Add Prims's Algorithm for MST (#248)

Author: Erennn7

graph_algorithms/prim_mst.r

@@ -0,0 +1,255 @@
+# Prim's Algorithm for Minimum Spanning Tree (MST)
+#
+# Prim's algorithm finds the MST of a connected, weighted, undirected graph.
+# It starts from an arbitrary node and repeatedly adds the smallest edge
+# connecting the growing MST to a new vertex.
+#
+# Time Complexity: O(V^2) using adjacency matrix, O((V + E) log V) with priority queue
+# Space Complexity: O(V) for key, parent, and visited arrays
+#
+# Applications:
+# - Network design (telecommunications, computer networks)
+# - Circuit design and electrical grids
+# - Clustering algorithms in machine learning
+# - Transportation and logistics planning
+# - Approximation algorithms for NP-hard problems
+
+#' Compute the Minimum Spanning Tree using Prim's algorithm
+#' @param graph Adjacency matrix of the graph (use 0 or Inf for no edge)
+#' @param start_vertex Starting vertex (default is 1)
+#' @return List with MST edges, total weight, and parent array
+prim_mst <- function(graph, start_vertex = 1) {
+  # Validate input
+  if (!is.matrix(graph)) {
+    stop("Error: Input must be a matrix")
+  }
+  
+  if (nrow(graph) != ncol(graph)) {
+    stop("Error: Graph must be a square adjacency matrix")
+  }
+  
+  n <- nrow(graph)
+  
+  if (start_vertex < 1 || start_vertex > n) {
+    stop(sprintf("Error: start_vertex must be between 1 and %d", n))
+  }
+  
+  # Initialize arrays
+  key <- rep(Inf, n)          # Minimum weight to include vertex
+  parent <- rep(NA, n)        # Store MST edges
+  in_mst <- rep(FALSE, n)     # Vertices included in MST
+  
+  # Start from specified vertex
+  key[start_vertex] <- 0
+  parent[start_vertex] <- -1  # Root of MST
+  
+  # Build MST with n vertices
+  for (count in 1:n) {
+    # Pick vertex u not in MST with minimum key value
+    min_key <- Inf
+    u <- NA
+    
+    for (v in 1:n) {
+      if (!in_mst[v] && key[v] < min_key) {
+        min_key <- key[v]
+        u <- v
+      }
+    }
+    
+    # Check if graph is disconnected
+    if (is.na(u)) {
+      warning("Graph is disconnected. MST is incomplete.")
+      break
+    }
+    
+    # Include u in MST
+    in_mst[u] <- TRUE
+    
+    # Update key and parent for adjacent vertices of u
+    for (v in 1:n) {
+      # Check if there's an edge, v is not in MST, and edge weight is smaller
+      if (graph[u, v] != 0 && graph[u, v] != Inf && 
+          !in_mst[v] && graph[u, v] < key[v]) {
+        key[v] <- graph[u, v]
+        parent[v] <- u
+      }
+    }
+  }
+  
+  # Construct MST edges and calculate total weight
+  mst_edges <- list()
+  total_weight <- 0
+  edge_count <- 0
+  
+  for (v in 1:n) {
+    if (!is.na(parent[v]) && parent[v] != -1) {
+      edge_count <- edge_count + 1
+      mst_edges[[edge_count]] <- list(
+        from = parent[v],
+        to = v,
+        weight = graph[parent[v], v]
+      )
+      total_weight <- total_weight + graph[parent[v], v]
+    }
+  }
+  
+  return(list(
+    edges = mst_edges,
+    total_weight = total_weight,
+    parent = parent,
+    num_edges = edge_count
+  ))
+}
+
+#' Print MST in a formatted way
+#' @param mst Result from prim_mst function
+#' @param graph Original graph (for verification)
+print_mst <- function(mst, graph = NULL) {
+  cat("Minimum Spanning Tree:\n")
+  cat(strrep("=", 50), "\n\n")
+  
+  if (length(mst$edges) == 0) {
+    cat("No edges in MST (graph may be disconnected)\n")
+    return()
+  }
+  
+  cat(sprintf("%-10s %-10s %-10s\n", "Edge", "Vertices", "Weight"))
+  cat(strrep("-", 50), "\n")
+  
+  for (i in seq_along(mst$edges)) {
+    edge <- mst$edges[[i]]
+    cat(sprintf("%-10d %-10s %-10g\n", 
+                i, 
+                paste0(edge$from, " -- ", edge$to),
+                edge$weight))
+  }
+  
+  cat(strrep("-", 50), "\n")
+  cat(sprintf("Total Weight: %g\n", mst$total_weight))
