atof:convert string to double

#include<ctype.h>

/*atof: convert string s to double */
double atof(char s[]){
      double val,power;
      int i, sign;

      for(i=0;isspace(s[i]);i++) /*skip white space */
           ;
      sign = (s[i] == '-') ? -1 : 1;
      if (s[i] == '+' || s[i] == '-')
          i++;
      for (val = 0.0; isdigit(s[i]);i++)
          val = 10.0 *val + (s[i] - '0');
      if (s[i] == '.')
          i++;
      for(power = 1.0; isdigit(s[i]);i++){
          val = 10.0 * val + (s[i] - '0');
          power *= 10.0;
      }
      return sign *val/power;
}