ASCII to Float in C

atof.c

/* Onlu needed for printf() */
#include <stdio.h>

/* Boolean types - comment if not needed */
typedef int bool;
#define true 1
#define false 0
/* !Boolean types */

/* There are also built-in functions for these */
bool isdigit(char c) {
   return c >= '0' && c <= '9';
}

bool isspace(char c) {
   return c == ' ';
}

/* Definition of atof */
double atof(char s[]) {
    double val, power;
    int i, sign;

    // Skip initial whitespace
    for (i = 0; isspace(s[i]); i++);

    // Look for sign prefix
    sign = (s[i] == '-') ? -1 : 1;
    if (s[i] == '+' || s[i] == '-') ++i;

    // Compute the value for all digits before the point
    for (val = 0.0; isdigit(s[i]); ++i) {
        val = 10.0 * val + (s[i] - '0');
    }

    // Compute the fractional power for all digits after the point
    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));
}

int main(int argc, char** argv) {
  double res;

  res = atof("0.123456");
  printf("0.123456 is now %f\n", res);

  res = atof("-123.234534");
  printf("-123.234534 is now %f\n", res);

  res = atof("19293949");
  printf("19293949 is now %f\n", res);

  res = atof("1233333333.1");
  printf("1233333333.1 is now %f\n", res);

  // Rounding error!
  res = atof("-0.12399999");
  printf("-0.12399999 is now %f\n", res);

  return 0;
}