# C code for Prim’s Algorithm

Saurabh | Mar 06, 2010 | Comments 3

Prim’s algorithm is one of the two commonly used algorithms for finding a Minimum Spanning Tree. The coding of the other one, Kruskal algorithm has already been presented here. Here the C code for implementing this algorithm has been presented . The code operates on the Input File which can be downloaded and must be in the same directory in which the program is to be executed. This code can be made to operate on any graph which is represented in adjacency matrix of size n*n and the 1st row 1st column contains the value of n, as represented in the file input file, dist.txt.

```
#include "stdio.h"
/*
The input file (dist.txt) look something like this
4
0 0 0 21
0 0 8 17
0 8 0 16
21 17 16 0
The first line contains n, the number of nodes.
Next is an nxn matrix containg the distances between the nodes
NOTE: The distance between a node and itself should be 0
*/
int n; /* The number of nodes in the graph */
int weight[100][100]; /* weight[i][j] is the distance between node i and node j;
if there is no path between i and j, weight[i][j] should
be 0 */
char inTree[100]; /* inTree[i] is 1 if the node i is already in the minimum
spanning tree; 0 otherwise*/
int d[100]; /* d[i] is the distance between node i and the minimum spanning
tree; this is initially infinity (100000); if i is already in
the tree, then d[i] is undefined;
this is just a temporary variable. It's not necessary but speeds
up execution considerably (by a factor of n) */
int whoTo[100]; /* whoTo[i] holds the index of the node i would have to be
linked to in order to get a distance of d[i] */
/* updateDistances(int target)
should be called immediately after target is added to the tree;
updates d so that the values are correct (goes through target's
neighbours making sure that the distances between them and the tree
are indeed minimum)
*/
void updateDistances(int target) {
int i;
for (i = 0; i < n; ++i) if ((weight[target][i] != 0) && (d[i] > weight[target][i])) {
d[i] = weight[target][i];
whoTo[i] = target;
}
}
main() {
FILE *f = fopen("dist.txt", "r");
int i,j,total,treeSize;
fscanf(f, "%d", &n);
for (i = 0; i < n; ++i)
for (j = 0; j < n; ++j)
fscanf(f, "%d", &weight[i][j]);
fclose(f);
/* Initialise d with infinity */
for (i = 0; i < n; ++i)
d[i] = 100000;
/* Mark all nodes as NOT beeing in the minimum spanning tree */
for (i = 0; i < n; ++i)
inTree[i] = 0;
/* Add the first node to the tree */
printf("Adding node %c\n", 0 + 'A');
inTree[0] = 1;
updateDistances(0);
total = 0;
for (treeSize = 1; treeSize < n; ++treeSize) {
/* Find the node with the smallest distance to the tree */
int min = -1;
for (i = 0; i < n; ++i) if (!inTree[i]) if ((min == -1) || (d[min] > d[i]))
min = i;
/* And add it */
printf("Adding edge %c-%c\n", whoTo[min] + 'A', min + 'A');
inTree[min] = 1;
total += d[min];
updateDistances(min);
}
printf("Total distance: %d\n", total);
return 0;
}
```

**Filed Under**: Algorithms • C • Codes

**About the Author**: Software Engineer - Search & Recommendation at Rovi

the programe can not running.

now ok.

i run it with wxdev.

So how does it check that it doesn´t create circles