Read an Integer from the User

[dcs]

Read an Integer from the User, Part 1

Obtaining user input can be done in many surprisingly different ways. This code is somewhere in the middle: safer than scanf("%d", &n), but not bulletproof. It is meant to be a simple but relatively safe demonstration.

The function mygeti reads user input from the stdin into a string using fgets. Then it attempts to convert this string to an integer using sscanf. If both functions succeed, a 1 is returned; if either fails, a 0 is returned.

#include <stdio.h>

int mygeti(int *result)
{
        char buff [ 13 ]; /* signed 32-bit value, extra room for '\n' and '\0' */
        return fgets(buff, sizeof buff, stdin) && sscanf(buff, "%d", result) == 1;
}

int main(void)
{
        int value;
        do {
                fputs("Enter an integer: ", stdout);
                fflush(stdout);
        } while ( !mygeti(&value) );
        printf("value = %d\n", value);
        return 0;
}

/* my output
Enter an integer: one
Enter an integer:
Enter an integer: 42
value = 42

Enter an integer: 24f
value = 24

Enter an integer:    125 help
value = 125
*/

Leading whitespace, and other trailing characters are some things not handled. Those issues are handled in Read an Integer from the User, Part 2. Reading a Floating-Point Value from the User can be done similarly.

[dcs]

Read an Integer from the User, Part 2

Some issues, such as leading whitespace and trailing characters that cannot be part of a number, were not handled in Read an Integer from the User, Part 1. Here such issues receive lip service.

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

int mygeti(int *result)
{
        char c, buff [ 13 ]; /* signed 32-bit value, extra room for '\n' and '\0' */
        return fgets(buff, sizeof buff, stdin) && !isspace(*buff) &&
        sscanf(buff, "%d%c", result, &c) == 2 && (c == '\n' || c == '\0');
}

int main(void)
{
        int value;
        do {
                fputs("Enter an integer: ", stdout);
                fflush(stdout);
        } while ( !mygeti(&value) );
        printf("value = %d\n", value);
        return 0;
}

/* my output
Enter an integer: one
Enter an integer:
Enter an integer: f123
Enter an integer: 123f
Enter an integer:  123
Enter an integer: 123
Enter an integer: 1.23
Enter an integer: -42
value = -42
*/

/* note: this line in the above has a space character following the 123
Enter an integer: 123
*/

[dcs]

Read an Integer from the User, Part 3

This is a strtol version of Read an Integer from the User, Part 2.

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

int mygeti(int *result)
{
   char *end, buff [ 13 ];
   fgets(buff, sizeof buff, stdin);
   *result = strtol(buff, &end, 10);
   return !isspace(*buff) && end != buff && (*end == '\n' || *end == '\0');
}

int main(void)
{
   int value;
   do
   {
      fputs("Enter an integer: ", stdout);
      fflush(stdout);
   } while ( !mygeti(&value) );
   printf("value = %d\n", value);
   return 0;
}

/* my output
Enter an integer: one
Enter an integer:
Enter an integer: f123
Enter an integer: 123f
Enter an integer:  123
Enter an integer: 123
Enter an integer: 1.23
Enter an integer: -42
value = -42
*/