c - Program keeps returning Segmentation Fault -

i'm new person loves play around coding. going through course on edx, , 1 of exercises need complete has small code snippet keeps on giving segmentation fault. have taken out faulty bit (everything else compiles nicely)

#include <stdio.h> #include <string.h> #include <cs50.h> #include <ctype.h> #include <stdlib.h>  int main (int argc, string argv[]) {     if (argc == 2 && isalpha(argv[1]))     {         int = 0;          while (argv[1][a] == '\0')         {             a++;             printf("%c\n", argv[1][a]);         }     }     else     {         printf("usage: ./programname 1-alphabetical word\n");         return 1;     } } 

the problem seems here: argv[1][a] can't life of me find out what, , how fix it.

(1) isalpha(argv[1]) wrong. function expects single character, passing pointer-to-string. not give kind of expected result, , it's undefined behaviour bargain. either need loop , check each character, use more high-level library function check entire string, or - quick , sense-changing fix - check 1st character bluepixy suggested: isalpha( argv[1][0] ) or isalpha( *argv[0] ).

(2) while condition wrong. telling loop while current character nul. nothing non-empty strings , hit next problem #3 empty one. presumably meant while (argv[1][a] != '\0'), i.e. loop until nul byte reached.

(3) increment index a before trying printf() it. index out of range right now, if input string empty, body executes , index beyond terminating nul. if loop condition fixed, miss 1st character , print terminating nul, neither of make sense. should increment a once have verified in-range , done need it. so, printf() it, then increment it.

2 , 3 seem soluble using for loop instead of manually splitting initialisation, testing, , incrementing of loop variable. should use correct type indexing: in case wanted print string millions or billions of characters, int not wide enough, , it's not style. so:

#include <stddef.h> /* size_t */  (size_t = 0; argv[1][a] != '\0'; ++a) {     printf("%c\n", argv[1][a]); }