Split main.cpp into a header file and main.cpp

[propvgvnda]

Hello, everyone. I need to make a header file with 3 functions, "generating new key", "encryption" and "decryption". Then, just include this in the main.cpp so that it works the same way, can you help me?

  #include<iostream>
#include<string.h>
 
using namespace std;
 
int main(){
    char msg[] = "ATTACKATDAWN";
    char key[] = "LEMON";
    int msgLen = strlen(msg), keyLen = strlen(key), i, j;
 
    char newKey[msgLen], encryptedMsg[msgLen], decryptedMsg[msgLen];
 
    //generating new key
    for(i = 0, j = 0; i < msgLen; ++i, ++j){
        if(j == keyLen)
            j = 0;
 
        newKey[i] = key[j];
    }
 
    newKey[i] = '\0';
 
    //encryption
    for(i = 0; i < msgLen; ++i)
        encryptedMsg[i] = ((msg[i] + newKey[i]) % 26) + 'A';
 
    encryptedMsg[i] = '\0';
 
    //decryption
    for(i = 0; i < msgLen; ++i)
        decryptedMsg[i] = (((encryptedMsg[i] - newKey[i]) + 26) % 26) + 'A';
 
    decryptedMsg[i] = '\0';
 
    cout<<"Original Message: "<<msg;
    cout<<"\nKey: "<<key;
    cout<<"\nNew Generated Key: "<<newKey;
    cout<<"\nEncrypted Message: "<<encryptedMsg;
    cout<<"\nDecrypted Message: "<<decryptedMsg;
 
	return 0;
}

[Handy Andy]

Hello propvgvnda,

I need to make a header file with 3 functions

You do not put functions in a header file. I am thinking along the lines of "Functions.cpp", for the functions, "proto.hpp", for the prototypes of the functions and "main.cpp" for the main program.

I am thinking that the 3 sections:

//generating new key

//encryption

//decryption

are to be your 3 functions.

I have not worked on this yet, but this is my initial thought:

Proto.hpp

#ifndef PROTO_HPP
#define PROTO_HPP

void GenerateNewKey();  // <--- The return value and the parameters may need to change.

#endif // !PROTO_HPP 

And Functions.cpp

#include <iostream>

#include "Proto.hpp"

void GenerateNewKey()
{
    // <--- Add code from "main" here.
}

My only question is why do you need to use "string.h" and C-strings when a "std::string" will work better?

I would discus where you define "i" and "j", but for the moment with your code it works.

Andy

[propvgvnda]

I don't know how to implement this with only strings.

[seeplus]

What C++ compiler are you using - as this is invalid standard C++

char newKey[msgLen], encryptedMsg[msgLen], decryptedMsg[msgLen]

The size of an array must be able to be determined by the compiler at compile time. strlen() is a run-time function.

If you're using C++17, then

int msgLen = strlen(msg), keyLen = strlen(key), i, j;
 
char newKey[std::size(msg)], encryptedMsg[std::size(msg)], decryptedMsg[std::size(msg)];

[Handy Andy]

Hello propvgvnda,


That is to bad that you do not know about "std::string"s yet. It makes it much easier.

The first problem: char newKey[msgLen], encryptedMsg[msgLen], decryptedMsg[msgLen];. This is known as a VLA, (variable length array), and is not allowed in C++ and not allowed by most newer compilers. A few very old compilers may allow this though.

What is in the []s needs to be a constant number like 12 or a variable defined as a constant like:
constexpr int MSGLEN{ 12 }; or you could just "const".

If you have a need for using char newKey[msgLen] you could use "magLen" to create a dynamic array using the "new" key word. But do not forget to include "delete" to free the memory before the program ends.

Before I can continue I need to know if you know how to create a dynamic array.

It just hit me. You say char newKey[msgLen] which would create an array of 12 elements, but you did not allow for the "\0" at the end. what you would need is: char newKey[msgLen + 1] to allow for the '\0' at the end.

Using "std::string"s your code would change to this:

std::string msg{ "ATTACKATDAWN" };
std::string key{ "LEMON" };
size_t i, j;
 
std::string newKey, encryptedMsg, decryptedMsg;

//generating new key
for (i = 0, j = 0; i < msg.size(); ++i, ++j)
{

    if (j == key.size())
        j = 0;

    newKey += key[j];
}

Here you do not have to worry about sizes or arrays, but you can still access the string the same as you would an array.

Something to think about and if you want some information:
http://www.cplusplus.com/reference/string/string/
https://en.cppreference.com/w/cpp/string
https://www.learncpp.com/cpp-tutorial/4-4b-an-introduction-to-stdstring/
or the home page https://www.learncpp.com Very useful.

I will try working with what you started with.

Andy

[seeplus]

std::size(msg) includes the terminating \0