Thread: checks for bad input

well, there are certain levels of verification - a weak version might be something like:

Code:

int isInteger(const char * inp) {
 int i, j, found = FALSE;
 const char * comp = "0123456789";
 for(i = 0; inp[i] != '\0'; ++i) {
 found = FALSE;
  for(j = 0; comp[j] != '\0'; ++j) {
   if(inp[i] == comp[j]) {
    found = TRUE;
    break;
    } 
   }
  if(found == FALSE) {
   break;
   } 
  }  
 return found;
 }

obviously, a really robust one would account for valid numerical limits, etc.

The standard C Library has some useful functions, see ctype.h

For example:

#include <ctype.h>
int isdigit(int c);

Returns 1 if c is in the set '0' - '9'. Otherwise it returns 0.

URL ref.:
http://www.infosys.utas.edu.au/info/...b.html#ctype.h

If you don't want to use ctype.h's isdigit() you could do:

Code:

int isInteger(const char *inp)
{
  int i;

  for(i = 0;inp[i];i++)
    if(inp[i] <= '0' || inp[i] >= '9')
      break;
  return !inp[i];
}

Of course...it would be better just #include <ctype.h> and then replace if(inp[i] <= '0' || inp[i] >= '9') with if(isdigit(inp[i]))

EDIT: Beaten to the punch on isdigit(). Now I know how quzah feels.

>Of course...it would be better just #include <ctype.h> and then replace
Of course, it would be even better to go all out and perform a stronger check:

Code:

#include <errno.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int is_integer ( const char *s )
{
  char *end;
  long result;
  
  errno = 0;
  result = strtol ( s, &end, 0 );

  /* Incomplete conversion */
  if ( *s != '\0' && *end != '\0' )
    return 0;
  /* Overflow/Underflow */
  if ( ( result == LONG_MAX || result == LONG_MIN ) && errno == ERANGE )
    return 0;
  /* Out of range */
  if ( result > INT_MAX || INT_MIN > result )
    return 0;

  return 1;
}

int main ( void )
{
  char buffer[BUFSIZ];

  while ( fgets ( buffer, sizeof buffer, stdin ) != NULL ) {
    buffer[strcspn ( buffer, "\n" )] = '\0';
    printf ( "%s -- %d\n", buffer, is_integer ( buffer ) );
  }

  return 0;
}

Originally Posted by Prelude

>Of course...it would be better just #include <ctype.h> and then replace
Of course, it would be even better to go all out and perform a stronger check:

That's true, but the OP didn't mention anything about converting the string to an integer, just to check if the string was all digits. That's why I didn't check limits, etc. It might even be prohibitive to what he's trying to do.

Thanks guys a lot! I am working on these now. I really appreciate. I am a beginner, so it will take some time

>just to check if the string was all digits
Well, technically the question was how to check if a string represents a numerical value. In that case even my stronger check isn't sufficient because floating-point values are numeric and can contain characters other than digits and signs (which I might add, you neglected to check for ). There is also the issue of formatting. -1234 is a valid number but 123-4 is not unless - is being used as a decimal marker. So just because the string contains the requisite characters doesn't mean that it's a valid number. Checking limits was just an after thought because all examples prior had implemented an is_integer rather than is_numeric. The key point is that strtol handles all of the nasty format checking.