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

#define MAXOP      100
#define NUMBER       0
#define IDENTIFIER   1
#define ENDSTRING    2
#define TRUE         1
#define FALSE        0
#define MAX_ID_LEN  32
#define MAXVARS     30


struct varType{
 char name[MAX_ID_LEN];
 double val;
};


int Getop(char s[]);
void push(double val);
double pop(void);
void showTop(void);
void duplicate(void);
void swapItems(void);
void clearStacks(struct varType var[]);
void dealWithName(char s[],  struct varType var[]);
void dealWithVar(char s[], struct varType var[]);

int pos = 0;
struct varType last;

int main(void)
{
 int type;
 double op2;
 char s[MAXOP];
 struct varType var[MAXVARS];

 clearStacks(var);

 while((type = Getop(s)) != EOF)
 {
  switch(type)
  {
  case NUMBER:
   push(atof(s));
   break;
  case IDENTIFIER:
   dealWithName(s, var);
   break;
  case '+':
  push(pop() + pop());
  break;
  case '*':
   push(pop() * pop());
   break;
  case '-':
   op2 = pop();
   push(pop()- op2);
   break;
  case '/':
   op2 = pop();
   if(op2)
    push(pop() / op2);
   else
    printf("nError: division by zero!");
   break;
  case '%':
   op2 = pop();
   if(op2)
    push(fmod(pop(), op2));
   else
    printf("nError: division by zero!");
   break;
  case '?':
   showTop();
   break;
  case '#':
   duplicate();
   break;
  case '~':
   swapItems();
   break;
  case '!':
   clearStacks(var);
   break;
  case 'n':
   printf("nt%.8gn", pop());
   break;
  case ENDSTRING:
   break;
  case '=':
   pop();
   var[pos].val = pop();
   last.val = var[pos].val;
   push(last.val);
   break;
  case '<':
   printf("The last variable used was: %s (value == %g)n",
     last.name, last.val);
   break;

  default:
   printf("nError: unknown command %s.n", s);
   break;
  }
 }
 return EXIT_SUCCESS;
}

#define MAXVAL 100

int sp = 0;          
double val[MAXVAL];  

/* push: push f onto stack. */
void push(double f)
{
 if(sp < MAXVAL)
  val[sp++] = f;
 else
  printf("nError: stack full can't push %gn", f);
}

/*pop: pop and return top value from stack.*/
double pop(void)
{
 if(sp > 0)
 {
  return val[--sp];
 }
 else
 {
  printf("nError: stack emptyn");
  return 0.0;
 }
}

void showTop(void)
{
 if(sp > 0)
  printf("Top of stack contains: %8gn", val[sp-1]);
 else
  printf("The stack is empty!n");
}


void duplicate(void)
{
 double temp = pop();

 push(temp);
 push(temp);
}

void swapItems(void)
{
 double item1 = pop();
 double item2 = pop();

 push(item1);
 push(item2);
}


void clearStacks(struct varType var[])
{
 int i;
 sp = 0;

 for( i = 0; i < MAXVARS; ++i)
 {
  var[i].name[0] = '';
  var[i].val = 0.0;
 }
}

void dealWithName(char s[], struct varType var[])
{
 double op2;

 if(!strcmp(s, "sin"))
  push(sin(pop()));
 else if(!strcmp(s, "cos"))
  push(cos(pop()));
 else if (!strcmp(s, "exp"))
  push(exp(pop()));
 else if(!strcmp(s, "pow"))
 {
  op2 = pop();
  push(pow(pop(), op2));
 }

 else
 {
  dealWithVar(s, var);
 }
}


void dealWithVar(char s[], struct varType var[])
{
 int i = 0;

 while(var[i].name[0] != '' && i < MAXVARS-1)
 {
  if(!strcmp(s, var[i].name))
  {
   strcpy(last.name, s);
   last.val = var[i].val;
   push(var[i].val);
   pos = i;
   return;
  }
  i++;
 }

 /* variable name not found so add it */
 strcpy(var[i].name, s);
 /* And save it to the last variable */
 strcpy(last.name, s);
 push(var[i].val);
 pos = i;
}

int getch(void);
void unGetch(int);

/* Getop: get next operator or numeric operand. */
int Getop(char s[])
{
 int i = 0;
 int c;
 int next;

 /* Skip whitespace */
 while((s[0] = c = getch()) == ' ' || c == 't')
 {
  ;
 }
 s[1] = '';

 if(isalpha(c))
 {
  i = 0;
  while(isalpha(s[i++] = c ))
  {
   c = getch();     
  }
  s[i - 1] = '';
  if(c != EOF)
   unGetch(c);
  return IDENTIFIER;
 }

 /* Not a number but may contain a unary minus. */
 if(!isdigit(c) && c != '.' && c != '-')
 {
  /* 4-6 Deal with assigning a variable. */
  if('=' == c && 'n' == (next = getch()))
  {
   unGetch('');
   return c;
  }
  if('' == c)
   return ENDSTRING;

  return c;                 
 }

 if(c == '-')
 {
  next = getch();
  if(!isdigit(next) && next != '.')
  {
   return c;
  }
  c = next;
 }
 else
 {
  c = getch();
 }

 while(isdigit(s[++i] = c))
 {
  c = getch();
 }
 if(c == '.')                 /* Collect fraction part. */
 {
  while(isdigit(s[++i] = c = getch()))
   ;
 }
 s[i] = '';
 if(c != EOF)
  unGetch(c);
 return NUMBER;
}

#define BUFSIZE 100

int buf[BUFSIZE];
int bufp = 0;

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

/* unGetch: push character back on input. */
void unGetch(int c)
{
 if(bufp >= BUFSIZE)
  printf("nUnGetch: too many charactersn");
 else
  buf[bufp++] = c;
}

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