Feb 7, 2012

K & R C Programs Exercise 7-5.

K and R C, Solution to Exercise 7-5:
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.
Rewrite the postfix calculator of chapter 4 to use scanf and/or sscanf to do the input number conversion.
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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   */
#define MAXVAL 100
int gettop(char []);
void push(double);
double pop(void);
int sp = 0;          /* Next free stack position. */
double val[MAXVAL];  


//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);
   break;

  }
 }
 return 0;
}




/* Getop: get next operator or numeric operand. */
int getop(char s[])
{

 int i = 0;
 int c;
 int rc;
 static char lastc[] = " ";
 sscanf(lastc,"%c", &c);
 lastc[0] = ' ';

 /* Skip whitespace */
 while((s[0] = c ) == ' ' || c == '\t')
  if(scanf("%",&c) == EOF)
   c = EOF;
 s[1] = '\0';

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


 if(isdigit(c))
  do{
   rc = scanf("%c",&c);
   if(!isdigit(s[++i] = c))
    break;
  }while(rc != EOF)

   if(c == '.')
    do{
     rc = scanf("%c",&c);
     if(!isdigit(s[++i] = c))
      break;
    }while(rc != EOF)

     s[i] = '\0';
 if(rc != EOF)
  lastc[0] = c;
 return NUMBER;

}



/* 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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