CSCI 261 - Programming Concepts - Maynooth 2022

Lab 2C - The Secret Moosage

This lab is due by Friday, July 01, 2022, 11:59 PM.
As with all labs you may, and are encouraged, to pair program a solution to this lab. If you choose to pair program a solution, be sure that you individually understand how to generate the correct solution.

Concepts

The focus of this assignment is on how to read data from an "input file stream" or ifstream object and write data to an "output file stream" or ofstream object.

Working with Data

Reading From Files

Today's class discussed how data is often treated as "streams" of information that can be read a piece at a time. The files we will read in CSCI 261 are simple text files; for lab today, the simple text file contains a series of characters. Remember that whenever you work with a file stream as input, we call them ifstream objects.

There will always be four things you will do whenever working with an ifstream . Open the file, check for an error, read its data, and close the file. The typical pattern for this is as follows: 

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ifstream myCatsAgesIn("filename"); // declare ifstream object and open the file

    // check for an error
    if ( myCatsAgesIn.fail() ) {
        cerr << "Error opening input file";
        return -1;
    }

    // read the data and do something with it
    int age;
    while( !myCatsAgesIn.eof() ) {
        myCatsAgesIn >> age;
        cout << "One cat is " << age << " years old.\n";
    }

    myCatsAgesIn.close(); // close the file
    return 0;
}

Remember, once you have an ifstream object (like myCatsAgesIn shown above) you use it in a manner similar to usingcin.

Writing To Files

There will always be four things you will do whenever working with an ofstream.

  1. Open the file
  2. Check for an error
  3. Write its data
  4. Close the file

The typical pattern for this is as follows:

#include <fstream>
#include <iostream>
using namespace std;

int main() {
    // declare ofstream object and open the file
    ofstream myCatsAgesOut("filename");

    // check for an error
    if ( myCatsAgesOut.fail() ) {
        cerr << "Error opening output file";
        return -1;
    }

    // write the data
    myCatsAgesOut << 5 << endl;
    myCatsAgesOut << 8 << endl;

    // close the file
    myCatsAgesOut.close();

    return 0;
}

Remember, once you have an ofstream object (like myCatsAgesOut shown above) you use it in a manner similar to using cout .

Instructions

The cows have been kidnapped by aliens! The only clue to their whereabouts is a strange "ciphered" message, stored in the file secretMessage.txt. Fortunately, our in-house cryptanalysis expert, D. Cipher, has discovered the key:

"The key isn't very advanced. To decipher the message," she says, " you should take each character and replace all ~ (tilde) characters with a space, and shift all other characters up by one."

Your goal for this assignment is to create a program that reads the ciphered text file and then writes a deciphered version to a file called decipheredMessage.txt. For each character in the file, your program should implement the following replacement algorithm:

To see if your implementation works, you should be able to open the file decipheredMessage.txt and see the information about the missing cows.

Hints

Where to place the input file

Any input file needs to be placed at the project level, which should be the same directory as your main.cpp file. You should see both your input file AND main.cpp file in the same directory.

Reading whitespace characters

In order to capture and replace whitespace characters, you will not use the >> operator with the input filestream. Instead, you will use the get() function like this: 

while ( secretMessage.get(c) ) {
    // c is now assigned the next character from the file
}

This example assumes your ifstream is called secretMessage and you have a char variable declared that is called c.

Selection statement

Note that one requirement is to model the logic of the deciphering algorithm using a proper if/else-if/else construct. A switch statement could also be used.

Casting to char

Remember, cout and ofstream objects are sensitive to the datatype of the value to be printed or written to a file. Consider the following: 

cout << ('a' + 2);

What is printed to the screen? The number 99. Why? Because a char plus an int yields an int, and then the int is "sent" to cout. To print the character c to the screen, you will need to use casting, like this: 

cout << (char)('a' + 2);

Ahhh, much better.

Lab Submission

Submit your main.cpp, Makefile file(s).

You will submit your solution to this lab with the rest of Set2. Detailed instructions for doing this are posted in Assignment 2.


This lab is due by Friday, July 01, 2022, 11:59 PM.
As with all labs you may, and are encouraged, to pair program a solution to this lab. If you choose to pair program a solution, be sure that you individually understand how to generate the correct solution.