Read a Floating-Point Value from the User

[dcs]

Read a Floating-Point Value 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("%lf", &n), but not bulletproof. It is meant to be a simple but relatively safe demonstration. Note also that there would be slight differences for using float instead of double.

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

#include <stdio.h>

int mygetd(double *result)
{
   char buff [ 32 ]; /* modify array size to suit your needs */
   return fgets(buff, sizeof buff, stdin) && sscanf(buff, "%lf", result) == 1;
}

int main(void)
{
   double value;
   do {
      fputs("Enter a floating-point number: ", stdout);
      fflush(stdout);
   } while ( !mygetd(&value) );
   printf("value = %g\n", value);
   return 0;
}

/* my output
Enter a floating-point number: one
Enter a floating-point number: 
Enter a floating-point number: f12.3
Enter a floating-point number: -45.67
value = -45.67

Enter a floating-point number: -12.3f
value = -12.3

Enter a floating-point number:    125 help
value = 125
*/

Leading whitespace, and other trailing characters are some things not handled. Those issues are handled in Read a Floating-Point Value from the User, Part 2 and Read a Floating-Point Value from the User, Part 3.

[dcs]

Read a Floating-Point Value 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 a Floating-Point Value from the User, Part 1. Here such issues receive lip service. See also Read a Floating-Point Value from the User, Part 3.

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

int mygetd(double *result)
{
   char c, buff [ 32 ];
   return fgets(buff, sizeof buff, stdin) && !isspace(*buff) &&
   sscanf(buff, "%lf%c", result, &c) == 2 && (c == '\n' || c == '\0');
}

int main(void)
{
   double value;
   do
   {
      fputs("Enter a floating-point number: ", stdout);
      fflush(stdout);
   } while ( !mygetd(&value) );
   printf("value = %g\n", value);
   return 0;
}

/* my output
Enter a floating-point number: one
Enter a floating-point number:
Enter a floating-point number: f12.3
Enter a floating-point number: -45.67
value = -45.67

Enter a floating-point number: -12.3f
Enter a floating-point number:    125 help
Enter a floating-point number: 1.2.3
Enter a floating-point number: 1.23
value = 1.23
*/

[dcs]

Read a Floating-Point Value from the User, Part 3

This snippet uses strtod to be a little more stringent.

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

int mygetd(double *result)
{
   char *end, buff [ 32 ];
   return fgets(buff, sizeof buff, stdin) && !isspace(*buff) &&
          (*result = strtod(buff, &end)), ( *end == '\n' || *end == '\0' );
}

int main(void)
{
   double value = -12.3;
   do
   {
      fputs("Enter a floating-point number: ", stdout);
      fflush(stdout);
   } while ( !mygetd(&value) );
   printf("value = %g\n", value);
   return 0;
}

/* my output
Enter a floating-point number: one
Enter a floating-point number:
Enter a floating-point number: f12.3
Enter a floating-point number: -45.67
value = -45.67
 
Enter a floating-point number: -12.3f
Enter a floating-point number:    125 help
Enter a floating-point number: 1.2.3
Enter a floating-point number: 1.23
value = 1.23
*/