Extend atof to handle scientific notation of the form 123.45e-6

#include <ctype.h>

/*atof: convert string s to double */
double atof(char s[]){
     double val, power;
     int exp, 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;
     }
      val = sign * val / power;
    if (s[i] == 'e' || s[i] == 'E'){
        sign = (s[++i] == '-') ? -1 : 1;
        if (s[i] =='+' || s[i] == '-')
             i++;
        for(exp = 0; isdigit(s[i]); i++)
            exp = 10 * exp + (s[i] - '0');
        if (sign == 1)
            while (exp-- > 0)   /* positive exponent */
                  val *= 10;
       else
           while (exp-- > 0)  /* negative exponent   */
                val /= 10;
    }
     return val;
}