vigenere

#include <stdlib.h>
#include <stdio.h>
#include <cs50.h> //gets string from user with GetString()
#include <ctype.h>
#include <string.h>

//takes in char a, and increments it by k if it is a letter
char crypt(char a, char k);

int main(int argc, string argv[])
{ 
    if (argc!=2)        //checks for parameters
    {
        printf("Invalid parameters.\n");
        return 1;
    }
    string k = argv[1];
    int klength = strlen(k);
    for (int i=0; i<klength; i++) //checks that all key values are chars
    {
        if (!isalpha(k[i]))
        {
            printf("Invalid parameters.  Contains non-characters.\n");
            return 1;
        }
    }

    string sentence = GetString();
    
    for(int i=0, j=0, slength = strlen(sentence); i<slength; i++)
    {
        if (isalpha(sentence[i]))
        {
            sentence[i] = crypt(sentence[i], k[j%klength]);
            j++;
        }
    }
    printf("%s\n", sentence);
}

char crypt(char a, char k)
{
    int base;           //base allows for the use of MOD on letter values of 0-25
    k= tolower(k);      //converts k to proper offset value
    k-=97;
    if (isalpha(a))
    {
        if(islower(a))      
            base = 97;
        else
            base= 65;
        return (a-base+k)%26+base;      
    }
    else
        return a;   
}