Graph Operations And Representation

Sample Graph Problems Path problems. Connectedness problems. Spanning tree problems. Graph coloring problems. Flow network problems.

Path Finding Path between 1 and 8. Path length is 20. 2 3 8 1 10 4 5 9 11 6 7 Vertices represent cities and edges represent highways. Edge weights are distances or driving times. Depending on the context, path length may either be the number of edges on the path or the sum of the weights of the edges on the path. Path length is 20.

Another Path Between 1 and 8 2 3 8 10 1 4 5 9 11 6 7 Since a graph may have more than one path between two vertices, we may be interested in finding a path with a particular property. For example, find a path with minimum length Path length is 28.

Example Of No Path 2 3 8 10 1 4 5 9 11 6 7 No path between 2 and 9.

Connected Graph Undirected graph. There is a path between every pair of vertices.

Example Of Not Connected 2 3 8 10 1 4 5 9 11 6 7

Connected Graph Example 2 3 8 10 1 4 5 9 11 6 7 Determine whether an undirected graph is connected.

Connected Components A subgraph in which any two vertices are connected to each other by paths  It is connected to no additional vertices in the supergraph A vertex with no incident edges is itself a connected component Source: https://en.wikipedia.org/wiki/Connected_component_(graph_theory)

Connected Components

Connected Components 2 3 8 10 1 4 5 9 11 6 7

Connected Component A maximal subgraph that is connected. Cannot add vertices and edges from original graph and retain connectedness. A connected graph has exactly 1 component.

Not A Component 2 3 8 10 1 4 5 9 11 6 7 Determine connected components of an undirected graph

Connected Components Several algorithms exists A straightforward approach: To find all the connected components of a graph, loop through its vertices, starting a new breadth first or depth first search whenever the loop reaches a vertex that has not already been included in a previously found connected component. Source: https://en.wikipedia.org/wiki/Connected_component_(graph_theory) Reference: John Hopcroft and Robert Tarjan. 1973. Algorithm 447: efficient algorithms for graph manipulation. Commun. ACM 16, 6 (June 1973), 372-378. DOI=http://dx.doi.org/10.1145/362248.362272

Connected Components Source: https://www.cs.umd.edu/class/sum2005/cmsc451/components.pdf

Connected Components Source: https://www.cs.umd.edu/class/sum2005/cmsc451/components.pdf

Connected Components Source: https://www.cs.umd.edu/class/sum2005/cmsc451/components.pdf

Connected Components Source: https://www.cs.umd.edu/class/sum2005/cmsc451/components.pdf

Communication Network 2 3 8 10 1 4 5 9 11 6 7 Each edge is a link that can be constructed (i.e., a feasible link).

Communication Network Problems Is the network connected? Can we communicate between every pair of cities? Find the components. Want to construct smallest number of feasible links so that resulting network is connected.

Cycles And Connectedness 2 3 8 10 1 4 5 9 11 6 7 Removal of an edge that is on a cycle does not affect connectedness.

Cycles And Connectedness 2 3 8 1 10 4 5 9 11 6 7 Connected subgraph with all vertices and minimum number of edges has no cycles.

Tree Connected graph that has no cycles. n vertex connected graph with n-1 edges. In graph terminology, the term rooted tree is used to denote what we were earlier calling a tree (Chapter 12).

Spanning Tree Subgraph that includes all vertices of the original graph. Subgraph is a tree. If original graph has n vertices, the spanning tree has n vertices and n-1 edges.

Minimum Cost Spanning Tree 2 3 8 10 1 4 5 9 11 6 7 Tree cost is sum of edge weights/costs.

A Spanning Tree Spanning tree cost = 51. 2 4 3 8 8 1 6 10 2 4 5 4 4 3 9 8 11 5 6 2 7 6 7 Spanning tree cost = 51.

Minimum Cost Spanning Tree 2 4 3 8 8 1 6 10 2 4 5 4 4 3 5 9 8 11 5 6 2 In the communication networks area, we are interested in finding minimum cost spanning trees. 7 6 7 Spanning tree cost = 41.

A Wireless Broadcast Tree 2 4 3 8 8 1 6 10 2 4 5 4 4 3 5 9 8 11 5 6 2 Edge cost is power needed to reach a node. If vertex 1 broadcasts with power 2, only vertex 4 is reached. If it broadcasts with power 4, both 2 and 4 are reached. Min-broadcast rooted spanning tree is NP-hard. Cost of tree of previous slide becomes 26. 7 6 7 Source = 1, weights = needed power. Cost = 4 + 8 + 5 + 6 + 7 + 8 + 3 = 41.

Graph Representation Adjacency Matrix Adjacency Lists Linked Adjacency Lists Array Adjacency Lists

Adjacency Matrix 0/1 n x n matrix, where n = # of vertices A(i,j) = 1 iff (i,j) is an edge 1 2 3 4 5 2 3 1 4 5 1 1 1 1 1 1 1 1 1 1

Adjacency Matrix Properties 2 3 1 4 5 Diagonal entries are zero. Adjacency matrix of an undirected graph is symmetric. A(i,j) = A(j,i) for all i and j.

Adjacency Matrix (Digraph) 2 3 1 4 5 Diagonal entries are zero. Adjacency matrix of a digraph need not be symmetric.

Adjacency Matrix n2 bits of space For an undirected graph, may store only lower or upper triangle (exclude diagonal). (n-1)n/2 bits O(n) time to find vertex degree and/or vertices adjacent to a given vertex.

Adjacency Lists Adjacency list for vertex i is a linear list of vertices adjacent from vertex i. An array of n adjacency lists. aList[1] = (2,4) aList[2] = (1,5) aList[3] = (5) aList[4] = (5,1) aList[5] = (2,4,3) 2 3 1 4 5

Linked Adjacency Lists Each adjacency list is a chain. 2 3 1 4 5 aList[1] aList[5] [2] [3] [4] 2 4 1 5 5 5 1 2 4 3 Array length n simply means we need an array with n spots. A direct implementation using a Java array would need n+1 spots, because spot 0 would not be utilized. However, by using spot 0 for vertex 1, spot 1 for vertex 2, and so on, we could get by with a Java array whose length is actually n. Array Length = n # of chain nodes = 2e (undirected graph) # of chain nodes = e (digraph)

Array Adjacency Lists Each adjacency list is an array list. aList[1] 2 3 1 4 5 aList[1] aList[5] [2] [3] [4] Array Length = n # of list elements = 2e (undirected graph) # of list elements = e (digraph)

Weighted Graphs Cost adjacency matrix. C(i,j) = cost of edge (i,j) Adjacency lists => each list element is a pair (adjacent vertex, edge weight)

Number Of C++ Classes Needed Graph representations Adjacency Matrix Adjacency Lists Linked Adjacency Lists Array Adjacency Lists 3 representations Graph types Directed and undirected. Weighted and unweighted. 2 x 2 = 4 graph types 3 x 4 = 12 C++ classes

Abstract Class Graph template<class T> class graph { public: // ADT methods come here // implementation independent methods come here };

Abstract Methods Of Graph // ADT methods virtual ~graph() {} virtual int numberOfVertices() const = 0; virtual int numberOfEdges() const = 0; virtual bool existsEdge(int, int) const = 0; virtual void insertEdge(edge<T>*) = 0; virtual void eraseEdge(int, int) = 0; virtual int degree(int) const = 0; virtual int inDegree(int) const = 0; virtual int outDegree(int) const = 0;

Abstract Methods Of Graph // ADT methods (continued) virtual bool directed() const = 0; virtual bool weighted() const = 0; virtual vertexIterator<T>* iterator(int) = 0; virtual void output(ostream&) const = 0;