Description
The purpose of this assignment is to implement an ADT for a Directed Graph (digraph) and associated
operations in C. PA3 will utilize your List ADT once again, and some parts of this project will look very
familiar if you completed PA2. You should read the Graphs.pdf file handout that’s on
ResourcesàGeneral Resources, as well as appendices B.4 and B.5 from CLRS, Chapter 22 of CLRS, and
class Lectures 8, 9, 15, etc.
The Adjacency List representation of a digraph also consists of an array of Lists; this representation was
discussed in Lectures 8 and 9. However, the edges in a digraph are directed (like one-way streets); there
may be an edge (3, 1) between vertex 3 and vertex 1 although there is no edge (1, 3) between vertex 1 and
vertex 3. You will create a Digraph ADT that represents a directed graph as an array of Lists. As in PA2,
each vertex is associated with an integer label in the range 1 to numVertices, where numVertices is the
number of vertices in the digraph.
Your client program will use this Digraph ADT to perform various operations on the digraph that are
described below. The client program using your Digraph ADT will be called DigraphProperties, and
will take two command line arguments (here % denotes the unix prompt):
% DigraphProperties input_file output_file
For PA3, you are to write a makefile that creates the executable file, called DigraphProperties, similar to
the makefiles that you used for PA1 and PA2. Include a clean utility in your makefile.
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Input and Output File Formats
The first line of the input file describes the digraph, and the other lines describe operations to be
performed on the digraph. Most of these operations simply print values returned by functions
implemented in a Digraph ADT that you will implement in Digraph.c and Digraph.h
The first line starts with an integer that we’ll call numVertices that tells you the number of vertices in the
digraph. The rest of that line gives pair of distinct numbers in the range 1 to numVertices, separated by a
space. These numbers are the vertices for an edge. There is a comma between numVertices and the first
edge, and a comma between edges. We’ll also put a space after each comma for readability. All edges
are directed. Here’s an example that represents digraph on Slide 14 of Lecture 8:
4, 1 3, 2 4, 3 4, 4 3, 3 2
Note that the same digraph could be represented by each of the following input lines:
4, 3 2, 4 3, 1 3, 3 4, 2 4
4, 3 2, 4 3, 1 3, 3 4, 2 4, 1 3
even though the edge (1, 3) appears in the last digraph twice.
However, the following digraph is different:
4, 3 2, 4 3, 1 3, 3 4, 2 4, 3 1
because edges {1, 3} and {3, 1} are different edges.
The rest of the lines in the input file that correspond to digraph operations on operands. Each line begins
with a keyword, which is followed by the operands. You should output the entire input line into the
output file as a separate line. That line should be followed by another line that just has the result of the
operation. If an input line does not follow one of the specified formats, the output on the second line
should just say ERROR, and processing of the input file should continue. For example, it’s an error if the
operation isn’t one of the legal operations (capitalization matters), or if the operation has the wrong
number of operands, or if an operand that’s supposed to be a vertex isn’t a number between 1 and
numVertices.
So the output file should always have twice as many lines as the input file (not counting the line that
describes the digraph). For example, if an input file has 7 lines, the first of which is the digraph, then the
output should have 12 lines in it, containing each digraph operation lines, each followed immediately by
the result of that operation (or ERROR).
As in PA2: If the digraph in the first line of the input file is illegal, then you should just print out that
first line, followed by another line with the word ERROR on it. Do not process any subsequent lines in
the input file. What’s an illegal digraph? A digraph is illegal if the number of vertices isn’t positive, or if
an edge specifies a vertex that doesn’t exist in the digraph. For example, if there are 6 vertices, both
vertex 0 and vertex 7 are illegal. Also, there shouldn’t be any “self-loop” edges between a vertex and
itself, so an edge (3, 3) would also make the digraph illegal.
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Here are the operation tokens, their operands, and descriptions of the results. All of these operations
simply print values returned by functions implemented in the Digraph ADT that you will implement in
Digraph.c and Digraph.h.
• PrintDigraph takes no operands. It prints out the current digraph, in the same format as an input line.
