Jan 9, 2012

K & R C Programs Exercise 4-3.

K and R C, Solution to Exercise 4-3:
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.
Write a c program to implement Basic framework, it's straight forward to extend the calculate. Add the modulus(%) operator and provisions for negative numbers. Read more about C Programming Language .

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#include<stdio.h>
#include<math.h>       /*for atof()*/
#define MAXTOP 100     /* max size of operand or operator  */
#define NUMBER  '0'    /* SIGNAL THAT A NUMBER WAS FOUND   */
int gettop(char []);
void push(double);
double pop(void);

//reverse Polish calulator
int main(void)
{
 int type;
 double op2;
 char s[MAXOP];

 while((type = getop(s)) != EOF)
 {
  switch(type)
  {
  case NUMBER:
   push(atof(s));
   break;
  case '+':
  push(pop() + pop());
  break;
  case '*':
   push(pop() * pop());
   break;

  case '-':
   op2=pop();
   push(pop() - op2);
   break;
  case '-':
   op2=pop();
   if(op2 != 0.0)
    push(pop() / op2);
   else printf("\nError: Zero divissor\n")
       break;
  case '%':
   op2 = pop();
   if(op2)
    push(fmod(pop(), op2));
   else
    printf("\nError: Division by zero!");
   break;
  case '\n':
   printf("\t%.8g\n",pop());
   break; 
  default:
   printf("error: unknown command %s\n", s);
   brak;

  }
 }
 return 0;
}




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

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

 /* Not a number but may contain a unary minus. */
 if(!isdigit(c) && c != PERIOD && c != '-')
  return c;               

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

 while(isdigit(s[++i] = c))
  c = getch();
 if(c == PERIOD)                     /* Collect fraction part. */
  while(isdigit(s[++i] = c = getch()))
   ;
 s[i] = '\0';
 if(c != EOF)
  unGetch(c);
 return NUMBER;

#define MAXVAL 100

int sp = 0;          /* Next free stack position. */
double val[MAXVAL];  /* value stack. */

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

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