Caesar

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


int main(int argc, string argv[])
{
    if(argc==2)
    {
        for(int j=0;j<strlen(argv[1]);j++)
        {
            if(argv[1][j]>='0' && argv[1][j]<='9')
            {
                //printf("%c\n", argv[1][j]);
            }
            else
            {
                printf("Only type one key.\n");
                return 1;
            }
        }

       int k = strtol(argv[1], NULL, 10);
        //printf("%i\n", k);

        if (k>0 && k<27)
        {
            //printf("good key.\n");
        }
        else if (k>26)
        {
            do
            {
                k = k - 26;
            }
            while(k>26);
            //printf("key is actually %i\n", k);
        }
        else
        {
            printf("key must be positive.\n");
            return 1;
        }

        string plain=get_string("plaintext: ");
        printf("ciphertext: ");

        for(int i=0;i<=strlen(plain);i++)
        {
            if (plain[i] >= 'a' && plain[i] <= 'z')
            {
                int cipher= (int)plain[i] + k;
                if (cipher>122)
                {
                    cipher = cipher - 26;
                }
                char c=(char)cipher;
                printf("%c",c);
            }
            else if (plain[i] >= 'A' && plain[i] <= 'Z')
            {
                int cipher= (int)plain[i] + k;
                if (cipher>90)
                {
                    cipher = cipher - 26;
                }
                char c=(char)cipher;
                printf("%c",c);
            }
            else
            {
                int cipher= plain[i];
                char c=(char)cipher;
                printf("%c",c);
            }
        }
        printf("\n");
        return 1;
    }

    else
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }

}