K and R C, Solution to Exercise 4-6:
K and R C Programs Exercises provides the solution to all the exercises in the C Programming Language (2nd Edition). You can learn and solve K&R C Programs Exercise.
C Program to Add commands for handling variables.(It’s easy to provide twenty-six variables with single-letter names.). Add a variable for the most recently printed value.

/***********************************************************
* You can use all the programs on  www.c-program-example.com
* for personal and learning purposes. For permissions to use the
* programs for commercial purposes,
* contact info@c-program-example.com
* To find more C programs, do visit www.c-program-example.com
* and browse!
* 
*                      Happy Coding
***********************************************************/

#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] = '