diff --git a/GreedyAlgorithms/Prims_Algorithm.py b/GreedyAlgorithms/Prims_Algorithm.py new file mode 100644 index 00000000..4e136332 --- /dev/null +++ b/GreedyAlgorithms/Prims_Algorithm.py @@ -0,0 +1,81 @@ +# A Python program for Prim's Minimum Spanning Tree (MST) algorithm. +# The program is for adjacency matrix representation of the graph + +import sys # Library for INT_MAX + +class Graph(): + + def __init__(self, vertices): + self.V = vertices + self.graph = [[0 for column in range(vertices)] + for row in range(vertices)] + + # A utility function to print the constructed MST stored in parent[] + def printMST(self, parent): + print "Edge \tWeight" + for i in range(1, self.V): + print parent[i], "-", i, "\t", self.graph[i][ parent[i] ] + + # A utility function to find the vertex with + # minimum distance value, from the set of vertices + # not yet included in shortest path tree + def minKey(self, key, mstSet): + + # Initilaize min value + min = sys.maxint + + for v in range(self.V): + if key[v] < min and mstSet[v] == False: + min = key[v] + min_index = v + + return min_index + + # Function to construct and print MST for a graph + # represented using adjacency matrix representation + def primMST(self): + + # Key values used to pick minimum weight edge in cut + key = [sys.maxint] * self.V + parent = [None] * self.V # Array to store constructed MST + # Make key 0 so that this vertex is picked as first vertex + key[0] = 0 + mstSet = [False] * self.V + + parent[0] = -1 # First node is always the root of + + for cout in range(self.V): + + # Pick the minimum distance vertex from + # the set of vertices not yet processed. + # u is always equal to src in first iteration + u = self.minKey(key, mstSet) + + # Put the minimum distance vertex in + # the shortest path tree + mstSet[u] = True + + # Update dist value of the adjacent vertices + # of the picked vertex only if the current + # distance is greater than new distance and + # the vertex in not in the shotest path tree + for v in range(self.V): + # graph[u][v] is non zero only for adjacent vertices of m + # mstSet[v] is false for vertices not yet included in MST + # Update the key only if graph[u][v] is smaller than key[v] + if self.graph[u][v] > 0 and mstSet[v] == False and key[v] > self.graph[u][v]: + key[v] = self.graph[u][v] + parent[v] = u + + self.printMST(parent) + +g = Graph(5) +g.graph = [ [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]] + +g.primMST(); + +# Contributed by Divyanshu Mehta