Thread: Validating an input

I want to accept an input which is a string, and from there, verify if the entry is an integer.

To do this, probably I will have to divide the string and evaluate whether each character is within the ascii range for numbers, can anybody help me by giving me ideas on how i can achieve this, and is this possible? crashproof?

how about looking up the function atoi ?
that might be helpful

>is this possible?
Certainly.

>crashproof?
More difficult. This is a good start (not really elegant, but it works):

Code:

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

static size_t numberOfDigits(int val)
{
    size_t n = 0;

    while (val)
    {
        ++n;
        val /= 10;
    }

    return n;
}

static int validNumber(char *s)
{
    size_t len = strlen(s);

    if (len > numberOfDigits(INT_MAX))
        return 0;
    
    while (*s)
    {
        if (!isdigit(*s++))
            return 0;
    }

    return 1;
}

int main(void)
{
    char buff[BUFSIZ];
    int  num;

    printf("Enter a number: ");
    fflush(stdout);

    if (fgets(buff, sizeof (buff), stdin))
    {
        char *newline = strrchr(buff, '\n');

        if (newline)
            *newline = '\0';

        if (validNumber(buff))
        {
            num = (int)strtol(buff, 0, 0);
            printf("Your number (%d) was valid\n", num);
        }
    }

    return 0;
}

Another possibility is to ignore prevalidation and use the return codes from strtol to get your bulletproof conversion (code "borrowed" from Jack Klein ):

Code:

/* file usestrtol.c
*
*   15-Jul-2002
*   (C) 1999 by Jack Klein
*   All rights reserved
*
*   License granted for free non-commercial use
*
*   Description:
*
*   this program demonstrates the use of the standard
*   library function strtol() prototpyed in <stdlib.h>
*   to obtain and validate a signed integer value from the
*   standard input stream
*
*   C++ notes:
*
*   On newer, ANSI/ISO Standard Conforming C++ compilers,
*   the function strtol() is prototpyed in <cstdlib> and
*   is in the std namespace
*
*   The strtol() function operates with an array of constant
*   characters as its source which could be read with the
*   cin.getline() member function instead of the C library
*   fgets() function
*
*   It can even be called with the pointer returned by the
*   c_str() member function for a std::string
*/

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

int main(void)
{
    char buff [25];
    char *end_ptr;
    long long_var;
    int int_var;
    for ( ; ; )
    {
        printf("Enter an int, return only to quit: ");
        fflush(stdout);
        fgets(buff, sizeof buff, stdin);
        
        if (buff [0] == '\n')
        {
            break;
        }
        
        errno = 0;
        
        long_var = strtol(buff, &end_ptr, 0);
        
        if (ERANGE == errno)
        {
            puts("number out of range\n");
        }
        else if (long_var > INT_MAX)
        {
            printf("%ld too large!\n", long_var);
        }
        else if (long_var < INT_MIN)
        {
            printf("%ld too small!\n", long_var);
        }
        else if (end_ptr == buff)
        {
            printf("not valid numeric input\n");
        }

        
        /* here is where you might want to add code to  */
        /* test for extra non-numeric characters at the */
        /* end of the input string, if you want that to */
        /* be an error                                  */
        /*                                              */
        /* if you want to generate this error, remove   */
        /* the two preprocessor lines that start with   */
        /* #if 0 and #endif                             */
        
        #if 0
        else if ('\0' != *end_ptr)
        {
             printf("extra characters on input line\n");
        }
        #endif
        
        else
        {
            int_var = (int)long_var;
            printf("The number %d is OK!\n", int_var);
        }
    }
    return 0;
}

>how about looking up the function atoi ?
atoi is far from crashproof. It results in undefined behavior if anything untoward happens, better to use strtol and cast to an int.

Here's another shot at it. I didn't see that it had to be an integer (so it checks for real numbers, though thats not hard to modify). It also checks for a prepending +/- on the number. Also, not exactly elegant, but a start.