Each edge should be printed out exactly once, following the same format as the input line for the
digraph, starting with the number of vertices, then a comma, then the edges.
For PA3, you must print the edges sorted by source and then by destination. If u < v, print all the
edges whose source is vertex u before all the edges whose source is vertex v. Also, if (u, v1) and
(u, v2) are edges, and v1 < v2, print (u, v1) before (u, v2).
Strong suggestion: Keep the destinations for each vertex as a sorted list. That will make it easy to
print out all the edges in the current digraph sorted as described.
• GetOrder takes no operands. It returns the total number of vertices in the current digraph.
• GetSize takes no operands. It returns the total number of edges in the current digraph.
• GetOutDegree takes a vertex u as operand. It returns the number of vertices that are outgoing
neighbors of u in the current digraph. The out degree of a vertex u is the number of vertices v such
that (u, v) is an edge in the digraph.
• AddEdge takes two vertices u and v as operands. It adds the edge (u, v) to the current digraph. If that
edge didn’t exist in the current digraph, it returns 0. If that edge already existed in the current digraph,
it returns 1, since no action was taken. (This is not treated as an error.)
• DeleteEdge takes two vertices u and v as operands. It deletes the edge (u, v) from the current digraph.
If that edge already existed in the current digraph, it returns 0. If that edge didn’t exist in the current
digraph, it returns 1, since no action was taken. (This is not treated as an error.)
• Distance takes a pair of vertices u and v as operands, and outputs the distance from u to v in the
current digraph, and INF if there is no path from u to v. Of course, this must be a directed path. Yes,
if the operands u and v are the same vertex, then there is a path from u to v that has no edges in it, so
the output will be 0. The distance from u to v is the length of the shortest path from u to v.
• Acyclic takes no operands. It outputs YES if the current digraph is acyclic, and NO otherwise.
• TopoSort takes no operands. It outputs a topological sort of the current digraph, if the current digraph
is acyclic. (There may be more than one correct topological sort.) However, if the digraph is not
acyclic, it outputs IMPOSSIBLE.
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Example: For an input file that consists of the following lines:
4, 1 3, 2 4, 3 4, 4 3, 3 2
PrintDigraph
GetSize
Distance 1 4
Distance 3 1
Acyclic
TopoSort
GetOutDegree 4
GetOutDegree 5
DeleteEdge 2 4
Acyclic
DeleteEdge 1 2
DeleteEdge 3 2
AddEdge 1 2
Acyclic
TopoSort
the output file should be:
PrintDigraph
4, 1 3, 2 4, 3 2, 3 4, 4 3
GetSize
5
Distance 1 4
2
Distance 3 1
INF
Acyclic
NO
TopoSort
IMPOSSIBLE
GetOutDegree 4
1
GetOutDegree 5
ERROR
DeleteEdge 2 4
0
Acyclic
NO
DeleteEdge 1 2
1
DeleteEdge 4 3
0
AddEdge 1 2
0
Acyclic
YES
TopoSort
1 3 2 4
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Your Digraph ADT will be implemented in files Digraph.c and Digraph.h. Digraph.c defines a struct
called DigraphObj, and Digraph.h should define a type called Digraph that points to this struct.
Your DigraphObj struct is required to have the following fields. It is okay for you to have other fields, but
you should document them in your README file.
• numVertices, the number of vertices in the digraph, called the order of the digraph.
• numEdges, the number of edges in the digraph, called the size of the digraph.
• An array of Lists, whose jth element contains the outgoing neighbors (destinations) from source
vertex j, the vertices k such that (j, k) is an edge in the digraph.. This is the outgoing Adjacency
List for vertex j. You’ll use your List ADT from PA1 and PA2 to represent the outgoing
Adjacency List for each vertex. We strongly suggest that you keep the outgoing neighbors in
sorted order, to make printing the digraph simpler.
• Other arrays similar to the marks that you used in PA2. You’ll have to determine yourselves what
arrays (if any) you’ll need for PA3.
Since indexes of C arrays begin at 0, you might choose to use arrays that have length numVertices +1, but
only use indices 1 through numVertices. That’s so that array indices can be directly identified with vertex
numbers (and index 0 is ignored). Of course, if you prefer to have arrays of length numVertices, rather
than numVertices + 1,that works, as long as you always remember to look at position j-1 when you’re
accessing information about vertex j.
