Exercise 4-2. Extend atof to handle scientific notation of the form 123.45e-6

Exercise 4-2. Extend atof to handle scientific notation of the form 123.45e-6 where a floating-point number may be followed by e or E and an optionally signed exponent.

#include <stdio.h>

#include <ctype.h>

#define MAXLINE 100

/*rudimentary calculator*/

int main()

{

    double atof(char[]);

    char line[MAXLINE];

    int getlin(char line[], int max);

    while(getlin(line, MAXLINE) > 0)

        printf("\t%g\n", atof(line));

    return 0;

}

/*atof: convert string s to double*/

double atof(char s[])

{

    double val, power;

    int i, sign, exp;

    

    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')

    {

        i++;

        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)

            for(; exp > 0; exp--)

                val *=10;

        else

            for(; exp > 0; exp--)

                val /= 10;

    }

    return val;

}

/* getlin */

int getlin(char line[], int max)

{

    int c, i;

    

    i = 0;

    while(--max > 0 && (c=getchar()) != EOF && c != '\n')

        line[i++] = c;

    if(c == '\n')

        line[i++] = c;

    line[i] = '\0';

    return i;

}

Result: