Encoded supplied input according to vigenere crytography, key is suppiled at command line.

vigenere.c

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

int main(int argv,char *argc[])
{
    if(argv!=2)
    {
        printf("Usage: ./vigenere k\n");
        return 1;
    }
    string key=argc[1];
    
    
    int i;
    int flag = 1;
    for(i=0; i< strlen(key) && flag == true; i++ )
        {
            if( isalpha(key[i]))
              flag = 1;
             else
                flag = 0;
                
                key[i]=tolower(key[i]);
        }  
       // printf("%s\n",key);
        if (flag==0)
        { printf("Error\n");    return 1; }

    string plain;
    char cipher[100];
    printf("plaintext: ");
    plain=get_string();
    int plain_length=strlen(plain),key_length=strlen(key);
    int j=0;
    
    for(i=0;i<plain_length;i++)
    {
            //printf("%s\n",cipher);
        if(isalpha(plain[i])&&isupper(plain[i]))
       {
           //printf("%d\n",(key[(j++)%key_length])-97);
           cipher[i]=(  (  (  plain[i]  +  (key[(j++)%key_length]-97)  )  ) -65)%26 + 65;
       }
       else if(isalpha(plain[i])&&islower(plain[i]))
       {
            cipher[i]=(((plain[i]+(key[(j++)%key_length]-97)))-97)%26+97;
       }
       else
       {
           cipher[i]=plain[i];
       }
    }
    cipher[i]='\0';
    printf("ciphertext: %s\n",cipher);
     
     return 0;
}