1. CSE 373 Data Structures Lecture 13
Graph Terminology
CSE 373
Data Structures
Lecture 13

2. Graph Terminology - Lecture 13
Reading
Reading
Section 9.1
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Graph Terminology - Lecture 13

3. Graph Terminology - Lecture 13
What are graphs?
Yes, this is a graph….
But we are interested in a different kind of “graph”
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Graph Terminology - Lecture 13

4. Graph Terminology - Lecture 13
Graphs
Graphs are composed of
Nodes (vertices)
Edges (arcs)
node
edge
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Graph Terminology - Lecture 13

5. Graph Terminology - Lecture 13
Varieties
Nodes
Labeled or unlabeled
Edges
Directed or undirected
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Graph Terminology - Lecture 13

6. Graph Terminology - Lecture 13
Motivation for Graphs
node
node
Consider the data structures we have looked at so far…
Linked list: nodes with 1 incoming edge + 1 outgoing edge
Binary trees/heaps: nodes with 1 incoming edge + 2 outgoing edges
B-trees: nodes with 1 incoming edge + multiple outgoing edges
Value
Next
Value
Next 94 10 97
3 5 96 99
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Graph Terminology - Lecture 13

7. Graph Terminology - Lecture 13
Motivation for Graphs
How can you generalize these data structures?
Consider data structures for representing the following problems…
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Graph Terminology - Lecture 13

8. CSE Course Prerequisites at UW
461
322
373
143
321
326
415
142
410
370
341
413
417
378
Nodes = courses
Directed edge = prerequisite
421
401
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Graph Terminology - Lecture 13

9. Graph Terminology - Lecture 13
Representing a Maze S S B E E
Nodes = rooms
Edge = door or passage
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Graph Terminology - Lecture 13

10. Representing Electrical Circuits
Battery
Switch
Nodes = battery, switch, resistor, etc.
Edges = connections
Resistor
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Graph Terminology - Lecture 13

11. Graph Terminology - Lecture 13
Program statements x1 x2
x1=q+y*z + -
x2=y*z-q
Naive: q * * q
y*z calculated twice y z x1 x2
common
subexpression + -
eliminated: q * q
Nodes = symbols/operators
Edges = relationships y z
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Graph Terminology - Lecture 13

12. Graph Terminology - Lecture 13
Precedence
S1 a=0;
S2 b=1;
S3 c=a+1
S4 d=b+a;
S5 e=d+1;
S6 e=c+d; 6 5
Which statements must execute before S6?
S1, S2, S3, S4 4 3
Nodes = statements
Edges = precedence requirements 2 1
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Graph Terminology - Lecture 13

13. Information Transmission in a Computer Network
Sydney 56
Tokyo
Seattle
Seoul
128
New York 16
181 30
140
L.A.
Nodes = computers
Edges = transmission rates
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14. Traffic Flow on HighwaysUW
Nodes = cities
Edges = # vehicles on
connecting highway
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15. Graph Terminology - Lecture 13
Graph Definition
A graph is simply a collection of nodes plus edges
Linked lists, trees, and heaps are all special cases of graphs
The nodes are known as vertices (node = “vertex”)
Formal Definition: A graph G is a pair (V, E) where
V is a set of vertices or nodes
E is a set of edges that connect vertices
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Graph Terminology - Lecture 13

16. Graph Terminology - Lecture 13
Graph Example
Here is a directed graph G = (V, E)
Each edge is a pair (v1, v2), where v1, v2 are vertices in V
V = {A, B, C, D, E, F}
E = {(A,B), (A,D), (B,C), (C,D), (C,E), (D,E)} B C A F D E
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Graph Terminology - Lecture 13

17. Directed vs Undirected Graphs
If the order of edge pairs (v1, v2) matters, the graph is directed (also called a digraph): (v1, v2)  (v2, v1)
If the order of edge pairs (v1, v2) does not matter, the graph is called an undirected graph: in this case, (v1, v2) = (v2, v1) v2 v1 v1 v2
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18. Undirected Terminology
Two vertices u and v are adjacent in an undirected graph G if {u,v} is an edge in G
edge e = {u,v} is incident with vertex u and vertex v
The degree of a vertex in an undirected graph is the number of edges incident with it
a self-loop counts twice (both ends count)
denoted with deg(v)
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Graph Terminology - Lecture 13

19. Undirected Terminology
B is adjacent to C and C is adjacent to B
(A,B) is incident to A and to B B C
Self-loop A F D E
Degree = 0
Degree = 3
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20. Graph Terminology - Lecture 13
Directed Terminology
Vertex u is adjacent to vertex v in a directed graph G if (u,v) is an edge in G
vertex u is the initial vertex of (u,v)
Vertex v is adjacent from vertex u
vertex v is the terminal (or end) vertex of (u,v)
Degree
in-degree is the number of edges with the vertex as the terminal vertex
out-degree is the number of edges with the vertex as the initial vertex
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Graph Terminology - Lecture 13

21. Graph Terminology - Lecture 13
Directed Terminology
B adjacent to C and C adjacent from B B C A F D E
In-degree = 0
Out-degree = 0
In-degree = 2
Out-degree = 1
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Graph Terminology - Lecture 13

22. Graph Terminology - Lecture 13
Handshaking Theorem
Let G=(V,E) be an undirected graph with |E|=e edges. Then
Every edge contributes +1 to the degree of each of the two vertices it is incident with
number of edges is exactly half the sum of deg(v)
the sum of the deg(v) values must be even
Add up the degrees of all vertices.
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Graph Terminology - Lecture 13

23. Graph Representations
Space and time are analyzed in terms of:
Number of vertices = |V| and
Number of edges = |E|
There are at least two ways of representing graphs:
The adjacency matrix representation
The adjacency list representation
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Graph Terminology - Lecture 13

24. Graph Terminology - Lecture 13
Adjacency Matrix
A B C D E F B A B C D E F C A F D E
1 if (v, w) is in E
M(v, w) =
0 otherwise
Space = |V|2
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Graph Terminology - Lecture 13

25. Adjacency Matrix for a Digraph
A B C D E F B A B C D E F C A F D E
1 if (v, w) is in E
M(v, w) =
0 otherwise
Space = |V|2
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Graph Terminology - Lecture 13

26. Graph Terminology - Lecture 13
Adjacency List
For each v in V, L(v) = list of w such that (v, w) is in E a b A B C D E F
list of
neighbors B B D C A A C B D E F D E A C E C D
Space = a |V| + 2 b |E|
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Graph Terminology - Lecture 13

27. Adjacency List for a Digraph
For each v in V, L(v) = list of w such that (v, w) is in E a b A B C D E F B B D C A C F D E D E E
Space = a |V| + b |E|
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Graph Terminology - Lecture 13