Vigenere table! I did it!

[TimmyFangs]

I am just sharing this because I'm proud of figuring it out myself :-)

  #include <iostream>
using namespace std;

int main()
{
    int alpha[26][26];
    for(int i=0; i<26; i++)
    {
        for(int j=0; j<26; j++)
        {
            alpha[i][j]=(i+j)+65;
            if(alpha[i][j]>=91)
            {
                alpha[i][j]-=26;
            }
            cout<<(char) alpha[i][j];
        }
        cout<<endl;
    }
    return 0;
}

[keskiverto]

Good. Now read about operator %. Would calculating remainder (of something) allow a sightly different solution?

[JLBorges]

> I'm proud of figuring it out myself :-)

Well done.

Now, consider this: C++ does not specify that the character 'A' must have a value of 65; nor does it specify that the upper case characters in the basic execution character set must have contiguous values.

Other than that a. the null character has a value of zero, b. the value of each character after 0 in the list of decimal digits shall be one greater than the value of the previous, and c. the values of the characters in the basic execution character set are non-negative, "the values are locale specific".

Learning to write robust, portable code is not difficult; unlearning bad programming habits (often encouraged/insisted-on by 'career' teachers) is hard.

#include <iostream>
#include <string>

int main()
{
    constexpr std::size_t N = 26 ;
    std::string alpha[N] = { "ABCDEFGHIJKLMNOPQRSTUVWXYZ" } ;

    for( std::size_t i = 1 ; i < N ; ++i )
        alpha[i] = alpha[i-1].substr(1) + alpha[i-1][0] ;

    for( const auto& row : alpha ) std::cout << row << '\n' ;
}

[TimmyFangs]

I can think of using that by calculating everything in one go and using%26 to break up the line into rows. I'm sure you can use % to calculate the letters more efficiently, but I'm at a loss on how to do that currently. What I don't understand is how to use % when starting the alphabet over, going from z to a.

[TimmyFangs]

JLBorges,
The more I look at these forums, the more bad habits I realize that I've been taught. I will look up a couple of things in your code that I'm not familiar with and hopefully learn more. Thank you for your reply!

[integralfx]

I can think of using that by calculating everything in one go and using%26 to break up the line into rows. I'm sure you can use % to calculate the letters more efficiently, but I'm at a loss on how to do that currently. What I don't understand is how to use % when starting the alphabet over, going from z to a.

Not nearly as elegant or as bug/error proof as JLBorges' solution.

const int NUM_LETTERS{ 26 };
for( int i{}; i < NUM_LETTERS; i++ ) {
    for( int j{}; j < NUM_LETTERS j++ ) {
        alpha[i][j] = 'A' + (i + j) % NUM_LETTERS;
    }
}

[48T7M4Gy]

Yes, well done OP.
Here's another dimension that might be of interest.

#include <iostream>

int main()
{
    char alphabet[26] = {0};
    
    // SETUP ALPHABET ARRAY
    for ( char i = 'A'; i <= 'Z'; i++)
    {
        alphabet[i - 'A'] = i;
    }
    
    // DISPLAY ARRAY CONTENTS
    for ( char i = 'A'; i <= 'Z'; i++)
    {
        std::cout << alphabet[i - 'A'];
    }
    std::cout << '\n';
    
    // TEST A SINGLE CHARACTER
    char test = 'T';
    std::cout << alphabet[test - 'A'] << '\n';
    
    // SET VIGNERE SHIFT (SAY, T -> P, AND DISPLAY ALPHABET)
    int shift = 'T' - 'P' ;
    
    for ( char i = 'A'; i <= 'Z'; i++)
    {
        std::cout << i << " -> " << alphabet[(i - shift + 'A') % 26] << '\n';
    }
    
    return 0;
}

[TimmyFangs]

Taking into account what JLBorges said (which I agree with),I'm trying to figure out now how to do a vigenere cipher without the bad programming habits. I'm still not quite understanding the algorithm. I know how to use the table on paper, but coming up with an algorithm and coding it is difficult.

[48T7M4Gy]

1. There are 26 characters, each with an index number in the array between 0 and 25 inclusive
2. Take any plaintext character and find its corresponding index number, i
3. Add the shift number to i to encrypt the plaintext character
4. If the shifted index number is > 25 then subtract 25 ( hence %, modular arithmetic, because the index number must always remain in the 0 to 25 range.

[48T7M4Gy]

http://www.cplusplus.com/forum/beginner/203513/