1 /*
2  * Exercise 4-2. Extend atof to handle scientific notation of the form
3  *
4  * 123.45e-6
5  *
6  * where a floating point number may be followed by e or E and an optionally
7  * signed exponent.
8  */
9 
10 #include <stdio.h>
11 #include <ctype.h>
12 #include <math.h>
13 
14 #define MAXLINE 100
15 
16 /* rudimentary calculator */
17 
18 double pow(double x, double y);
19 
20 int main() {
21 	double atof(char []);
22 	char line[MAXLINE];
23 	int getlin(char line[], int max);
24 
25 	while (getlin(line, MAXLINE) > 0)
26 		printf("\t%f\n", atof(line));
27 	return 0;
28 }
29 
30 /* getlin: get line into s, return length */
31 int getlin(char s[], int lim) {
32 	int c, i;
33 	i = 0;
34 	while (--lim > 0 && (c=getchar()) != EOF && c != '\n')
35 		s[i++] = c;
36 	if (c == '\n')
37 		s[i++] = c;
38 	s[i] = '\0';
39 	return i;
40 }
41 
42 /* atof: convert string s to double */
43 double atof(char s[]) {
44 	double val, power;
45 	int i, sign, expsign, exp;
46 
47 	for (i = 0; isspace(s[i]); i++) /* skip white space */
48 		;
49 	sign = (s[i] == '-') ? -1 : 1;
50 	if (s[i] == '+' || s[i] == '-')
51 		i++;
52 	for (val = 0.0; isdigit(s[i]); i++)
53 		val = 10.0 * val + (s[i] - '0');
54 	if (s[i] == '.')
55 		i++;
56 	for (power = 1.0; isdigit(s[i]); i++) {
57 		val = 10.0 * val + (s[i] - '0');
58 		power *= 10.0;
59 	}
60 	if (s[i] == 'e' || s[i] == 'E') {
61 		i++;
62 		expsign = (s[i] == '-') ? -1 : 1;
63 		if (s[i] == '+' || s[i] == '-')
64 			i++;
65 		for (exp = 0; isdigit(s[i]); i++)
66 			exp = 10 * exp + (s[i] - '0');
67 	} else {
68 		exp = 0;
69 		expsign = 1;
70 	}
71 	return sign * val * pow(10,expsign*exp) / power;
72 }
73 
74 int atoi(char s[]) {
75 	double atof(char s[]);
76 	return (int) atof(s);
77 }