Substitution Cipher Program Problem...

Hi guys. I was bored today and wanted to make a program that did substitution ciphers. I made it and am now testing the actual Algorithm for it but it is not working. It will replace the first letter its told to but if you are substituting 2 letters for 2 other letters or even the whole alphabet for something else for a true substitution cipher it will not work. I know i should be using Strings instead of Char Arrays and that some of the Variable names arent great just remember i was writing this up quick and i am not familiar with strings enough to implement them in this situation. Thanks for any and all help you can give.

Code:

#include <iostream>
#include <fstream>
#include <string>
#include <windows.h>

using namespace std;

int main()
{
    //single-letter substitutions cipher program
    //infinite loop
    while(1)
    {
        //clear screen
        system("cls");
        //title
        cout<<"Mono-Alphabetic Substitution Cipher\n";
        cout<<"===================================\n\n";
        //menu
        cout<<"(1) Substitute\n";
        cout<<"(2) Exit\n\n";
        //choice
        int mchoice;
        cout<<"Choice: ";
        cin>>mchoice;
        cin.ignore();
        //evaluate
        if(mchoice==1)
        {
            //encrypt
            char ptext[50];
            char lettersto[30];
            char replacements[30];
            cout<<"Text to encrypt: ";
            cin.getline(ptext, 50);
            cout<<"Letters to replace in text:       ";
            cin.getline(lettersto, 26);
            cout<<"Replace with what (respectively): ";
            cin.getline(replacements, 26);
            //begin replacement
            int i=0;
            int j=0;
            int k;
            int n;
            int L=(strlen(ptext));
            int M=(strlen(lettersto));
            //check some stuff
            if(M != (strlen(replacements)))
            {
                cout<<"Replacement Mismatch\n";
                cin.ignore();
                break;
            }   
            //for loops
            //ENCRYPTION------------------------------
            for(i; i<M; i++)
            {
                k = int(lettersto[i]);
                for(j; j<L; j++)
                {
                    n = int(ptext[j]);
                    if(k == n)
                    {
                        //replace letter
                        ptext[j] = replacements[i];
                    }
                }
            }
            //ENCRYPTION-----------------------------
        
            cout<<ptext;
            cout<<"\n\n";           
            cin.ignore();
            continue;
        }
        if(mchoice==2)
        {
            //exit
            cout<<"Exitting...";
            Sleep(1500);
            break;
        }
        else{
            cout<<"\nIncorrect Choice!!!\n";
            cin.ignore();
        }    
    }
    return 0;
}

Your for loops don't initialize the counter variable to 0. It looks like you try to do this elsewhere, but it causes a problem in the nested loop because the second time through j is not reset to 0.

Once again, many thanks for all of your help Daved, i really appreciate it!

You'll need <cstring> for strlen(), and <sstdlib> for system.

Writing a function called say substitute() would help simplify main()

The initialization part of a for loop can be empty (actually, all parts can):

Code:

for(i; i<M; i++)

->

Code:

for( ; i<M; i++)

You might need to read/write your files in binary.

Oh, and strlen() returns type size_t, not int.

Thanks guys for all of the suggestions, the program is running fine. Like i said earlier though it was just for fun Thanks again everyone!

Perhaps someone could help me in my attempt to learn strings and show me how to make this program using strings? Thanks!

Maybe you should post your C-string code first.

>> Perhaps someone could help me in my attempt to learn strings and show me how to make this program using strings?

Convert

Code:

char ptext[50];

to

Code:

string ptext;

Do the same with the other string variables.

Convert

Code:

cin.getline(ptext, 50);

to

Code:

getline(cin, ptext);

Do the same for the other calls to getline.

Convert

Code:

strlen(ptext)

to

Code:

ptext.length()

or

Code:

ptext.size()

Do the same for all calls to strlen.

Your original program should have had #include <cstring> because you used strlen. The new code with C++ strings won't use strlen, so you won't need <cstring>. Your original code didn't use the C++ string class, so it didn't need #include <string>. The new code will use the C++ string class, so you should keep that in there.

You might be able to take advantage of built-in replace functionality to make your encryption code simpler, but it should still work the way it is with the C++ string class.

Oh yes, and you don't need your continue statement.

>> Oh yes, and you don't need your continue statement.

This is incorrect. If you take it out the program won't work properly.

Yes, you're right. You'd also need to make the if(mchoice==2) into an else if. Never mind then.