Communication between towers
In the Lord of the Rings trilogy, there is a scene where the first beacon is lit in the
towers of Minas Tirith. The second beacon then sees the fire, and knows to light its
fire to send a signal to the third beacon, and so forth. This was a means of
communicating in the days before telegraphs were invented as it was much faster
than sending a human rider to deliver a message. Communication towers were
equipped with signaling mechanisms, such as mirrors, that could spell out messages
using the positions of the mirrors.
Today, there are several examples of communication networks that are conceptually
similar, but much more technically advanced, that route messages through multiple
hubs between the sender and the receiver. For example, when you type a URL into a
web browser, a request is sent through a network of service providers to the
destination, and then packets of information are sent back to your machine. If I type
www.google.com from my home in Boulder, my request follows this path:
1 192.168.2.1 (192.168.2.1)
2 c-24-9-60-1.hsd1.co.comcast.net (220.127.116.11)
8 18.104.22.168 (22.214.171.124)
9 126.96.36.199 (188.8.131.52)
10 den03s06-in-f16.1e100.net (184.108.40.206)
Each IP address is a hop in the network for my request, which is received at each
service provider and then forwarded to the next service provider in the network,
depending on the final destination of the message.
(Note: I got this path by typing traceroute www.google.com in a terminal window.
From campus, you will see a different path.)
Build your own communications network
In this assignment, you’re going to simulate a communications network using a
linked list. Each node in your linked list will represent a city and you need to be able
to send a message between nodes from one side of the country to the other and back
again. Your program also needs to provide the capability to update the network by
adding cities, and deleting cities, and still be able to transmit the message.
(Note: I’ll refer to the linked list as “the network” throughout this document.)
Include the following cities in your network:
Implement each city as a struct with a name, a pointer connecting it to the next city
in the network and the previous city in the network, and a place to store the
message being sent. (You can assume the message is a string.) When you initially
build your network, the order of the cities should be the same as the order listed
above. After the network is built, you will provide the option of adding additional
First, display a menu
When your program starts, you should display a menu that presents the user with
options for how to run your program. The menu needs to look like the one shown
The user will select the number for the menu option and your program should
respond accordingly to that number. Your menu options need to have the following
1. Build Network: This option builds the linked list using the cities listed above
in the order they are listed. Each city needs to have a name, a pointer to the
next city, and a message value, which will initially be an empty string. This
option should be selected first to build the network, and can be selected
anytime the user wants to rebuild the starting network after adding cities. As
part of the Build Network functionality, you should print the name of each
city in the network once the network is built in the following format:
NULL <- Los Angeles <-> Phoenix <-> Denver <-> Dallas <-> St. Louis <-> Chicago
<-> Atlanta <-> Washington, D.C. <-> New York <-> Boston -> NULL
2. Print Network Path: This option prints out the linked list in order from the
head to the tail by following the next pointer for each city. You should print
the name of each city. Printing the path could be very useful to you when
debugging your code. The format should be the same as the format in Build
3. Transmit Message Coast-to-Coast-to-Coast: The message that you need to
transmit is in the file messageToTransmit.txt on Canvas. Your program needs
to handle the filename as a command line argument. Your program needs to
open this file and read word by word and transmit the message starting at
the beginning of the network and ending at the end of the network, and then
sends the message back again to the beginning of the network. Using the
cities in this write-up, the message would go from Los Angeles to Boston,
passing through each city along the way, and then back to Los Angeles,
passing through each city along the way. When a city receives the message,
you should print
When a city receives a word, the word should be deleted from the sender
city, i.e set the message for the sender city to an empty string. Here is a
screenshot of the output I get after transmitting the first two words in the
Note: The name of the file that contains the message should be a
command line argument to your program.
4. Add City: This option allows the user to add a new city to the network. If the
user selects this option, then they should be prompted for the name of the
city and the city that the new city should follow in the network. For example,
if the user wants to add Tucson after Phoenix in the network, then the first
four cities in the network would be:
Los Angeles <-> Phoenix <-> Tucson <-> Denver…
You don’t need to print anything when you add a new city, just call the Print
Network function again from the menu if you want to verify that the city has
If the user wants to add a new city to the head of the network, e.g. replace Los
Angeles as the starting city, then they should type “First” when prompted for
the previous city and your code should handle this special case.
