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I'm having issues on how to start the shift value part of this assignment. I've gotten the first part of the assignment and making sure the argc is indeed a word or letter and not an integer. Could anyone help me work it out, so I can learn how to approach this in the future and this assignment? My first step would be to break the shift value down in two steps by 1 single letter and a phrase. But would it be beneficial to try and combine both single letter and phrases with an equation or better to try and break it down the problem?

// takes single character as an input
int shift(char c);
int main(int argc, string argv[])
{
    // checking the argument if the statement has 2 arguments
    if (argc != 2)
    {
        printf("Usage: ./vigenere keyword ");

        return 1;
    }
    else
    {
        // making sure the keyword is A-Z 
        for (int i = 0, n = strlen(argv[1]); i < n; i++)
        {
            // if it is not alphabetical 
            if (!isalpha(argv[1][i]))
            {
                printf("Usage: ./vigenere keyword ");

                return 1;
            }
        }

    }
    // asking for the plaintext to be encoded by the key 
    string p = get_string("plaintext: ");
    printf("ciphertext: ");
    // storing the key 






}
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I am not sure what you mean by combining a single letter and a phrase. The vignette is similar to the Cesar cipher. There is a key and a word that needs to be encrypted. If you manage to encrypt it all with one number let’s say change hello then you just need to create a variable that houses the key and changes according to how many times the key has been used or what location it is at. There are many other approaches but a good start is to make a simple encryption code that changes everything like the Caesar by one simple key and then make the key more complex.

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