• view:  The user types the name of a file, and the file is read and dipslayed on the screen.
• mangle: A file is read, and an encrypted/decrypted version is written, as discussed below.

Encrypting text. We use a simple "transposition cipher" here, where each letter is replaced by the letter k steps further along in the alphabet.For instance, if k is 2, each 'a' is replaced by 'c', each 'b' is replaced by 'd', etc. Only printable characters are replaced. Printable characters are those between 32 (for space: ' ') and 126 (for tilde: '~') in the ASCII chart. (You can get an ASCII table here, but you won't actually need it to do this assignment.) The transpose() function below returns the character produces by shifting ch through k places. It shifts characters "cyclically", wrapping from the end of the printable characters to the beginning. Thus the next character after '~' is '  '.

char transpose(char ch, int k)
{ // If ch is printable (lies between ASCII ' ' and '~'), shift it k places cyclically.
    if(ch < ' ' || ch > '~') return ch; // no change since nonprintable
    int c = ch + k; // shift it: use int to avoid overflow
    while(c > '~') c -= 95; //cycle it back into printable range
    while(c < ' ') c += 95; // 95 = '~' - ' ' + 1
    return c;
}

So to "mangle" a file follow these steps:
 1. Prompt the user to type in:

a). A filename for the source file
b). A filename for the destination file
c). A shift number, k;

3. Close both files

Here is a skeleton of the process (statements in brackets, like [[do]], are pseudocode)

void doMangle()
{
 string inFilename, outFilename;
 [[prompt user for names of source file and destination file]]
 [[ read the file names using getline(cin,inFilename), etc. ]]
 // c_str() converts a string to a c-string, (as required by the inf constructor)
 ifstream inf(inFilename.c_str()); // try to open selected file
 if(!inf.is_open())  //test whether the file was found in the current directory
 {
        cout << "can't find that one!"; //nope, it wasn't found
        return;
 }
 [[ open the destination file in same manner]]
 [[prompt user for k, and get it]]

 char ch;
 while((ch = inf.get())!= EOF)// get next character from the source file
 {
   char newCh= transpose(ch,k);
   cout << newCh; //echo encryprted character to the screen
   outf << newCh; // and stick it in the destination file
 }
 inf.close(); // gracefully close the files
 outf.close();
}

You may find the following function helpful; It returns true if N is a prime number. Alert: There is one error in this algorithm! Study it carefully and correct the error.

bool isPrime(long N)
{
    if(N == 1 || N == 2) return true;
    if(N % 2 == 0) return false;
    for(long k = 3; k*k < N; k+=2)
        if(N % k == 0) return false;
    return true;
}

Lots more primes can be found here. Look here for the story of the largest prime number known to date, (which is 909,526 digits long!), found last year by Roland Clarkson (a 19 year-old student at California State University Dominguez Hills).

If the new word exists in the table, the corresponding numTimes[] is incremented. For instance, if the word is "banana" and words[23] contains "banana", then numTimes[23] is incremented by 1. If the new word is not in the table, it is put in the table, as long as the table size has not reached 500.

Once the file has been read, the table is printed: each word in the table is printed along with its occurrence count, as in:
boy:4 banana:12 and:45 not:6 encyclopedia:12, etc.

• A cover page for the project, with brief explanation, and a description of any special things you did.
• A clean listing of your program - well annotated;
• FTP your .cpp file to the FTP site, or submit a diskette containing the program, both source code and executable.