Exercise 5-1. As written, getint treats a + or - not followed by a digit as a valid representation of zero. Fix it to push such a character back on the input.

/* Exercise 5-1. As written, getint treats a + or - not followed by a digit as a valid representation of zero. 
   Fix it to push such a character back on the input. */

#include <stdio.h>
#include <ctype.h>
#define SIZE 5

int getint(int *pn);

int main()
{
    int n, a, array[SIZE];

    for (n = 0; n < SIZE && (a = getint(&array[n])) != EOF; n++)
        printf("array[%d] = %d, a = %d\n", n, array[n], a);

    return 0;
}

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

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

    while (isspace(c = getch())) /* skip white space */
        ;
    if (!isdigit(c) && c != EOF && c != '+' && c != '-') 
    {
        ungetch(c); /* it is not a number */
        return 0;
    }
    sign = (c == '-') ? -1 : 1;
    if (c == '+' || c == '-')
    {
        c = getch();
        if (!isdigit(c))
        {
            ungetch(c); /* it is not a number */
            return 0;
        }
    }
    for (*pn = 0; isdigit(c); c = getch())
        *pn = 10 * *pn + (c - '0');
    *pn *= sign;
    if (c != EOF)
        ungetch(c);

    return c;
}

#define BUFSIZE 100

char buf[BUFSIZE]; /* buffer for ungetch */
int bufp = 0; /* next free position in buf */

int getch(void) /* get a (possibly pushed-back) character */
{
    return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c) /* push character back on input */
{
    if (bufp >= BUFSIZE)
        printf("ungetch: too many characters\n");
    else
        buf[bufp++] = c;
}