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#include <stdio.h>
#include <string.h>
/* The C Programming Language: 2nd Edition
*
* Exercise 4-12: Adapt the ideas of printd() to write a recursive version of
* itoa; that is, convert an integer into a string by calling a recursive
* routine.
*
* Answer: Making itoa() recursive isn't too much work; it helps knowing how to
* use static variables; they make it possible to increment the position in the
* string without using an external variable. Additionally, you'll need to
* understand flow control to prevent the sub-routines from executing further
* than needed. As a result, itoa() gets called multiple times, but if it
* calls itself, it immediately returns so it won't venture further. This is
* to make sure i is reset to zero only after the number is finished, so the
* next call starts at the beginning of the string.
*
* Normally, recursive functions shouldn't worry about state, but it's necessary
* in this version of itoa.
*/
void itoa(int, char[]);
void reverse(char[]);
int main(void) {
char foo[40] = "";
itoa(829048, foo);
printf("%s\n", foo);
itoa(-4021, foo);
printf("%s\n", foo);
return 0;
}
void itoa(int num, char target[]) {
static int i = 0;
static int neg = 0;
if (num < 0) {
neg = 1;
num = -num;
}
if (num /= 10 > 0) {
target[i++] = (num % 10) + '0';
num /= 10;
itoa(num, target);
return;
} else {
if (neg == 1) {
neg = 0;
target[i++] = '-';
target[i] = '\0';
} else {
target[i] = '\0';
}
reverse(target);
}
i = 0;
}
void reverse(char s[]) {
int c, i, j;
for (i = 0, j = strlen(s)-1; i < j; i++, j--) {
c = s[i];
s[i] = s[j];
s[j] = c;
}
}
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