Shift deciphering tool (cesar cipher) - c++

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.

Answer 2

It seems to me you forget to ofstream::close your output file.
http://www.cplusplus.com/reference/iostream/ofstream/close/

Answer 3

There's a few serious problems I noticed.
1) You're not wrapping around when shifting. So if shift = 20 and you get the character 'y', when you do this:
asciiCode = static_cast<int>(ch); //Change it to the ASCII number
asciiCode += shift; //Do the shift
ch = static_cast<char>(asciiCode);
asciiCode becomes 'y' + 20 == 141, which is off the end of standard 128 bit ascii. You need to take the number mod, I guess, 128. So:
asciiCode = static_cast<int>(ch); //Change it to the ASCII number
asciiCode += shift; //Do the shift
asciiCode %= 128; // Wrap around
ch = static_cast<char>(asciiCode);
2) But this still doesn't address why you're only shifting alphabetic characters, yet shifting those letters to non-letter characters in the result.
And why are you counting them in an array of size 128, instead of 52? Or just 26?
Even more so, why are you generating the shift value in-code? You need to know that value to decode later...
In short, I reccommend you do it this way:
Translate characters to the numbers 0-51, rather than 0-127 (or 0-25)
When applying the shift, add it to the 0-51 value, then take that % 52 to get the new character.
Consider manually inputting the shift value.
This is how a Ceasar Cypher generally works.
3) As to the printing problem, have you tried debugging?
I can't speak about Bloodshed C++, but even just adding a cout statement would be really helpful. For instance:
std::cout << ch; //Print to cout for debugging
outfile << ch; //Print to the outfile
would let you know if the error is in the outfile processing.