CSC 213 – Large Scale Programming

Today’s Goals  Examine new properties of DirectedGraph s  What reaching & reachable mean for a Graph  How humans go about computing these properties  Algorithms for computers to compute these  Examine what meant by DAG and why you care  Simple ways to develop schedules will be examined  How can algorithm tell when a schedule impossible

Directed Graph  Only directed edges  Replace undirected edge with 2 directed edges  Relationships go in only one direction  One-way streets  Flights  Scheduling A C E B D

Directed Graph

Directed Graph Properties  Each edge goes one-way  (a,b) connects a to b does not connect  (a,b) does not connect b to a  Can discuss in-edges & out-edges  (a,b) is out-edge for a  (a,b) is in-edge for b  Adjacency-based Graph classes can change  Use 2 Sequences for adjacency-list vertices  Define source & target dimension in adjacency-matrix incidentEdges returns both in-edges & out-edges a c e b d

Reachability  e, a, d reachable from c a c e b d f

Reachability  e, a, d reachable from c  c reaches e a c e b d f

Reachability  e, a, d reachable from c  c reaches e  c reaches e & e incident upon d, a a c e b d f

Reachability  e, a, d reachable from c  c reaches e  c reaches e & e is incident upon d, a  d, a out-edges to c only a c e b d f

Reachability  a, c, d, e, f reachable from b a c e b d f

Reachability  a, c, d, e, f reachable from b  Path exists from b to every vertex a c e b d f

Reachability  a, c, d, e, f reachable from b  Path exists from b to every vertex  Actually have multiple paths to most vertices a c e b d f

Transitive Closure of G  Transitive closure of G usually written as G* b a d c e GG*G*

Transitive Closure of G  Transitive closure of G usually written as G*  Identical vertex sets in G & G*  G & G* have different edge sets, however b a d c e GG*G* b a d c e

Transitive Closure of G  Transitive closure of G usually written as G*  Identical vertex sets in G & G*  G & G* have different edge sets, however  Edge in G* if target reachable from source in G b a d c e GG*G* b a d c e

Transitive Closure of G  Transitive closure of G usually written as G*  Identical vertex sets in G & G*  G & G* have different edge sets, however  Edge in G* if target reachable from source in G b a d c e GG*G* b a d c e

Transitive Closure of G  Transitive closure of G usually written as G*  Identical vertex sets in G & G*  G & G* have different edge sets, however  Edge in G* if target reachable from source in G b a d c e GG*G* b a d c e

Transitive Closure of G  Transitive closure of G usually written as G*  Identical vertex sets in G & G*  G & G* have different edge sets, however  Edge in G* if target reachable from source in G b a d c e GG*G* b a d c e

Transitive Closure of G  Transitive closure of G usually written as G*  Identical vertex sets in G & G*  G & G* have different edge sets, however  Edge in G* if target reachable from source in G b a d c e GG*G* b a d c e

Computing Transitive Closure  Use dynamic programming to compute this  This solution known as Floyd-Warshall Algorithm  But how fast is it?

Floyd-Warshall’s Algorithm  Number G ’s vertices from 1 to n  Algorithm will compute n directed graphs  Set G 0 =G to initialize this algorithm  Graph of transitive closure is end result ( G n = G* )  All n directed graphs have same vertices  G k contains all edges in G k-1 (and G k-2, G k-3,…, G 0 )  G k also has edge ( v i, v j ) if edges ( v i, v k ) & ( v k, v j ) in G k-1  Takes O(n 3 ) time with adjacency matrix  Better to use “brute force” if few edges exist

Floyd-Warshall’s Algorithm  Number G ’s vertices from 1 to n  Algorithm will compute n directed graphs  Set G 0 =G to initialize this algorithm  Graph of transitive closure is end result ( G n = G* )  All n directed graphs have same vertices  G k contains all edges in G k-1 (and G k-2, G k-3,…, G 0 )  G k also has edge ( v i, v j ) if edges ( v i, v k ) & ( v k, v j ) in G k-1  Takes O(n 3 ) time with adjacency matrix  Better to use “brute force” if few edges exist

Floyd-Warshall Example – G 0 V6V6 V7V7 V5V5 V4V4 V1V1 V3V3 V2V2

V6V6 V7V7 V5V5 V4V4 V3V3 V2V2

V6V6 V7V7 V5V5 V4V4 V3V3 V2V2

V6V6 V7V7 V5V5 V4V4 V3V3 V2V2

V6V6 V7V7 V5V5 V4V4 V3V3 V2V2

V6V6 V7V7 V5V5 V4V4 V1V1 V3V3 V2V2

V6V6 V7V7 V5V5 V4V4 V1V1 V3V3

V6V6 V7V7 V5V5 V4V4 V1V1 V3V3

V6V6 V7V7 V5V5 V4V4 V1V1 V2V2

V6V6 V7V7 V5V5 V4V4 V1V1 V2V2

V6V6 V7V7 V5V5 V4V4 V1V1 V2V2

V6V6 V7V7 V5V5 V4V4 V1V1 V2V2

V6V6 V7V7 V5V5 V4V4 V1V1 V2V2

V6V6 V7V7 V5V5 V1V1 V3V3 V2V2

V6V6 V7V7 V5V5 V1V1 V3V3 V2V2

V6V6 V7V7 V5V5 V1V1 V3V3 V2V2

V6V6 V7V7 V5V5 V1V1 V3V3 V2V2

V6V6 V7V7 V5V5 V1V1 V3V3 V2V2

V6V6 V7V7 V5V5 V1V1 V3V3 V2V2

V6V6 V7V7 V4V4 V1V1 V3V3 V2V2

V6V6 V7V7 V4V4 V1V1 V3V3 V2V2

V6V6 V7V7 V4V4 V1V1 V3V3 V2V2

V6V6 V7V7 V4V4 V1V1 V3V3 V2V2

V7V7 V5V5 V4V4 V1V1 V3V3 V2V2

V7V7 V5V5 V4V4 V1V1 V3V3 V2V2

V6V6 V5V5 V4V4 V1V1 V3V3 V2V2

Floyd-Warshall Example – G * V6V6 V7V7 V5V5 V4V4 V1V1 V3V3 V2V2

DAG Directed Acyclic Graph  Often called a DAG  Number & sort vertices  Topological order found  Each edge ( v i, v j ) has i < j  Finds valid schedules…  …or proves cannot exist! b a d c e G (is a DAG) b a d c e A valid ordering of G

DAG Directed Acyclic Graph  Often called a DAG  Number & sort vertices  Topological order found  Each edge ( v i, v j ) has i < j  Finds valid schedules…  …or proves cannot exist! b a d c e G (is a DAG) b a d c e Another valid ordering of G

Topological Sorting  Edges connect smaller to larger vertex numbers wake up study eat class study homework work study go to bed dream about classwork Professor’s expectation of student’s day

Topological Sort Algorithm Algorithm topologicalSort( Graph G) H  // Make a copy of G m  new Vertex[ G.numVertices() ] stack  new … Stack () // Push onto stack any vertex in H with no outgoing edges n  G.numVertices() while n ≥ 1 do v  stack.pop() n  n – 1 m[n]  v foreach e in v.incidentEdges() ≠ 0 do w  H.opposite(e, v) // Remove e from H if w has no outgoing edges then stack.push(w) return m

Topological Sorting Example

For Next Lecture  Weekly assignment out & due tomorrow  Programming assignment #3 plans due Friday  Please do not wait to start working on these ideas  For Wednesday, read &  Find quickest way to get someplace, can it be done?  How can we find the spanning tree that weighs least?