S10 Answer

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

double stringToDouble( char *p )
{
  double intPart=0.0, fractionPart=0.0;
  int multiplier = 1;  /* change to -1 if string starts with minus */

  /* Find the sign for the entire number */
  if ( *p == '+' ) p++;
  if ( *p == '-' )
  {
    multiplier = -1;
    p++ ;
  }

  /* Convert the integer part of the number */
  while ( *p >= '0' && *p <= '9' )
  {
    intPart *= 10;
    intPart += *p - '0';
    p++ ;
  }

  /* Convert the fractional part of the number */
  if ( *p == '.' )
  {
    double divisor = 1.0;
    p++ ;
    while ( *p >= '0' && *p <= '9' )
    {
      divisor *= 10.0;
      fractionPart += (*p - '0')/divisor;
      p++ ;
    }
  }

  /* Put the pieces together */
  return  (intPart + fractionPart)*multiplier ;
}

int main(int argc, char *argv[])
{
  char *trials[] =
  {
   "1",
   "1.2",
   "+1",
   "-8.000",
   "+2003.99",
   "-3.45",
   "98.76  ",
   "12..5",
   "rats",
   "+95.M",
   "+ 234",
   ".999",
   "+1.345E+005"  /* Our function will not work for this */
  };

  int j ;
  for ( j=0; j<sizeof(trials)/sizeof(char *); j++ )
  {
    printf("%s is converted into: %lf\t\tshould be: %lf\n", trials[j],
         stringToDouble( trials[j]), atof( trials[j]) );
  }

  system("PAUSE");
  return 0;
}

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