atof.c

//* gcc -o output atof.c && output

#include <ctype.h>
#include <stdio.h>

#define MAXLINE 100

double atof(char s[]);

int main(void) {
    // rudimentary calculator:
    double sum, atof(char s[]);
    char line[MAXLINE];
    int getline(char line[], int max);

    sum = 0;
    while (getline(line, MAXLINE) > 0) {
        printf("\t%g\n", sum += atof(line));
    }

    return 0;
}

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;
}