# Assignment #4 Pointers to structs

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COMP2401 – Assignment #4
In this assignment you will gain additional practice in using pointers to structs as well as allocating
dynamic memory and creating linked lists.
Consider a robotic environment where obstacles are represented as a set of rectangles as shown
below. The robot would like to travel through the environment as efficiently as possible (i.e., shortest
route) without hitting any obstacles.
To find a good route through the environment, we will assume that the robot is a single point in size
(This can be easily adjusted later by “growing” the obstacles according to the robot’s shape so that
the robot does not bump into the obstacles when it gets close… although we will ignore this “growing”
concept for this assignment). One algorithm to find the best route through the obstacles is based on
computing the shortest path in a graph of edges that connects the obstacle corners.
On the next page, you will see screen snapshots showing is what is known as a “complete graph” of
the vertices (i.e., corners) of the obstacles. It is essentially a graph in which each vertex is connected
to all other vertices. If there are N rectangular obstacles, then there are exactly 4N*4N=16N2 edges
of this complete graph. That can be a lot of edges. Finding the shortest path in this graph can be
done. But it is better to reduce the graph size beforehand. As you can see, there are many edges
of the graph that cross through the obstacles. Obviously, these are not edges that can be travelled
on by the robot, as it will intersect/hit the obstacles.
Instead, we want to remove all edges of this graph that intersect with an obstacle. That will
significantly reduce the number of edges in the graph, making it faster to find the shortest path.

How do we determine if a line segment intersects a rectangle ? We can just check to see if that edge
intersects/crosses any of the obstacle edges.
Consider the two line segments shown on the right. We can
compute ua and ub below:

Then, the line segments intersect if 0 < ua < 1 and 0 < ub < 1. This does not include the case where
the line segments intersect at a vertex.
We can use this simple intersection test to determine if an edge of the graph intersects a rectangle by
checking all 4 sides of the rectangle. You will have to check for the special case of a edge that
connects diagonal vertices of the same obstacle … as this is not a valid edge either.
Once we eliminate all edges that are invalid, we should have a graph that looks like this:
This reduced graph has 168 edges (instead of the complete one which had 756 edges). Each edge
shown above is actually two edges in the graph… representing travel in both directions (i.e., one
edge from vertex v1 to vertex v2 and another from vertex v2 to vertex v1). In addition, there are
edges in both directions along the obstacle borders.
The shortest path from one vertex to another vertex within this graph is guaranteed to be a sequence
of consecutive edges along this reduced graph. We will not be computing the shortest path in this
assignment. Instead, we are just interested in computing this reduced graph, which is called a
visibility graph.
Follow the steps indicated to complete this assignment. You will begin with some code that already
A file called display.c contains code for opening a graphics window and drawing the obstacles,
vertices and edges of the graph. A corresponding display.h file is also available for you to use. You
MUST NOT modify either of these two files, yet they must be submitted along with your assignment
when you hand it in.
A program called plannerTester.c has also been written for you. It produces the example
environment shown earlier. For debugging purposes, here are the obstacle coordinates:
The program creates the obstacles, and then attempts to create the vertices, the complete graph of
edges, the reduced visibility graph … and then cleans up by freeing allocated memory.
Sadly, most of the code is missing. That’s where you come in. The code is to be written in a file
called pathPlanner.c. Complete the functions as needed. You MUST NOT modify this
plannerTester.c program but you MUST include it in your final submission.
Another test program, called bigEnvironment.c has also been created for you. It also MUST NOT
be modified and must be submitted with your assignment. It creates and environment of 50 random
rectangles and then calls your pathPlanner functions as well. On the next page it shows a sample
output of the produced visibility graph … although it is random each time.
• To begin, create a proper makefile (with a make clean as well) that compiles and generates
the executables for the two test programs. You can have the makefile generate both test
program executables by just adding an extra gcc -o line to the makefile. The makefile MUST
make proper use of dependencies so that if a needed source code file or header file is altered,
then the object file will be recompiled. You will need to include the -lX11 library in your linking
lines. That is a “minus lowercase L” at the front.
• You will need to create an obstacles.h file that defines the following typedefs:
o An Obstacle type that maintains the x, y, width and height of a rectangular object.
Examine the display.c code to see how these attributes must be defined.
o A Vertex type that maintains the x,y coordinate of the vertex as well as a linked list of
neighbouring vertices. That is, each vertex keeps a linked list of vertices that it
connects to in the graph. You should keep pointers to the first neighbour in the list (i.e.,
the head) and the last neighbour in the list (i.e., the tail). The vertex should also keep a
pointer to the obstacle that it belongs to (so that we can ask later if two vertices are
corners on the same obstacle). Again, see the display.c code for how some of these
vertex attributes must be defined. Note that the neighbours are actually Neighbour
types … not Vertex types.
o A Neighbour type that represents an item in the linked list of vertex neighbours. It
should keep a pointer to a Vertex that it represents, as well as a pointer to the next
Neighbour in the list.
o Finally, an Environment type should be created that maintains a pointer to a
dynamically-allocated array of Obstacle types and the number of obstacles that have
been allocated. It should also maintain a pointer to a dynamically-allocated array of
Vertex types and the number of vertices that have been allocated. You can check the
test program and display.c code to make sure that you are defining this properly.
o You may add other things to this header file as/if you need them.
• Create all remaining necessary functions and procedures in a pathPlanner.c file. The
functions required are indicated by the code in the test program. Note that each time the test
program displays something, it waits for the user to press ENTER. You may want to write
these functions in steps … by starting with the commenting out of code that you do not want to
test. So, you can first make sure that all your vertices are created properly … then your
complete graph … and finally your visibility graph. You should write the code as efficiently as
possible. You MUST NOT hardcode any arrays. The vertices of the environment MUST be
allocated dynamically, although the size will always be 4 times the number of obstacles. As
for the neighbours of a vertex …they must be created individually … you may NOT create an
array of neighbours at any time … they MUST be represented as a linked-list.
• Complete your code as necessary so that a call to valgrind returns 0 errors and 0 memory
leaks. Make sure that your code is efficient and that you are making good use of your written
functions.
________________________________________________________________________________
IMPORTANT SUBMISSION INSTRUCTIONS:
Submit all of your c source code files as a single tar file containing:
1. A Readme text file containing
• a list of source files submitted
• any specific instructions for compiling and/or running your code
2. All of your .c source files and all other files needed for testing/running your programs.
3. Any output files required, if there are any.
The code MUST compile and run on the course VM.
• If your internet connection at home is down or does not work, we will not accept this as a reason for
handing in an assignment late … so make sure to submit the assignment WELL BEFORE it is due !
• You WILL lose marks on this assignment if any of your files are missing. So, make sure that you hand
in the correct files and version of your assignment. You will also lose marks if your code is not written
neatly with proper indentation and containing a reasonable number of comments. See course
notes for examples of what is proper indentation, writing style and reasonable commenting).

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