Homework 1, Let's Write Some Functions.

Short Description:

Write an interactive caesar cipher program. You must use functions.

Goals

When you finish this homework, you should have:

Have a basic idea of how to work with functions.
Have written a C++ program using functions.

Formal Description

We will work on this program jointly in class. I expect you to use the code that we develop in class when working on this program.

Implement a generic Caesar Cipher

The Caesar Cipher is a substitution cipher named after Julius Caesar. By default, to encode each letter is shifted by 3: ie A -> D, B->E and so forth. At the end of the alphabet X->A, Y->B and Z->C. This represents a shift of 3 ('A'+3 = 'D'). Decryption is done by subtracting 3. For more detail as well as a history of this cipher see Wikipedia.

In the modern area, rot13 is an equivalent cipher where 13, rather than 3 is added for encryption and subtracted for decryption.

For this program, we will write a version which the user may specify any value for the shift.

Specifically, the program should work as follows.

Ask the user if they wish to encrypt a message. The answer should be yes or no.
- Prompt with "Do you wish to encrypt a message (yes/no)? "
- Read in the entire line of the response.
- Print a newline after you read in the line.
- Convert the input line to all lower case. (Yes, YeS and YES are all ok answers).
Check the prompt
- For an answer of no, exit the program.
- For an answer of yes, continue.
- For all other answers ask again.
If the user wishes to encrypt a message
- ask for the shift amount.
  - Prompt the user with "Enter the shift amount => "
  - Read in the integer, no error checking required.
  - Do a cin.ignore to ignore the rest of the line.
  - Return the equivelent of the nubmer between 0 and 25.
- Ask for the message, all on one line.
  - Prompt with "Enter the message to encrypt on one line: "
  - Read in the entire line of the response.
  - Print a newline after the response.
- Encrypt the message as follows:
  - Any alphabetic character should be shifted by the given amount.
    - Upper/lower case should be preserved.
  - Any non-alphabetic character should be preserved.
- Print out the encrypted message.
  - Print the encrypted message on a line ending with a newline.
  - Print a blank line (newline only)
- Return to step 1.

Notes on Makefile

Make is a utilitiy that builds an executable. It allows programmers to easly rebuild a project involving many files. We will be breaking our programs into multiple files this semester so we will learn to use make.

The key to make is a configuration file for each project called Makefile. No extensions, capatal M. This file contains the rules for building the executable.

For this project use this Makefile.

Download the file to your directory.
Call your source code caesar.cpp
Compile with make

Please note, makefiles are tricky, it is better if you download this file rather than try to retype it. A tab is required at the beginning of the last line.

Notes

You must use functions.
You must use the provided makefile.
Your program MUST compile with the flags supplied in the makefile without errors or warnings on cslab103.

Required Files

A single tar file containing the source code and makefile for this program.

Submission

Submit the assignment to the D2L folder Homework 1 by the due date.