Code:

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

int main()
{
   char str[100];
   int is_number = 1;
   int decimal = 0;
   char* s;

   fgets(str, sizeof str, stdin);

   /* If the first character is not a digit, a +, or a
 -, then its not a number (continue no more). 
Actually, for real numbers, the first character 
could be a '.' as well, but I forgot about that. */
   if(!isdigit(str[0]) && str[0] != '-' && str[0] != '+')
   {
     is_number = 0;
   }
   else
   {
      /* Loop over the string until hitting a NULL terminator. */
     for(s = &str[1]; *s != '\0'; ++s)
     {
         /* If it isn't a digit, and it isn't a decimal 
(note, if it is a decimal, but it is the second 
one it fails as well), then its not a number, 
and break the loop */
       if(!isdigit(*s) &&
	  (*s == '.' && decimal != 0))
       {
	 is_number = 0;
	 break;
       }

         /* If it is a decimal, and it is the first one,
 then its okay, but register the decimal, and 
continue. */
       if(*s == '.')
       {
	 decimal = 1;
       }
     }
   }

   /* A result of 0 states that it was not a 
number, a result of 1 states that it was. The 
atoi is for comparison to that function 
(seeing what it outputs if you did not enter a 
number). */
   printf("Result: %d\n%d", is_number, atoi(str));

   return 0;
}

i see, thank you all so much, but could you help me by inserting some comments, i understand the syntax of the code, but i am having trouble with identifying the purpose of certain segments of code, the purpose of some lines...

How about a simple version, very similar to Zach's:

Code:

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#define MAXBUF 20    /* define maximum input size */
int main()
{
    char buf[MAXBUF];  /* input buffer */
    char *p;           /* temp buffer pointer */
    int    err = 0;    /* start with err = false */

    printf("Enter: ");
    fgets(buf, MAXBUF, stdin);  /* read a line */
    p = buf;                /* set the temp pointer */
    while (*p)              /* end when *p is '\0' */
    {
        if (!isdigit(*p))   /* is the character not a digit? */
        {                   /* no it's not -- set err to true */
            err = 1;
             break;         /* exit the loop */
        }
         p++;            /* next character */
    }

/* test err for 0/1 or false/true here */
}

i see, thanx! will try that out, looks easier to understand for me...

also, how can i detect if the user has pressed the esc button?

also, i tried using ur code WaltP, the 'isdigit' couldn't detect watever integer i entered as in put...is there anything wrong with it?

Originally posted by fkheng
also, i tried using ur code WaltP, the 'isdigit' couldn't detect watever integer i entered as in put...is there anything wrong with it?

isdigit() only returns TRUE if the character passed is '0' - '9'. So my routine simply tells you whether the input is a number or not. any non-digit character will set err to 1 (true).

If it is a number you will still have to convert the string into a number yourself.

OOPS Warning...

I forgot the following statement after the fputs():
buf[strlen(buf)-1] = '\0';

That will remove the \n from the buffer.

er...i don't see an fputs anywhere in the code, hehe...

also, where should i convert the string to integer?
coz the while loop needs the string in order to loop thru each character, so i can't convert it to integer at that point yet...

any ideas?

Originally posted by fkheng
er...i don't see an fputs anywhere in the code, hehe...

Sorry, I mistyped. fgets(), hoho

also, where should i convert the string to integer?
coz the while loop needs the string in order to loop thru each character, so i can't convert it to integer at that point yet...

I'm not going to write the whole thing for you! Think about it.

okay, but despite adding the clearing buffer line, my getchar() statement sitll seesm to leave an extra line and skip some other getchar statement of the loop...

how can i get aroun dthis?

Originally posted by fkheng
okay, but despite adding the clearing buffer line, my getchar() statement sitll seesm to leave an extra line and skip some other getchar statement of the loop...

how can i get aroun dthis?

Well you could actually post your code so we could tell you what's wrong. Because in this entire thread you have yet to post it a single time.

Quzah.