Ex_4.4_rpn_top_two_elements.c

/* Add commands to 
  - print top element of the stack,without poping
  - duplicate it
  - swap the top two elements
  - Clear the stack  */

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

#define MAXOP 100
#define NUMBER '0'

int getop(char []);
void push(double);
double pop(void);

/* reverse polish calculator */

int main(void)
{
	int type;
	double op2,op1;
	char s[MAXOP];
	void clearsp(void);

	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("error:zero divisor\n");
						break;
				case '%':
						op2 = pop();
						if(op2 != 0.0)
							push(fmod(pop(),op2));
						else
							printf("erro:zero divisor\n");
						break;
				case '?':
						op2=pop();
						printf("\t%.8g\n",op2);
						push(op2);
						break;
				case 'c':
						clearsp();
						break;
				case 'd':
						op2=pop();
						push(op2);
						push(op2);
						break;
				case 's':
						op1=pop();
						op2=pop();
						push(op1);
						push(op2);
						break;	
				case '\n':
						printf("\t%.8g\n",pop());
						break;
				default:
						printf("error: unknown command %s\n",s);
						break;
		}
	}
	return 0;
}


#define MAXVAL 100

int sp = 0;
double val[MAXVAL];

void push(double f)
{
	if(sp < MAXVAL)
		val[sp++]=f;
	else
		printf("error:stack full, cant push %g\n",f);
}


double pop(void)
{
	if(sp>0)
		return val[--sp];
	else
	{
		printf("error: stack empty\n");
		return 0.0;
	}
}

void clearsp(void)
{
	sp = 0;
}

#include<ctype.h>

int getch(void);
void ungetch(int);

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

	while((s[0] = c = getch()) == ' ' || c =='\t')
		;
	s[1] = '\0';
	
	i = 0;
	if(!isdigit(c) && c!='.' && c!='-')
		return c;

	if(c=='-')
		if(isdigit(c=getch()) || c == '.')
			s[++i]=c;
		else
		{
			if(c!=EOF)
				ungetch(c);
			return '-';
		}
	
	if(isdigit(c))
		while(isdigit(s[++i] =c =getch()))
			;

	if(c=='.')
		while(isdigit(s[++i] = c=getch()))
			;
	
	s[i] = '\0';
	if(c!=EOF)
		ungetch(c);
	return NUMBER;
}

#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

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

void ungetch(int c)
{
	if(bufp >= BUFSIZE)
		printf("ungetch: too many characters\n");
	else
		buf[bufp++] = c;
}