#include <stdio.h>
#include <ctype.h>
/*count digits, white space, others*/
int atoi(char[]);
int main(void)
{
char ndigit[] = " -34";
printf("%d \n", atoi(ndigit));
return 0;
}
int atoi(char s[]) {
int i=0, n, sign;
for (i = 0; isspace(s[i]); i++) /* skip white space */
;
sign = (s[i] == '-') ? -1 : 1;
if (s[i] == '+' || s[i] == '-') /* skip sign */
i++;
for (n = 0; isdigit(s[i]); i++)
n = 10 * n + (s[i] - '0');
return sign * n;
}
/*The C Programming Language (second edition, Prentice Hall)
by Brian W. Kernighan and Dennis M. Ritchie*/
The structure of the program reflects the form of the input:
skip white space, if any
get sign, if any
get integer part and convert it
Each step does its part, and leaves things in a clean state for the next. The whole process terminates on the first character that could not be part of a number.
[The C Programming Language (second edition, Prentice Hall) by Brian W. Kernighan and Dennis M. Ritchie, p.61]
