5.1.c


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#include <stdio.h>
#include <ctype.h>

int getint(int *np);
char getch(void);
void ungetch(char input);

int 
main(void)
{
  int num;

  getint(&num);

  printf("%d\n", num);

  return 0;
}

int 
getint(int *np)
{
  int input, sign;

  /* skip space */
  while (isspace(input = getch()))
    ;

  if (!isdigit(input) && input != EOF && input != '+' && input != '-') {
    ungetch(input);
    return 0;
  }

  sign = (input == '-') ? -1 : 1;

  if (input == '+' || input == '-')
    while (isspace(input = getch()))
     ;

  for (*np = 0; isdigit(input); input = getch())
    *np = *np * 10 + input - '0';

  *np *= sign;

  if (input != EOF)
    ungetch(input);

  /* return last digit or EOF */
  return input;
}

static char buff = -1;

char 
getch(void)
{
  char temp;

  if (buff == -1)
    return getchar();

  temp = buff;
  buff = -1;
  return temp;
}

void
ungetch(char input)
{
  if (buff == -1)
    buff = input;
  else 
    printf("Err: buffer is full\n");
}