listing 1
#define MAX 100

char *p[MAX];
int spos = 0;
int rpos = 0;

/* Store an appointment. */
void qstore(char *q)
{
  if(spos==MAX) {
    printf("List Full\n");
    return;
  }
  p[spos] = q;
  spos++;
}

/* Retrieve an appointment. */
char *qretrieve()
{
  if(rpos==spos) {
    printf("No more appointments.\n");
    return '\0';
  }
  rpos++;
  return p[rpos-1];
}

listing 2
/* Mini Appointment-Scheduler */

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

#define MAX 100

char *p[MAX], *qretrieve(void);
int spos = 0;
int rpos = 0;
void enter(void), qstore(char *q), review(void), delete_ap(void);

int main(void)
{
  char s[80];
  register int t;

  for(t=0; t < MAX; ++t) p[t] = NULL; /* init array to nulls */

  for(;;) {
    printf("Enter, List, Remove, Quit: ");
    gets(s);
    *s = toupper(*s);

    switch(*s) {
      case 'E':
        enter();
        break;
      case 'L':
        review();
        break;
      case 'R':
        delete_ap();
        break;
      case 'Q':
        exit(0);
    }
  }
  return 0;
}

/* Enter appointments in queue. */
void enter(void)
{
  char s[256], *p;

  do {
    printf("Enter appointment %d: ", spos+1);
    gets(s);
    if(*s==0) break; /* no entry */
    p = (char *) malloc(strlen(s)+1);
    if(!p) {
      printf("Out of memory.\n");
      return;
    }
    strcpy(p, s);
    if(*s) qstore(p);
  } while(*s);
}

/* See what's in the queue. */
void review(void)
{
  register int t;

  for(t=rpos; t < spos; ++t)
    printf("%d. %s\n", t+1, p[t]);
}

/* Delete an appointment from the queue. */
void delete_ap(void)
{
  char *p;

  if((p=qretrieve())==NULL) return;
  printf("%s\n", p);
}

/* Store an appointment. */
void qstore(char *q)
{
  if(spos==MAX) {
    printf("List Full\n");
    return;
  }
  p[spos] = q;
  spos++;
}

/* Retrieve an appointment. */
char *qretrieve(void)
{
  if(rpos==spos) {
    printf("No more appointments.\n");
    return NULL;
  }
  rpos++;
  return p[rpos-1];
}

listing 3
void qstore(char *q)
{
  /* The queue is full if either spos is one less than rpos
     or if spos is at the end of the queue array and rpos
     is at the beginning.
  */
  if(spos+1==rpos || (spos+1==MAX && !rpos)) {
    printf("List Full\n");
    return;
  }

  p[spos] = q;
  spos++;
  if(spos==MAX) spos = 0; /* loop back */
}

char *qretrieve(void)
{
  if(rpos==MAX) rpos = 0; /* loop back */
  if(rpos==spos) {
    printf("No events to retrieve.\n");
    return NULL;
  }
  rpos++;
  return p[rpos-1];
}

listing 4
/* A circular queue example using a keyboard buffer. */
#include <stdio.h>
#include <conio.h>
#include <stdlib.h>

#define MAX 80

char buf[MAX+1];
int spos = 0;
int rpos = 0;

void qstore(char q);
char qretrieve(void);

int main(void)
{
  register char ch;
  int t;

  buf[80] = '\0';

  /* Input characters until a carriage return is typed. */
  for(ch=' ',t=0; t<32000 && ch!='\r'; ++t) {
    if(_kbhit()) {
      ch = _getch();
      qstore(ch);
    }
    printf("%d ", t);
    if(ch == '\r') {
      /* Display and empty the key buffer. */
      printf("\n");
      while((ch=qretrieve()) != '\0') printf("%c", ch);
      printf("\n");
    }
  }
  return 0;
}

/* Store characters in the queue. */
void qstore(char q)
{
  if(spos+1==rpos || (spos+1==MAX && !rpos)) {
    printf("List Full\n");
    return;
  }
  buf[spos] = q;
  spos++;
  if(spos==MAX) spos = 0; /* loop back */
}

/* Retrieve a character. */
char qretrieve(void)
{
  if(rpos==MAX) rpos = 0; /* loop back */
  if(rpos==spos) return '\0';

  rpos++;
  return buf[rpos-1];
}

listing 5
int stack[MAX];
int tos=0;   /* top of stack */

/* Put an element on the stack. */
void push(int i)
{

  if(tos >= MAX) {
    printf("Stack Full\n");
    return;
  }
  stack[tos] = i;
  tos++;
}

/* Retrieve the top element from the stack. */
int pop(void)
{
  tos--;
  if(tos < 0) {
    printf("Stack Underflow\n");
    return 0;
  }
  return stack[tos];
}

listing 6
int *p;   /* will point to a region of free memory */
int *tos; /* points to top of stack */
int *bos; /* points to bottom of stack */

/* Store an element on the stack. */
void push(int i)
{
  if(p > bos) {
    printf("Stack Full\n");
    return;
  }
  *p = i;
  p++;
}

/* Retrieve the top element from the stack. */
int pop(void)
{
  p--;
  if(p < tos) {
    printf("Stack Underflow\n");
    return 0;
  }
  return *p;
}

listing 7
/* A simple four-function calculator. */

#include <stdio.h>
#include <stdlib.h>

#define MAX 100

int *p;   /* will point to a region of free memory */
int *tos; /* points to top of stack */
int *bos; /* points to bottom of stack */

void push(int i);
int pop(void);

int main(void)
{
  int a, b;
  char s[80];

  p = (int *) malloc(MAX*sizeof(int)); /* get stack memory */
  if(!p) {
    printf("Allocation Failure\n");
    exit(1);
  }
  tos = p;
  bos = p + MAX-1;

  printf("Four Function Calculator\n");
  printf("Enter 'q' to quit\n");

  do {
    printf(": ");
    gets(s);
    switch(*s) {
      case '+':
        a = pop();
        b = pop();
        printf("%d\n", a+b);
        push(a+b);
        break;
      case '-':
        a = pop();
        b = pop();
        printf("%d\n", b-a);
        push(b-a);
        break;
      case '*':
        a = pop();
        b = pop();
        printf("%d\n", b*a);
        push(b*a);
        break;
      case '/':
        a = pop();
        b = pop();
        if(a==0) {
          printf("Divide by 0.\n");
          break;
        }
        printf("%d\n", b/a);
        push(b/a);
        break;
      case '.': /* show contents of top of stack */
        a = pop();
        push(a);
        printf("Current value on top of stack: %d\n", a);
        break;
      default:
        push(atoi(s));
    }
  } while(*s != 'q');

  return 0;
}

/* Put an element on the stack. */
void push(int i)
{
  if(p > bos) {
    printf("Stack Full\n");
    return;
  }
  *p = i;
  p++;
}

/* Retrieve the top element from the stack. */
int pop(void)
{
  p--;
  if(p < tos) {
    printf("Stack Underflow\n");
    return 0;
  }
  return *p;
}

listing 8
void slstore(struct address *i,
             struct address **last)
{
  if(!*last) *last = i; /* first item in list */
  else (*last)->next = i;
  i->next = NULL;
  *last = i;
}

listing 9
/* Store in sorted order. */
void sls_store(struct address *i, /* new element to store */
               struct address **start, /* start of list */
               struct address **last) /* end of list */
{
  struct address *old, *p;

  p = *start;

  if(!*last) { /* first element in list */
    i->next = NULL;
    *last = i;
    *start = i;
    return;
  }

  old = NULL;
  while(p) {
    if(strcmp(p->name, i->name)<0) {
      old = p;
      p = p->next;
    }
    else {
      if(old) { /* goes in middle */
        old->next = i;
        i->next = p;
        return;
      }
      i->next = p; /* new first element */
      *start = i;
      return;
    }
  }
  (*last)->next = i; /* put on end */
  i->next = NULL;
  *last = i;
}

listing 10
void display(struct address *start)
{
  while(start) {
    printf("%s\n", start->name);
    start = start->next;
  }
}

listing 11
struct address *search(struct address *start, char *n)
{
  while(start) {
    if(!strcmp(n, start->name)) return start;
    start = start->next;
  }
  return NULL;  /* no match */
}

listing 12
void sldelete(
     struct address *p, /* previous item */
     struct address *i, /* item to delete */
     struct address **start, /* start of list */
     struct address **last) /* end of list */
{
  if(p) p->next = i->next;
  else *start = i->next;

  if(i==*last && p) *last = p;
}

listing 13
void dlstore(struct address *i, struct address **last)
{

  if(!*last) *last = i; /* is first item in list */
  else (*last)->next = i;
  i->next = NULL;
  i->prior = *last;
  *last = i;
}

listing 14
/* Create a doubly linked list in sorted order. */
void dls_store(
  struct address *i,   /* new element */
  struct address **start, /* first element in list */
  struct address **last /* last element in list */
)
{
  struct address *old, *p;

  if(*last==NULL) { /* first element in list */
    i->next = NULL;
    i->prior = NULL;
    *last = i;
    *start = i;
    return;
   }

  p = *start; /* start at top of list */

  old = NULL;
  while(p) {
    if(strcmp(p->name, i->name)<0){
      old = p;
      p = p->next;
    }
    else {
      if(p->prior) {
        p->prior->next = i;
        i->next = p;
        i->prior = p->prior;
        p->prior = i;
        return;
      }
      i->next = p; /* new first element */
      i->prior = NULL;
      p->prior = i;
      *start = i;
      return;
    }
  }
  old->next = i; /* put on end */
  i->next = NULL;
  i->prior = old;
  *last = i;
}

listing 15