair_wins_i == max_pair_wins]
Nested loops checking preferences[i][j] > preferences[j][i] . If true, set pairs[pair_count].winner = i , pairs[pair_count].loser = j , and increment pair_count . 4. void sort_pairs(void)
bool has_incoming = false; for (int j = 0; j < candidate_count; j++)
bool is_source = true; for (int j = 0; j < candidate_count; j++) Mastering CS50 Tideman: A Complete Guide to the
return false;
Use a standard sorting algorithm like Selection Sort , Bubble Sort , or Merge Sort . Sort the pairs array in descending order of victory strength. 5. lock_pairs (The Hardest Part)
for (int i = 0; i < pair_count; i++)
Sort all pairings of candidates by the margin of victory, then lock them in a graph (draw an edge from winner to loser) only if it does not create a cycle. void sort_pairs(void) bool has_incoming = false; for (int
To truly master lock_pairs , you need to deeply understand cycle detection. The problem requires you to prevent any cycles in the final graph. A common initial approach is to only look at whether the loser of the current pair is already locked over the winner . However, this only catches direct cycles (A->B and B->A). The Tideman method requires checking for indirect cycles that span multiple edges.
The lock_pairs function builds the final graph by setting locked[i][j] = true .
The algorithm proceeds in several steps: