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Graph

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- Linear data structures:
- Array, linked list, stack, queue

- Non linear data structures:
- Tree, binary tree, graph and digraph

- A graph is a non-linear structure (also called a network)
- Unlike a tree or binary tree, a graph does not have a root
- Any node (vertex, v) can be connected to any other node by an edge (e)
- Can have any number of edges (E) and nodes (set of Vertex V)
- Application: the highway system connecting cities on a map

a graph data structure

u

- v

- G = (V,E)
- V is the vertex set (nodes set).
- Vertices are also called nodes and points.
- E is the edge set.
- Each edge connects two different vertices.
- Edges are also called arcs and lines.
- Directed edge has an orientation (u,v).

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- v

- v

- Undirected edge has no orientation (u,v).

- Undirected graph => no oriented edge.
- Directed graph => every edge has an orientation.

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G = (V, E)

V = {1,2,â€¦11}

E = { (2,1),

(4,1),

â€¦}

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G = (V, E)

V = {1,2,â€¦11}

E = { (1,2),

(1,4),

â€¦}

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- Vertex = city, edge = communication link.

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- Vertex = city, edge weight = driving distance/time.
- Find a shortest path from city 1 to city 5
- 1ïƒ 2ïƒ 5 (12)
- 1ïƒ 4ïƒ 6ïƒ 7ïƒ 5 (14)

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- Some streets are one way.
- Find a path from 1ïƒ 11

n = 4

n = 1

n = 2

n = 3

Has all possible edges.

- Each edge is of the form (u,v), u != v.
- Number of such pairs in an n vertex graph is n(n-1).
- Since edge (u,v) is the same as edge (v,u), the number of edges in a complete undirected graph is n(n-1)/2.
- Number of edges in an undirected graph is <= n(n-1)/2.

n = 4

n = 1

n = 2

n = 3

Has all possible edges.

- Each edge is of the form (u,v), u != v.
- Number of such pairs in an n vertex graph is n(n-1).
- Since edge (u,v) is not the same as edge (v,u), the number of edges in a complete directed graph is n(n-1).
- Number of edges in a directed graph is <= n(n-1).

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Number of edges incident to vertex.

degree(2) = 2, degree(5) = 3, degree(3) = 1

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Sum of degrees = 2e (e is number of edges)

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in-degree is number of incoming edges

indegree(2) = 1, indegree(8) = 0

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out-degree is number of outbound edges

outdegree(2) = 1, outdegree(8) = 2

each edge contributes 1 to the in-degree of some vertex and 1 to the out-degree of some other vertex

sum of in-degrees = sum of out-degrees = e,

where e is the number of edges in the digraph

Graph Operations And Representation

- Path problems.
- Connectedness problems.
- Spanning tree problems.

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Path Finding

Path between 1 and 8.

Path length is 20.

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Another Path Between 1 and 8

Path length is 28.

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No path between 2 and 9.

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

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- A maximal subgraph that is connected.
- A connected graph has exactly 1 component.

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Each edge is a link that can be constructed

- Is the network connected?
- Can we communicate between every pair of cities?

- Find the components.
- Want to construct smallest number of links so that resulting network is connected (finding a minimum spanning tree).

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Removal of an edge that is on a cycle does not affect connectedness.

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Connected subgraph with all vertices and minimum number of edges has no cycles.

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- Connected graph that has no cycles.
- n vertex connected graph with n-1 edges.

- Adjacency Matrix
- Adjacency Lists
- Linked Adjacency Lists
- Array Adjacency Lists

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- 0/1 n x n matrix, where n = # of vertices
- A(i,j) = 1 iff (i,j) is an edge

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- Diagonal entries are zero.
- Adjacency matrix of an undirected graph is symmetric.
- A(i,j) = A(j,i) for alliand j.

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- Diagonal entries are zero.
- Adjacency matrix of a digraph need not be symmetric.

- n2bits of space
- For an undirected graph, may store only lower or upper triangle (exclude diagonal).
- (n-1)n/2 bits

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- Adjacency list for vertex i is a linear list ofvertices 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)

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- Each adjacency list is a chain.

Array Length = n

# of chain nodes = 2e (undirected graph)

# of chain nodes = e (digraph)

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- Each adjacency list is an array list.

Array Length = n

# of list elements = 2e (undirected graph)

# of list elements = e (digraph)

- 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

};

http://www.boost.org/doc/libs/1_42_0/libs/graph/doc/index.html

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;