ex_4-2.c

   1 #include <stdio.h>
   2 #include <ctype.h>
   3 /*
   4
   5 Add support for scientific notation.
   6
   7 123.45e-6
   8 123.45E6
   9 -123.45e56
  10
  11 */
  12
  13 double atof(char s[])
  14 {
  15         double val, power;
  16         int i, sign;
  17         for(i=0; isspace(s[i]); i++) /* skip spaces */
  18                 ;
  19
  20         sign = (s[i] == '-') ? -1 : 1; // mark sign for number
  21         if(s[i] == '+' || s[i] == '-') // skip over signs
  22                 i++;
  23         for(val = 0.0; isdigit(s[i]); i++)
  24                 val = 10.0 * val + (s[i] - '0');
  25         if(s[i] == '.') // skip over decimals
  26                 i++;
  27
  28         for(power = 1.0; isdigit(s[i]); i++)
  29         {
  30                 val = 10.0 * val + (s[i] - '0');
  31                 power *= 10.0;
  32         }
  33         return sign * val / power;
  34 }
  35
  36 int main()
  37 {
  38         char s[] = "-10928473.25";
  39         double ans = atof(s);
  40         printf("%f\n", ans);
  41         return 0;
  42 }