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Your Digraph ADT should export the following operations through the file Digraph.h:
/*** Constructors-Destructors ***/
Digraph newDigraph(int numVertices);
// Returns a Digraph that points to a newly created DigraphObj representing a digraph which has
// n vertices and no edges.
void freeDigraph(Digraph* pG);
// Frees all dynamic memory associated with its Digraph* argument, and sets
// *pG to NULL.
/*** Access functions ***/
int getOrder(Digraph G);
// Returns the order of G, the number of vertices in G.
int getSize(Digraph G);
// Returns the size of G, the number of edges in G.
int getOutDegree(Digraph G, int u);
// Returns the number of outgoing neighbors that vertex u has in G, the number of vertices v such
// that (u, v) is an edge in G. Returns -1 if v is not a legal vertex.
List getNeighbors(Digraph G, int u);
// Returns a list that has all the vertices that are outgoing neighbors of vertex u, i.e.,
// a list that has all the vertices v such that (u, v) is an edge in G.
// There is no input operation that corresponds to getNeighbors.
/*** Manipulation procedures ***/
int addEdge(Digraph G, int u, int v);
// Adds v to the adjacency list of u, if that edge doesn’t already exist.
// If the edge does already exist, does nothing. Used when edges are entered or added.
// Returns 0 if (u, v) is a legal edge, and the edge didn’t already exist.
// Returns 1 if (u, v) is a legal edge and the edge did already exist.
// Returns -1 if (u, v) is not a legal edge.
int deleteEdge(Digraph G, int u, int v);
// Deletes v from the adjacency list of u, if that edge exists.
// If the edge doesn’t exist, does nothing. Used when edges are deleted.
// Returns 0 if (u, v) is a legal edge, and the edge did already exist.
// Returns 1 if (u, v) is a legal edge and the edge didn’t already exist.
// Returns -1 if (u, v) is not a legal edge.
You probably will want to have other functions in Digraph.c/Digraph.h that are similar to the unvisitAll,
getMark and setMark functions in PA). But you’ll have to figure out what those functions are yourselves.
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/*** Other operations ***/
void printDigraph(FILE* out, Digraph G);
// Outputs the digraph G in the same format as an input line, including the number of vertices
// and the edges. The edges should be in sorted order, as described above.
void distance(FILE* out, Digraph G, int u, int v);
// Outputs the distance between vertices u and v if there is a directed path between them in the
// digraph. Otherwise, outputs INF
void acyclic(FILE* out, Digraph G);
// Outputs YES if the digraph is acyclic. Otherwise, outputs NO.
void topoSort(FILE* out, Digraph G);
// Outputs a topological sort of the digraph. If the digraph is not acyclic, outputs IMPOSSIBLE.
In addition to the above prototypes, Digraph.h will define the type Digraph, as well as #define constant
macros for constants that you decide that you need to implement the functions described above.
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What does DigraphProperties.c do?
DigraphProperties reads in the first line of the input file, and uses newDigraph() and addEdge() to create a
digraph that corresponds to that line, as described above. When it reads in a subsequent line whose
operation token is one of GetOrder, GetSize or GetOutDegree, it calls the corresponding function in
Digraph.c (using the digraph that was read in as the digraph argument), and outputs the value returned.
For other operations tokens (Distance, Acyclic, TopoSort), it calls the corresponding function in Digraph.c
which performs its own output. Obvious requirements about number of operands should be checked in
DigraphProperties.c; obvious requirements about operand values in should be checked in Digraph.c. (If G
isn’t a digraph, your program has a bug, and should exit.)
About distance: Find the distance between two vertices of a digraph using BFS. See Section 22.2 of
CLRS.
About acyclic: A digraph is acyclic if and only if a DFS of the digraph produces no back edges, as
described in Section 22.3 of CLRS. You’ll have to figure out how to implement this.
About topological sort: Topological sort using DFS was discussed in Lecture 16 (February 15). See
Section 22.4 of CLRS.
