caesar.c

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

int main(int argc, string argv[])
{
    
     //if agrv[1] is not number, atoi(agrv[1]=0; 
    //however, need to add !isdigit(argv[1]), 
    //because Your return 1; will immediately end the program (ie, if the first character is a digit, your program will print success) 
    //so no further characters are checked.
    //The function atoi(const char *) will read characters from a string until it hits a non-digit character. 
    //That's why 20x succeeds, but x20 fails. With 20x it reads 2 then 0 then stops. With x20 it just hits the x and then stops.
    if (argc!=2)
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
//if input equals "./caesar 20x", the output still not equals "Usage: ./caesar key".
//isdigit(*argv[1]) will only determine whether the string argv[1] starts with a digit or not, it doesn't apply to the full string.

    for (int a=0, b=strlen(argv[1]); a<b; a++)
    {
        if(!isdigit(argv[1][a]))
        {
            printf("Usage: ./caesar key\n");
            return 1;
        }
    }    
    string p = get_string("plaintext: ");
    printf("ciphertext: ");
    int k = atoi(argv[1]);
    //int k = atoi(argv[1]) can not be in the top;
    //the mistakes is in the above assignment, if the there is no argv[1] so the OS will report error and terminates the program, 
    //so you have not to do k assignment to argv[1] unless you make sure there is argv[1].
    int m = k%26;
    for (int i=0, j=strlen(p); i<j; i++)
    {
        if (p[i]<='Z'&&p[i]>='A')
        {
            if (p[i]+m<='Z')
            {
                printf("%c", p[i]+m);
            }
            else
            {
                printf("%c", p[i]+m-26);
            }
        }
        else if (p[i]<='z'&&p[i]>='a')
        {
            if (p[i]+m<='z')
            {
                printf("%c", p[i]+m);
            }
            else
            {
                printf("%c", p[i]+m-26);
            }
        }
        else
        {
            printf("%c", p[i]);
        }
    }
    printf("\n");
    return 0;
}

//segmentation fault reason:
//a segmentation fault happens when you touch a memory location that you shouldn't touch.
//more likely is that you're passing a string instead of a char to a function that accepts a char (e.g., isalpha())