Takes user input and encrypts it with an alphabetical keyword.

/**
 * vigenere.c
 *
 * Koren Leslie Cohen
 *
 * Takes user input and encrypts it.
 *
 */

#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

// program to encrypt user string by keyword entered in command line
int main(int argc, string argv[])
{
    // declare variables
    int shift;
    int KeyValue;

    // only accept two command line arguments
    if (argc != 2)
    {
        printf("No key or incorrect key. Run program and enter single alphabetical keyword in command line.\n");
        return 1;
    }

    // capture the second command line argument as a string
    string key = argv[1]; 

    // iterate through the key to make sure it's alphabetical
    for (int n = 0, keylength = strlen(argv[1]); n < keylength; n++)
    {
        if ((key[n] >= '0') && (key[n] <= '9'))
        {
            printf("Sorry! You were supposed to enter a keyword with only letters.  Boohoo.\n");
            return 1;
        }
    }

    // get the plain text
    // doesn't pass check50 when printed - printf("Type a word to encrypt!\n");
    string PlainText = GetString();   

    // encrypt - iterate over the characters in string, printing each one encrypted
    for(int i = 0, j = 0, length = strlen(PlainText); i < length; i++, j++)
    {
        // start the key again if key shorter than PlainText
        if (j >= strlen(key))
        {
            j = 0;
        }

        // loop was "breaking" when I used "key[j]" for "A" or "a" due to the above
        // declared a variable of a different name to make the program run properly
        KeyValue = key[j]; 

        // skip key[j] if PlainText[i] is not an alpha character
        if (!isalpha(PlainText[i]))
        {
            j = (j - 1);
        }  

        // makes Aa = 0, Zz = 25 for the uppercase letters 
        if ((KeyValue >= 'A') && (KeyValue <=  'Z'))
        {
            KeyValue = (KeyValue -  'A');
        }

        // makes Aa = 0, Zz = 25 for the lowercase letters
        if ((KeyValue >= 'a') && (KeyValue <= 'z'))
        {
            KeyValue = (KeyValue - 'a');
        }

        // encryption
        shift = (PlainText[i] + KeyValue);

        // wrapping after Z for uppercase letters
        if (isupper(PlainText[i]) && (shift > 'Z'))
        {
            shift = (shift - 26);
        }

        // wrapping after z for lowercase letters
        if (islower(PlainText[i]) && (shift > 'z'))
        {
            shift = (shift - 26);
        }

        // test - printf("The ASCII value of %c is %d.\n\n", shift, shift);
        // test - printf("In calculating %c + %d...\n", PlainText[i], key[j]); 
        // test - printf("The value of PlainText %c is %d\n", PlainText[i], PlainText[i]);
        // test - printf("The value of key is %d\n", KeyValue); 

        // if character is alphabetical, print encrypted result
        if (isalpha(PlainText[i]))
        {
            printf("%c", shift);
        }

       // if non-alphabetical character, print as is   
        else
        {
            printf("%c", PlainText[i]);
        }

    }  
    printf("\n");
    return 0;
}

Result:

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Koren Leslie Cohen

Product manager at Facebook. Former senior product manager at Dollar Shave Club in Los Angeles and software engineer at J.Crew / Madewell in New York City. Recovering trial lawyer.

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Latest posts by Koren Leslie Cohen (see all)

• PM Career Story - April 28, 2022
• How to Transition into Product Management - December 26, 2017
• What I’ve Learned in My First Few Months as a Product Manager - October 14, 2015

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