exercise5-1.c

#include <ctype.h>

int getch(void);
void ungetch(int);

/*getint: get next integer from input into *pn */
int getint(int *pn)
{
    int c, sign, next;

    while (isspace(c = getch())) /* skip white space */
        ;
    if (!isdigit(c) && c != EOF && c != '+' && c != '-') {
        ungetch(c); /* it's not a number */
        return 0;
    }

    sign = (c == '-') ? -1 : 1;
    if (c == '+' || c == '-') {
        next = getch();
        if (isdigit(next))
            c = next;
        else {
            ungetch(c);
            if (next != EOF)
                ungetch(next);
            return c;
        }
    }

    for (*pn = 0; isdigit(c); c = getch())
        *pn = 10 * *pn + (c - '0');
    *pn *= sign;
    if (c != EOF)
        ungetch(c);
    return c;
}