Concerns about '\0' and '\n' inside a loop

Good afternoon fellas.

I'm concerned about the usage of a function I wrote to discover if a string is a number in addition with atoi. I thought I could do this:

Code:

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/* if it reached the null terminator, then all the chars where numbers. */
int isanumber(char* s) {
  int i;
  for(i = 0; isdigit(s[i]) && s[i] != '\0'; i++)
  ;
  if(s[i] == '\0')
    return 1;
  return 0;           
}

---

instead of, the only thing that worked was:

Code:

---


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

#ifndef MAXSIZE
  #define MAXSIZE 100
#endif 

int Atoi(char s[]);
int isanumber(char* s);

int Atoi(char s[])
{
  int i, n, sign;
  for(i = 0; isspace(s[i]); i++)
  ;
  sign = (s[i] == '-') ? -1 : 1;
  if(s[i] == '+' || s[i] == '-')
    i++;
  for(n = 0; isdigit(s[i]); i++)
    n = 10 * n + (s[i] - '0');
  return sign * n;
}

/* if it reached the line feed, then all the chars where numbers. */

int isanumber(char* s) {
  int i;
  for(i = 0; isdigit(s[i]) && s[i] != '\n'; i++)
  ;
  if(s[i] == '\n')
    return 1;
  return 0;           
}   

int main(void)
{
  char* strnum = malloc(MAXSIZE * sizeof(char));
  int num;
 
  do {
  printf("Enter a number: ");
  if(fgets(strnum, MAXSIZE, stdin))
  num = Atoi(strnum);
  } while(!(isanumber(strnum)));
 
  printf("You entered: %d\n", num);
 
  free(strnum);
  return 0;   
}

---

how's that?

What will happen if i input a negative number, for example -5, in your isnumber function?

I wonder if I fixed it ... I tested with a negative number and used valgrind.

Code:

---


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

#ifndef MAXSIZE
  #define MAXSIZE 100
#endif 

int Atoi(char s[]);
int isanumber(char* s);

int Atoi(char s[])
{
  int i, n, sign;
  for(i = 0; isspace(s[i]); i++)
  ;
  sign = (s[i] == '-') ? -1 : 1;
  if(s[i] == '+' || s[i] == '-')
    i++;
  for(n = 0; isdigit(s[i]); i++)
    n = 10 * n + (s[i] - '0');
  return sign * n;
}

/* if it reached the line feed, then all the chars where numbers. */

int isanumber(char* s) {
 
  int i = 0;
 
  while(s[i] != '\n')
  {
    if(isalpha(s[i]) || isblank(s[i]) )
      break;
   
    i++;   
  }       
 
  if(s[i] == '\n')
    return 1;
  return 0;           
}   

int main(void)
{
  char* strnum = malloc(MAXSIZE * sizeof(char));
  int num;
 
  do {
  printf("Enter a number: ");
  if(fgets(strnum, MAXSIZE, stdin))
  num = Atoi(strnum);
  } while(!(isanumber(strnum)));
 
  printf("You entered: %d\n", num);
 
  free(strnum);
  return 0;   
}

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It seems ok.Did you test it and find any problem?

The only thing that comes to mind, is that suppose that a user inputs an integer, he can overflow you.In the limits.h there are defined the maximum and minimum numbers an integer can get.

Quote:

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he can overflow you

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this has been worrying me a bit... do I have to use the constants from limits.h every time I want to protect my program from overflows ?

I did this:

Code:

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#ifndef MAXSIZE
    #define MAXSIZE UCHAR_MAX
#endif

---

is that right?

I don't know how to properly guard that code from overflow.

Quote:

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Originally Posted by thames

this has been worrying me a bit... do I have to use the constants from limits.h every time I want to protect my program from overflows ?

I did this:

Code:

---

#ifndef MAXSIZE
    #define MAXSIZE UCHAR_MAX
#endif

---

is that right?

---

UCHAR_MAX is the maximum value of an unsigned char (usually 255). So why do you want to limit the size of your string to it?

If you are concerned about overflow, just use the standard function strtol() (or strtoll() since C99) which handles overflow when converting numbers (strtol() returns LONG_MAX and sets errno to ERANGE if there is an overflow).

Bye, Andreas

many thanks Andreas

Code:

---


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

#ifndef MAXSIZE
  #define MAXSIZE 1000
#endif

int Atoi(char s[]);
int isanumber(char* s);

int Atoi(char s[])
{
  int i, n, sign;
  for(i = 0; isspace(s[i]); i++)
  ;
  sign = (s[i] == '-') ? -1 : 1;
  if(s[i] == '+' || s[i] == '-')
    i++;
  for(n = 0; isdigit(s[i]); i++)
    n = 10 * n + (s[i] - '0');
  return sign * n;
}

/* if it reached the line feed, then all the chars where numbers. */

int isanumber(char* s) {
 
  int i = 0;
 
  while(s[i] != '\n')
  {
    if(isalpha(s[i]) || isblank(s[i]) )
      break;
   
    i++;   
  }       
 
  if(s[i] == '\n')
    return 1;
  return 0;           
}   

int main(void)
{
  char* strnum = malloc(MAXSIZE * sizeof(char));
  long long int num;
 
  do {
  printf("Enter a number: ");
  if(fgets(strnum, MAXSIZE, stdin))
  num = strtoll(strnum, NULL, 10);
  } while(!(isanumber(strnum)));
 
  printf("You entered: %lld\n", num);
 
  free(strnum);
  return 0;   
}

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Since char may be either signed or unsigned, you should always cast to unsigned char when passing one to the is* and to* functions, like so:

Code:

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isspace((unsigned char) s[i])

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like this ?

edit:

what are the problems a signed char can cause?

Code:

---



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

#ifndef MAXSIZE
  #define MAXSIZE 1000
#endif

int Atoi(char s[]);
int isanumber(char* s);

int Atoi(char s[])
{
  int i, n, sign;
  for(i = 0; isspace(s[i]); i++)
  ;
  sign = (s[i] == '-') ? -1 : 1;
  if(s[i] == '+' || s[i] == '-')
    i++;
  for(n = 0; isdigit(s[i]); i++)
    n = 10 * n + (s[i] - '0');
  return sign * n;
}

/* if it reached the line feed, then all the chars where numbers. */

int isanumber(char* s) {
 
  int i = 0;
 
  while(s[i] != '\n')
  {
    if(isalpha((unsigned char) s[i]) || isblank( (unsigned char) s[i]) )
      break;
   
    i++;   
  }       
 
  if(s[i] == '\n')
    return 1;
  return 0;           
}   

int main(void)
{
  char* strnum = malloc(MAXSIZE * sizeof(char));
  long long int num;
 
  do {
  printf("Enter a number: ");
  if(fgets(strnum, MAXSIZE, stdin))
  num = strtoll(strnum, NULL, 10);
  } while(!(isanumber(strnum)));
 
  printf("You entered: %lld\n", num);
 
  free(strnum);
  return 0;   
}

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Do you have to write your own number checking function?

Here are two examples your function accepts as a number:

Code:

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Enter a number: 42-56
You entered: 42
Enter a number: #42
You entered: 0

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Bye, Andreas

Quote:

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Do you have to write your own number checking function?

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I don't have to, but I was curious about it. Thanks Andreas.