INFO: Workaround for Converting a Float/Double to a String (57244)

SUMMARY

The gcvt() function returns an exponential number in the string even if the number fits in the specified precision when the number is of the form 0.0x, where x is any digit(s). Because of this behavior with the gcvt() function, floating-point numbers cannot be converted to a string. Another function that converts a floating-point number to a string is fcvt(). Unfortunately, fcvt() does not do all the conversion for you because it leaves out both the decimal point and the sign of the number.

You can also use the sprintf() or printf() function with the "%lf" format specifier to obtain the correct results. However, if you do not want to use any printf() constructs, supporting code is needed to completely convert the floating-point number to a string. The following program shows one possible way this can be done, and the printf() statements can be replaced by puts() statements:

Sample Code

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>

#define PRECISION   8

char *double_to_char (double) ;

char *temp2 ;

void main (void)
{
   char buf[32];

   temp2 = double_to_char ((double) 0.0004567891) ;
   printf ("temp = %s\n", temp2) ;
   free (temp2) ;

   gcvt ((double) 0.0004567891, PRECISION, buf) ;
   printf ("temp = %s\n", buf) ;

   temp2 = double_to_char ((double) 123.564) ;
   printf ("temp = %s\n", temp2) ;
   free (temp2) ;

   temp2 = double_to_char ((double) -43.7864383846738) ;
   printf ("temp = %s", temp2) ;
   free (temp2) ;
}


/*  Translates a double to an ASCIIZ string. */ 

char *double_to_char (double number)
{
   char *buffer,
        *temp ;

   int  decimal_spot,
        sign,
        count,
        current_location = 0 ;

   temp = fcvt (number, PRECISION, &decimal_spot, &sign) ;

   if (strlen (temp) > PRECISION)
      buffer = (char *) malloc (strlen (temp) + 3) ;
   else
      buffer = (char *) malloc (PRECISION + 3) ;

   if (buffer == NULL)
   {
      printf ("Memory allocating attempt has failed in"
              "'double_to_char'\n") ;
      exit (-1) ;
   }

/* Add negative sign if required. */ 

   if (sign)
      buffer [current_location++] = '-' ;

/* Place decimal point in the correct location. */ 

   if (decimal_spot > 0)
   {
      strncpy (&buffer [current_location], temp, decimal_spot) ;
      buffer [decimal_spot + current_location] = '.' ;
      strcpy (&buffer [decimal_spot + current_location + 1],
                      &temp [decimal_spot]) ;
   }
   else
   {
      buffer [current_location] = '.' ;
      for(count = current_location;
             count<abs(decimal_spot)+current_location; count++)
         buffer [count + 1] = '0' ;
      strcpy (&buffer [count + 1], temp) ;
   }

   return (buffer) ;
}