Here is a screenshot showing the expected output for the add city
functionality when the user selects Add City from the menu.
5. Delete City: This option allows the user to delete a city from the network.
When the user selects this option, they should be prompted for the name of
the city to delete. Your code should then update the next and previous
pointers for the surrounding cities and free the memory for the deleted city.
6. Clear Network: This option allows the user to delete all cities in the network
starting at the head city. After this functionality executes, the head of the list
should be NULL and all cities should be deleted from the network. When a
city is deleted, print the name of the city just before freeing the memory.
Your clear network method should be called in the destructor of
CommunicationNetwork, in addition to being a menu option.
7. Quit: This option allows the user to exit the program.
For each of the options presented, after the user makes their choice and your code
runs for that option, you should re-display the menu to allow the user to select
Structuring your program
The specific cout statements to produce the expected output are shown in Appendix
The functionality for your network will be implemented in a class called
CommunicationNetwork. A suggested header file called, CommunicationNetwork.h
is provided for you on Canvas. You are welcome to structure your program
differently than the structure provided in CommunicationNetwork.h, or write
additional helper functions if needed. Your code needs to be readable, efficient, and
accomplish the task provided. You will be asked in interview grading to explain the
decisions you made.
Here are the prototypes for the methods included in the
CommunicationNetwork.h header file.
void CommunicationNetwork::addCity(string previousCity, string newCity)
/*Insert a new city into the linked list after the previousCity. The name of the new
city is in the argument newCity.
void CommunicationNetwork::transmitMsg(char *filename)
/*Open the file and transmit the message between all cities in the network word by
word. A word needs to be received at the end of the network before sending the next
word. Only one city can hold the message at a time; as soon as it is passed to the next
city, it needs to be deleted from the sender city. Once the message reaches the end of
the network, it needs to be transmitted back the other direction to the beginning of
/*Start at the head of the linked list and print the name of each city in order to the
end of the list. */
/*Build the initial network from the cities given in this writeup. The cities can be
fixed in the function, you do not need to write the function to work with any list of
void CommunicationNetwork::deleteCity(string cityNameDelete)
/*Find the city in the network where city name matches cityNameDelete. Change the
next and previous pointers for the surrounding cities and free the memory.
/*Delete all cities in the network, starting at the head city.
Suggestions for completing this assignment
There are several components to this assignment that can be treated independently.
My advice is to tackle these components one by one, starting with printing the menu
and getting user input. Next, build the network and print it. Then, add the
functionality to add additional cities.
Once you get one feature completed, test, test, test, to make sure it works before
moving on to the next feature.
There are several examples of how to work with linked lists in Chapter 5 in your
book, and we will also be covering these concepts in lectures this week.
Your code should be separated into three files – CommunicationNetwork.h,
CommunicationNetwork.cpp, and Assignment4.cpp. You need to write both the
CommunicationNetwork.cpp and the Assignment4.cpp files. You can compile your
code on the command-line using g++
g++ -std=c++11 CommunicationNetwork.cpp Assignment4.cpp -o Assignment4
and then run your program on the command-line using
Also, start early.
What to do if you have questions
There are several ways to get help on assignments in 2275, and depending on your
question, some sources are better than others. There is a discussion forum on Piazza
that is a good place to post technical questions, such as how to delete a node in a
linked list. When you answer other students’ questions on the forum, please do not
post entire assignment solutions. The CAs are also a good source of technical
information, especially questions about C++. If, after reading the assignment writeup, you need clarification on what you’re being asked to do in the assignment, the
TAs and the Instructor are better sources of information than the discussion forum
or the CAs.
Appendix A – cout statements
cout << "===CURRENT PATH===" << endl; cout<<"NULL <- "; cout << tmp->name << " <-> “; //for all nodes in network
cout << current->cityName << " -> “;
cout << "NULL" << endl; cout << "==================" << endl; Transmit Message cout<