+  cat(sprintf("Number of Edges: %d\n", mst$num_edges))
+  
+  # Verify MST properties
+  if (!is.null(graph)) {
+    n <- nrow(graph)
+    expected_edges <- n - 1
+    if (mst$num_edges == expected_edges) {
+      cat(sprintf("[OK] MST has correct number of edges (%d)\n", expected_edges))
+    } else {
+      cat(sprintf("[WARN] MST has %d edges, expected %d (graph may be disconnected)\n", 
+                  mst$num_edges, expected_edges))
+    }
+  }
+  cat("\n")
+}
+
+#' Create adjacency matrix from edge list
+#' @param edges List of edges, each with from, to, and weight
+#' @param n_vertices Number of vertices
+#' @return Adjacency matrix
+create_graph_from_edges <- function(edges, n_vertices) {
+  graph <- matrix(0, nrow = n_vertices, ncol = n_vertices)
+  
+  for (edge in edges) {
+    graph[edge$from, edge$to] <- edge$weight
+    graph[edge$to, edge$from] <- edge$weight  # Undirected graph
+  }
+  
+  return(graph)
+}
+
+# ========== Example 1: Basic 5-vertex graph ==========
+cat("========== Example 1: Basic 5-Vertex Graph ==========\n\n")
+graph1 <- matrix(c(
+  0, 2, 0, 6, 0,
+  2, 0, 3, 8, 5,
+  0, 3, 0, 0, 7,
+  6, 8, 0, 0, 9,
+  0, 5, 7, 9, 0
+), nrow = 5, byrow = TRUE)
+cat("Graph adjacency matrix:\n")
+print(graph1)
+cat("\n")
+mst1 <- prim_mst(graph1)
+print_mst(mst1, graph1)
+
+# ========== Example 2: 6-vertex graph ==========
+cat("========== Example 2: 6-Vertex Graph ==========\n\n")
+graph2 <- matrix(c(
+  0, 4, 0, 0, 0, 0,
+  4, 0, 8, 0, 0, 0,
+  0, 8, 0, 7, 0, 4,
+  0, 0, 7, 0, 9, 14,
+  0, 0, 0, 9, 0, 10,
+  0, 0, 4, 14, 10, 0
+), nrow = 6, byrow = TRUE)
+cat("Graph adjacency matrix:\n")
+print(graph2)
+cat("\n")
+mst2 <- prim_mst(graph2, start_vertex = 1)
+print_mst(mst2, graph2)
+
+# ========== Example 3: Using edge list representation ==========
+cat("========== Example 3: Graph from Edge List ==========\n\n")
+edges <- list(
+  list(from = 1, to = 2, weight = 1),
+  list(from = 1, to = 3, weight = 4),
+  list(from = 2, to = 3, weight = 2),
+  list(from = 2, to = 4, weight = 5),
+  list(from = 3, to = 4, weight = 3)
+)
+graph3 <- create_graph_from_edges(edges, 4)
+cat("Graph from edge list:\n")
+for (edge in edges) {
+  cat(sprintf("%d -- %d : %g\n", edge$from, edge$to, edge$weight))
+}
+cat("\nAdjacency matrix:\n")
+print(graph3)
+cat("\n")
+mst3 <- prim_mst(graph3)
+print_mst(mst3, graph3)
+
+# ========== Example 4: Disconnected graph ==========
+cat("========== Example 4: Disconnected Graph ==========\n\n")
+graph4 <- matrix(c(
+  0, 1, 0, 0,
+  1, 0, 0, 0,
+  0, 0, 0, 2,
+  0, 0, 2, 0
+), nrow = 4, byrow = TRUE)
+cat("Disconnected graph adjacency matrix:\n")
+print(graph4)
+cat("\n")
+mst4 <- prim_mst(graph4)
+print_mst(mst4, graph4)
+
+# ========== Example 5: Complete graph K4 ==========
+cat("========== Example 5: Complete Graph K4 ==========\n\n")
+graph5 <- matrix(c(
+  0, 1, 2, 3,
+  1, 0, 4, 5,
+  2, 4, 0, 6,
+  3, 5, 6, 0
+), nrow = 4, byrow = TRUE)
+cat("Complete graph K4:\n")
+print(graph5)
+cat("\n")
+mst5 <- prim_mst(graph5)
+print_mst(mst5, graph5)
+
+# ========== Notes ==========
+cat("========== Algorithm Properties ==========\n\n")
+cat("1. Prim's algorithm guarantees finding the MST for connected graphs\n")
+cat("2. Works only on undirected graphs with non-negative weights\n")
+cat("3. MST has exactly (V-1) edges for a connected graph with V vertices\n")
+cat("4. Total weight of MST is unique, but MST itself may not be unique\n")
+cat("5. Starting vertex doesn't affect the total weight of the MST\n\n")
+
+cat("========== Comparison with Kruskal's Algorithm ==========\n\n")
+cat("Prim's Algorithm:\n")
+cat("  - Starts from a vertex and grows the tree\n")
+cat("  - Better for dense graphs (many edges)\n")
+cat("  - O(V^2) with simple implementation\n\n")
+cat("Kruskal's Algorithm:\n")
+cat("  - Starts with edges sorted by weight\n")
+cat("  - Better for sparse graphs (few edges)\n")
+cat("  - O(E log E) or O(E log V)\n")