String terminator in character array in C

Question

I'm trying to make a program to decide the validity of a password, based on a set of rules.

Here is what I have:

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

int main()
{
  int uppercase = 0;
  int length = 0;
  int numbers = 0;
  int others = 0;
  char password[13];
  char yesOrNo;
  printf("Your password must be 8-12 characters long.\n"
         "It must contain at least one symbol, uppercase letter, and number.\n\n");
 COMEBACK:
  printf("Please enter your password:");
  scanf(" %s", &password);
  while (password != 'NULL') { // Tried 0 here, tried '\0', but to no avail.
    if (isalpha(password)) {
      length += 1;
      if (isupper(password)) {
        uppercase += 1;
      }
    }
    else if (isdigit(password)) {
      numbers += 1;
      length += 1;
    }
    else {
      length += 1;
    }
    // This is just a test, to see if it was working.
    printf("%d - %d - %d - %d --- %s",
           uppercase, length, numbers, others, password);
  }
  if ((uppercase > 0) && (numbers > 0)
      && (length >= 8) && (length <= 12) && (others > 0)) {
    printf("Good job, you've done your password correctly.");
  } else {
    printf("%d - %d - %d - %d --- %s \t Incorrect..",
           uppercase, length, numbers, others, password); // Same thing here.
    scanf("%s", &yesOrNo);
    switch (yesOrNo) {
    case 'y':
      goto COMEBACK;
      break;
    case 'n':
      printf("Sorry you're dumb man..");
      break;
    default:
      printf("Please enter a valid password.");
    }
  }
  return 0;
}

The problem I am having is, the while loop never ends because it can't seem to find the terminator for my password array. I've inputted '\0' as well as just '0'. But I still can't figure it out. Any help is appreciated. Thanks.

Answer 1

This code:

while (password != 'NULL') { 

should be generating compiler warnings galore. The multicharacter literal is non-portable and should not be compared with a pointer.

You might need:

char *ptr = password;
while (*ptr != '\0') {
    ...
    ptr++;
}

or (C99 or later):

for (char *ptr = password; *ptr != '\0'; ptr++)
{
    ...
}

and use *ptr to identify the character (or, often, (unsigned char)*ptr since plain char is often signed and isalpha() et al require a positive value or EOF as the input value). If you don't have C99, you can declare char *ptr; outside the loop and remove the char * inside the loop control.

You have:

if (isalpha(password)) {

Since password is an array, you're passing a fixed pointer to a function that requires a non-pointer (int) value. I'd probably add inside the loop:

{
    unsigned char uc = *ptr;
    if (isalpha(uc))
        ...

Note that you probably only need one length++; for all the cases.

Also note that no password will ever satisfy the 'at least one symbol' criterion because you never increment others.

And the goto can be replaced by a while() loop which can detect EOF as well:

while (scanf("%12s", password) == 1)
{
    length = others = uppercase = numbers = 0;  // Ignore previous attempts
    for (char *ptr = password; *ptr != '\0'; ptr++)
    {
        unsigned char uc = *ptr;
        length++;
        ...character testing code...
    }
    ...validity checking code...
}

---

While you're learning C, assume that the compiler warnings are solid errors. It knows more about C than you do at this stage.

Answer 2

password is an array. It will never be NULL. But it can contain a null-terminator, which is probably what you're after. Just check if password[0] == '\0'.