# Representing Graphs and Networks in C

A **Graph **is a data structure that consists of a set of nodes interconnected by directed or un-directed arcs. For a **DIRECTED Graph, **the ordered pair of nodes are written within angled brackets while for a **UN-DIRECTED Graph, **they are written within parenthesis. Consider a directed graph as depicted below –

Below are the main point to note about the above graph –

- NODE B is said to be
**ADJACENT**to A since there is an arc from A to B. **INDEGREE**– Number of arcs incident to a node. eg: indegree of B is 2**OUTDEGREE**– Number of arcs incident from a node. eg: outdegree of A is- Now there are
**2 CYCLES**from from A to A – 1 from E to E, 2 from D to D and 2 from B to B. **PATHS**from node A to D is of length 1 and length 2- There are 6 paths from A to C of lengths 2, 2, 3, 3, 6, 6 respectively.

Consider a RELATION, R = {<3,10>, <5,6>, <5,8>, <6,17>, <8,17>, <10,17>}. This relation could be represented as a **WEIGHTED GRAPH** or a **NETWORK** as shown below –

The weight of each arc is 1 which is value of rem(predecessor node/successor node). Relation is {(x,y) / y > x and rem. of y/x is 1}.

**General sequential representation of Graphs in C**

Below is a complete sequential representation of graphs in C. It is assumed that nodes have data and arcs are weighted.

# define MAXNODES 50 /*Declaration*/ struct node { int info; }; struct arc { int adj; int weight; }; struct graph { struct node nodes[MAXNODES]; struct arc arcs[MAXNODES][MAXNODES]; }g; /*Routine to add information to a node*/ void addinfo(struct graph g, int inf, int node1){ g.nodes[node1].info = inf; return; }/*end addinfo*/ /*Routine to join a weighted arc from node 1 to node 2*/ void joinwt(struct graph g, int node1, int node2, int Wt){ g.arcs[node1][node2].adj = TRUE; g.arcs[node1][node2].weight = Wt; }/*end joinwt*/ /*Routine to remove an arc*/ void remove(struct graph g, int node1, int node2){ g.arcs[node1][node2].adj = FALSE; g.arcs[node1][node2].weight = NULL; }/*end remove*/ /*Routine to check for adjacency*/ void adjacent(struct graph g, int node1, int node2){ return((g.arcs[node1][node2].adj == TRUE) ? TRUE : FALSE); g.arcs[node1][node2].weight = NULL; }/*end adjacent*/

In the sequential representation above, nodes and the information associated with them are stored in an array nodes[]. The **ADJACENCY ** or arcs are created or removed between two nodes i and j using 2D array g.arcs[i][j] where g.arcs[i][j].adj represents the adjacency (arc) and g.arcs[i][j] weight gives the associated weight. g.arcs[i][j] is calles the **ADJACENCY MATRIX **of order (MAXNODES – 1) x (MAXNODES – 1).

**TRANSITIVE CLOSURE – PATH MATRIX of length n **

If we consider that a graph is totally defined by the adjacency matrix such that the nodes have no info field and arcs are not weighted where **Adjacency Matrix ** a_{ij} is defined as –

**a _{ij} **= { 1, if there exists an adjacency or arc between two nodes i and j.

0, otherwise

}

For such graphs, we use the **TRANSITIVE PROPERTY OF GRAPHS** to check for the path between two nodes. Consider a graph completely represented as an adjacency matrix.

Now the adjacency matrix adj could be written as –

Likewise we could obtain the path matrices upto k where k is the max number of nodes from the boolean product of path matrix of length k-1 and adj.

**Matrix Path or Transitive Closure** = adj || adj2 || adj3 || ………………….. || adjk

Such a matrix path is often called the ** Transitive Closure ** of adj matrix. For the above gaph, the transitive closure is –

Transitive Closure (Matrix Path) = adj || adj2 || adj3 || adj4 || adj5 (k=5)

**Complete Adjacency Matrix representation of Graphs in C**

In such a case, we consider that the nodes have no info associated with it and arcs have no weights associated with them.

**Arcs with no weights**

# define MAXNODES 50 /*Declaration*/ int adj[MAXNODES][MAXNODES]; /*Routine joining arcs*/ void join(int adj[][MAXNODES], int node1, int node2){ adj[node1][node2] = TRUE; }/*end join*/ /*Routine for removing arcs*/ void remove(int adj[][MAXNODES], int node1, int node2){ adj[node1][node2] = FALSE; }/*end remove*/ /*Routine to check for adjacency*/ void adjacent(int adj[][MAXNODES], int node1, int node2){ return((adj[node1][node2] == TRUE) ? TRUE : FALSE); }/*end adjacent*/

**Arcs with weights**

# define MAXNODES 50 struct arc { int adj; int weight; }; /*Declaration*/ struct arc g[MAXNODES][MAXNODES]; /*Routine to join a weighted arc from node 1 to node 2*/ void joinwt(struct g[MAXNODES][MAXNODES], int node1, int node2, int Wt){ g[node1][node2].adj = TRUE; g[node1][node2].weight = Wt; }/*end joinwt*/ /*Routine to remove an arc*/ void remove(struct g[MAXNODES][MAXNODES], int node1, int node2){ g[node1][node2].adj = FALSE; g[node1][node2].weight = NULL; }/*end remove*