Question

I have built a Program to decipher shift ciphers (Cesar Cipher). It seems to take the Input and makes a output file but it is empty. I am thinking that something may be wrong with the Deciphering function and have tried changing multiple things to no avail. I use Bloodshed C++ for compiling and windows for a OS.

Thanks

kd7vdb

#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>

using namespace std;
const int ARRAYSIZE = 128;

void characterCount(char ch, int list[]);
void calcShift( int& shift, int list[]);
void writeOutput(ifstream &in, ofstream &out, int shift);

int main()
{
    int asciiCode = 0,
        shift = 0;
    string filename;
    char ch;
    ifstream infile;
    ofstream outfile;

    //input file

    cout << "Input file name: ";
    getline(cin, filename);

    infile.open(filename.c_str());

        if (!infile.is_open()) { 

            cout << "Unable to open file or it doesn't exist." << endl;

            return 1;

        }

    //output file

    cout << "Output file name: ";
    getline(cin, filename);



    outfile.open(filename.c_str());

    int list[ARRAYSIZE] = {0}; 

        while (infile.peek() != EOF) 
        {
            infile.get(ch);
            characterCount(ch, list); 
        }



    infile.clear();
    infile.seekg(0);

    calcShift (shift, list); //Calculate the shift based on the <strong class="highlight">most</strong> characters counted
    writeOutput(infile, outfile, shift); //Decypher and write to the other document

    return 0;
}

void characterCount(char ch, int list[])
{
        if (ch >= 'A' && ch <= 'z') //If the character is in the alphabet...
        {
            int asciiCode = 0;

            asciiCode = static_cast<int>(ch); //Change it to the ASCII number
            list[asciiCode]++; //And note it on the array
        }
}

void calcShift( int& shift, int list[])
{
    int maxIndex = 0, //Asuming that list[0] is the largest
        largest = 0;

        for (int i = 1; i < ARRAYSIZE; i++)
        {
            if (list[maxIndex] < list[i])
                    maxIndex = i; //If this is true, change the largest index
        }

    largest = list[maxIndex]; //When the maxIndex is found, then that has the largest number.

        if (largest >= 65 && largest <= 90) //Calculate shift with <strong class="highlight">E</strong> (for upper-case letters)
            shift = largest - 69;

        if (largest >= 97 && largest <= 122) //For lower-case letters (<strong class="highlight">e</strong>)
            shift = largest - 101;
}

void writeOutput(ifstream &infile, ofstream &outfile, int shift)
{
    char ch;
    int asciiCode = 0;

        while (infile.peek() != EOF) { //Until it is the end of the file...

            infile.get(ch); //Get the next character

                if (ch >= 'A' && ch <= 'z') //If the character is in the alphabet...
                {
                    asciiCode = static_cast<int>(ch); //Change it to the ASCII number
                    asciiCode += shift; //Do the shift
                    ch = static_cast<char>(asciiCode); //Change it to the shifted letter
                }

            outfile << ch; //Print to the outfile
        }
}

Answer 1

You have a couple of logical errors. I think your last two functions should be more like:

void calcShift( int& shift, int list[])
{
    int maxIndex = 0; //Asuming that list[0] is the largest

    for (int i = 1; i < ARRAYSIZE; i++)
    {
        if (list[maxIndex] < list[i])
                maxIndex = i; //If this is true, change the largest index
    }

    if (maxIndex >= 'A' && maxIndex <= 'Z') //Calculate shift with <strong class="highlight">E</strong> (for upper-case letters)
        shift = 'E' - maxIndex;

    if (maxIndex >= 'a' && maxIndex <= 'z') //For lower-case letters (<strong class="highlight">e</strong>)
        shift = 'e' - maxIndex;
}

void writeOutput(ifstream &infile, ofstream &outfile, int shift)
{
    char ch;

    while (infile.peek() != EOF) { //Until it is the end of the file...

        infile.get(ch); //Get the next character

            if (ch >= 'A' && ch <= 'Z') //If the character is in the alphabet...
            {
                ch = 'A' + (((ch - 'A') + shift + 26) % 26);
            }
            if (ch >= 'a' && ch <= 'z') //If the character is in the alphabet...
            {
                ch = 'a' + (((ch - 'a') + shift + 26) % 26);
            }

        outfile << ch; //Print to the outfile
    }
}

To summarise, you don’t need largest in calcShift, you need to subtract maxIndex from 'E' or 'e' to calculate the shift, and your calculation of the substituted char in writeOutput was pretty far off.

I’m not sure why you were getting an empty output file, but this works for me using MSVC.