Why isn't isalpha working?

Question

I'm working with C and I need to check that the user inputed second command line argument argv[1] is made up of only alphabetical charchaters and if not, to do what is inside the else loop. I used the is alpha function but when i compile and run the program no matter what my second command line argument is (alphabetical or otherwise), its always executing the "else loop". How do i fix this?

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

int main(int argc, string argv[])
{
  int a  = argc;    

  if (a != 2)
  {
    return 1;    
  }

  string b = argv [1]; 
  int c = strlen(b);
  string m;

  for (int i = 0; i < c; i++)
  {
    if (isalpha(b[c]))
    { 
      m = GetString();    
    }
    else
    {
      printf("Please provide a valid keyword\n");
      return 1;
    }         
  }
}

In the world of CS50, the `` header contains `typedef char *string;` — a design that is questionable, but that's the way the course defines it. — Jonathan Leffler, Aug 07 '16 at 20:11

score 10 · Answer 1 · edited May 23 '17 at 12:01

10

Try replacing

if (isalpha(b[c]))

with

if (isalpha(b[i]))

Currently you are checking the element at the index which is the result of strlen(b) at every iteration of your loop. Because array indices are zero based in C b[strlen(b)] is referencing '\0', the null terminator.

In reference to the Keith Thompson comment below and the answer to this question you should actually be casting the value passed to isalpha to an unsigned char to ensure that undefined behaviour is not invoked.

Thus you should change your code to

if (isalpha((unsigned char)b[i]))

to ensure there is no UB

edited May 23 '17 at 12:01

Community

1
1

answered Jul 25 '15 at 23:08

mathematician1975

21,161
6
59
101

8

`isalpha()` doesn't simply take a `char` argument. It takes an `int` argument whose value is converted from `unsigned char`. If plain `char` is signed (as it commonly is), then `isalpha(c)` has undefined behavior if `c < 0` (and `c != EOF`). Change `isalpha(b[i])` to `isalpha((unsigned char)b[i])`. (This is almost certainly not the cause of the OP's problem.) – Keith Thompson Jul 25 '15 at 23:16
Thanks, that helped :) – TeachMe Jul 25 '15 at 23:21
No you haven't. It should be `isalpha((unsigned char)b[i])`. – user207421 Jul 25 '15 at 23:38

score 0 · Answer 2 · edited Aug 07 '16 at 20:12

0

Use isalpha(b[i]) instead of isalpha(b[c]) like this:

if (isalpha(b[i]))
{ 
  m = GetString();    
}

edited Aug 07 '16 at 20:12

Jonathan Leffler

730,956
141
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1,278

answered Jul 26 '15 at 12:12

RatikBhat

61
11

Why isn't isalpha working?

2 Answers2