ex4_7.c

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

#define MAXSTACK 100
#define BUFSIZE 100
#define MAXOP 50
#define MAXVAR 26

#define NUMBER '0'
#define MATH 'm'
#define VAR 'v'
#define ASGN 'a'

double stack[MAXSTACK];
char buf[BUFSIZE];
double var[MAXVAR];

int buf_pos = 0;
int stack_pos = 0;

void push(double el)
{
  if (stack_pos < MAXSTACK) {
    stack[stack_pos++] = el;
  }
  else {
    printf("Error : STACK FULL\n");
  }
}

double pop(void)
{
  if (stack_pos > 0) {
    return stack[--stack_pos];
  }
  else {
    printf("Error : STACK EMPTY\n");
    return 0;
  }
}

double top(void)
{
  if (stack_pos > 0)
    return stack[stack_pos-1];
  else {
    printf("Error : STACK EMPTY\n");
    return 0;
  }
}

void duplicate(void)
{
  if (stack_pos < MAXSTACK) 
    stack[stack_pos++] = top();
  else
    printf("Error : STACK FULL\n");
}

void swap(void)
{
  double temp1, temp2;

  temp1 = pop();
  temp2 = pop();

  push(temp1);
  push(temp2);
}

void clear(void)
{
  stack_pos = 0;
  stack[stack_pos] = '\0';
}

int getch(void)
{
  return (buf_pos > 0) ? buf[--buf_pos] : getchar();
}

void ungetch(int c)
{
  if (buf_pos >= BUFSIZE) {
    printf("Error : BUFFER FULL\n");
  }
  else {
    buf[buf_pos++] = c;
  }
}

void ungets(char s[])
{
  // Using ungetch instead of directly accessing buf because
  // Don't want two functions accessing global variables
  // Still have to check if buf can hold s using buf_pos though
  
  int i, l = strlen(s);

  if (l + buf_pos >= BUFSIZE) {
    printf("Error : BUFFER CANNOT HOLD STRING");
  }
  else {
    for (i = l-1; i > 0; i--) {
      ungetch(s[i]);
    }
  }
}

int getop(char s[])
{
  int c, d, i;

  while ((c = s[0] = getch()) == ' ' || c == '\t')
    ;
  s[1] = '\0';

  if (!isdigit(c) && c != '.' && c != '-' && !isalpha(c))
    return c;

  i = 0;
  if (c == '-') {
    if ((d = getch()) == ' ')
      return c;
    else {
      s[++i] = d;
      c = d;
    }
  }
  
  if (isalpha(c)) {
    if (isalpha(d = getch())) {
      ungetch(d);
      s[++i] = getch();
      s[++i] = getch();
      s[++i] = '\0';
      return MATH;
    }
    else {
      d = getch();
      if (d == '=') {
	s[++i] = '\0';
	return ASGN;
      }
      else {
	ungetch(d);
	s[++i] = '\0';
	return VAR;
      }
    }
  }

  if (isdigit(c))
    while (isdigit((c = s[++i] = getch())))
      ;
  if (c == '.')
    while (isdigit((c = s[++i] = getch())))
      ;

  s[i] = '\0';

  if (c != EOF)
    ungetch(c);
  return NUMBER;
}

int main()
{
  char s[MAXOP];
  int type, ignore_newline = 0;
  double op2;
  
  while ((type = getop(s)) != EOF) {
    switch (type) {
    case NUMBER:
      push(atof(s));
      break;
    case ASGN:
      op2 = pop();
      var[s[0]-'a'] = op2;
      ignore_newline = 1;
      break;
    case VAR:
      push(var[s[0]-'a']);
      break;
    case MATH:
      if (strcmp(s, "sin") == 0)
	push(sin(pop()));
      else if (strcmp(s, "cos") == 0)
	push(cos(pop()));
      else if (strcmp(s, "tan") == 0)
	push(tan(pop()));
      else if (strcmp(s, "exp") == 0)
	push(exp(pop()));
      else if (strcmp(s, "pow") == 0) {
	op2 = pop();
	push(pow(pop(), op2));
      }
      else
	printf("Error : Invalid Math\n");
      break;
    case '+':
      push(pop() + pop());
      break;
    case '*':
      push(pop() * pop());
      break;
    case '-':
      op2 = pop();
      push(pop() - op2);
      break;
    case '/':
      op2 = pop();
      if (op2 == 0)
	printf("Error : DIVISION BY ZERO");
      else
	push(pop() / op2);
      break;
    case '%':
      op2 = pop();
      if (op2 == 0)
	printf("Error : MODULUS WITH ZERO");
      else
	push(fmod(pop(), op2));
      break;
    case '>':
      printf("%f\n", top());
      ignore_newline = 1;
      break;
    case '<':
      duplicate();
      ignore_newline = 1;
      break;
    case '@':
      swap();
      ignore_newline = 1;
      break;
    case '!':
      clear();
      ignore_newline = 1;
      break;
    case '\n':
      if (ignore_newline)
	ignore_newline = 0;
      else
	printf("\t%.8g\n", pop());
      break;
    default:
      printf("Error : INVALID ENTRY");
      break;
    }
  }
  return 0;
}