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#include <stdio.h>
#include <ctype.h>
/* The C Programming Language: 2nd Edition
*
* 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.
*
* Answer: It's relatively simple to extend atof. You need a variable to count
* the number of places the decimal needs to be moved, and in which direction.
* With a little clever manipulation of the `power` variable, you don't even
* need to touch the math that's used just before atof() returns. :)
*/
double atof(char s[]) {
double val, power;
int i, sign, esign, exp;
/* Skip whitespace */
for (i = 0; isspace(s[i]); i++) {
}
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;
}
/* Start the exponent checking... */
if (s[i] == 'e' || s[i] == 'E') {
i++;
esign = (s[i] == '-') ? 1 : 0;
if (s[i] == '+' || s[i] == '-') {
i++;
}
/* This allows us to count how many times we need to mess with
* the power variable */
exp = 0;
while (isdigit(s[i])) {
exp *= 10;
exp += (s[i] - '0');
i++;
}
/* And do our business, which is a bit tricky */
while (exp > 0) {
/* Because `power` is being divided with later, we need to reverse
* operands to ensure it shrinks and expands as needed. */
if (esign == 1) {
// A negative exponent means we need to divide by more
power *= 10.0;
} else {
// And a positive exponent means we need to divide by less
power /= 10.0;
}
exp--;
}
}
/* The math stays the same */
return sign * val / power;
}
int main(void) {
// Declare the function so there's no confusion over what it returns.
double atof(char s[]);
/* Should print "0.0000012345678900" */
printf("%20.16f\n", atof(" 123.456789e-8 "));
return 0;
}
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