ex_4-2.c

   1 #include <stdio.h>
   2 #include <ctype.h>
   3
   4 /*
   5
   6 Converts string of char into a double.
   7 Supports scientific notation, signed exponents.
   8
   9 Warning: very large or small numbers exibit rounding errors.
  10
  11 */
  12
  13 double atof(char s[])
  14 {
  15         long double val, power, expower;
  16         int i, sign, exsign;
  17         for(i=0; isspace(s[i]); i++) // skip spaces
  18                 ;
  19
  20         sign = (s[i] == '-') ? -1 : 1; // mark sign for number
  21         while(!isdigit(s[i])) // skip over signs and non numbers, dollar signs
  22                 i++;
  23
  24         val = 0.0;
  25         while(isdigit(s[i])) // calc val of nums before decimal
  26         {
  27                 val = 10.0 * val + (s[i] - '0');
  28                 i++;
  29         }
  30
  31         if(s[i] == '.') // skip over decimals
  32                 i++;
  33
  34         power = 1.0;
  35         while(isdigit(s[i])) // calc val of nums after decimal
  36         {
  37                 val = 10.0 * val + (s[i] - '0');
  38                 power *= 10.0;
  39                 i++;
  40         }
  41
  42         val = sign * val / power;
  43
  44         // check for exponent
  45         if(s[i] == 'e' || s[i] == 'E')
  46         {
  47                 ++i; // move past e or E
  48                 exsign = (s[i] == '-') ? -1 : 1; // mark sign for number
  49                 while(!isdigit(s[i])) // skip over sign(s)
  50                         i++;
  51
  52                 expower = 0.0;
  53                 while(isdigit(s[i]))
  54                 {
  55                         expower = 10.0 * expower + (s[i] - '0');
  56                         ++i;
  57                 }
  58         }
  59
  60         /* cannot get pow() to work in math.h ... improvising :) */
  61         if(exsign == 1)
  62         {
  63                 while(expower > 0) { val = val * 10; --expower; }
  64         } else {
  65                 while(expower > 0) { val = val / 10; --expower; }
  66         }
  67         return val;
  68 }
  69
  70 int main()
  71 {
  72         char s[] = "-123.456e-7";
  73         double ans = atof(s);
  74         printf("%f\n", ans);
  75
  76         char r[] = "123.45e-6";
  77         ans = atof(r);
  78         printf("%f\n", ans);
  79
  80         char t[] = "123.45E6";
  81         ans = atof(t);
  82         printf("%f\n", ans);
  83
  84         char u[] = "-123.45e56";
  85         ans = atof(u);
  86         printf("%f\n", ans);
  87         return 0;
  88 }