Prim's algorithm is a minimum spanning tree algorithm that takes a graph as input and finds the subset of the edges of that graph which

- form a tree that includes every vertex
- has the minimum sum of weights among all the trees that can be formed from the graph

## How Prim's algorithm works

It falls under a class of algorithms called greedy algorithms which find the local optimum in the hopes of finding a global optimum.

We start from one vertex and keep adding edges with the lowest weight until we we reach our goal.

The steps for implementing Prim's algorithm are as follows:

- Initialize the minimum spanning tree with a vertex chosen at random.
- Find all the edges that connect the tree to new vertices, find the minimum and add it to the tree
- Keep repeating step 2 until we get a minimum spanning tree

## Example of Prim's algorithm

## Prim's Algorithm pseudocode

The pseudocode for prim's algorithm shows how we create two sets of vertices U and V-U. U contains the list of vertices that have been visited and V-U the list of vertices that haven't. One by one, we move vertices from set V-U to set U by connecting the least weight edge.

```
T = ∅;
U = { 1 };
while (U ≠ V)
let (u, v) be the lowest cost edge such that u ∈ U and v ∈ V - U;
T = T ∪ {(u, v)}
U = U ∪ {v}
```

## Prim's Algorithm Implementation in C++

The program below implements Prim's algorithm in C++. Although adjacency matrix representation of graph is used, this algorithm can also be implemented using Adjacency List to improve its efficiency.

```
#include <iostream>
#include <cstring>
using namespace std;
#define INF 9999999
// number of vertices in grapj
#define V 5
// create a 2d array of size 5x5
//for adjacency matrix to represent graph
int G[V][V] = {
{0, 9, 75, 0, 0},
{9, 0, 95, 19, 42},
{75, 95, 0, 51, 66},
{0, 19, 51, 0, 31},
{0, 42, 66, 31, 0}
};
int main () {
int no_edge; // number of edge
// create a array to track selected vertex
// selected will become true otherwise false
int selected[V];
// set selected false initially
memset (selected, false, sizeof (selected));
// set number of edge to 0
no_edge = 0;
// the number of egde in minimum spanning tree will be
// always less than (V -1), where V is number of vertices in
//graph
// choose 0th vertex and make it true
selected[0] = true;
int x; // row number
int y; // col number
// print for edge and weight
cout << "Edge" << " : " << "Weight";
cout << endl;
while (no_edge < V - 1) {
//For every vertex in the set S, find the all adjacent vertices
// , calculate the distance from the vertex selected at step 1.
// if the vertex is already in the set S, discard it otherwise
//choose another vertex nearest to selected vertex at step 1.
int min = INF;
x = 0;
y = 0;
for (int i = 0; i < V; i++) {
if (selected[i]) {
for (int j = 0; j < V; j++) {
if (!selected[j] && G[i][j]) { // not in selected and there is an edge
if (min > G[i][j]) {
min = G[i][j];
x = i;
y = j;
}
}
}
}
}
cout << x << " - " << y << " : " << G[x][y];
cout << endl;
selected[y] = true;
no_edge++;
}
return 0;
}
```

On running the above code, we get the output as:

Edge : Weight 0 - 1 : 9 1 - 3 : 19 3 - 4 : 31 3 - 2 : 51

## Prim's vs Kruskal's Algorithm

Kruskal's algorithm is another popular minimum spanning tree algorithm that uses a different logic to find the MST of a graph. Instead of starting from an vertex, Kruskal's algorithm sorts all the edges from low weight to high and keeps adding the lowest edges, ignoring those edges that create a cycle.