C - Reverse Polish Calculator

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/*
 * A reverse Polish calculator
 *
 * ReversePolishCalculator.c - by FreeMan
 */

#include <stdio.h>
#include <stdlib.h> /* For atof() */

#define MAXOP 100 /* Max size of operand or operator */
#define NUMBER '0' /* Signal that a number was found */

int GetOp(char[]);
void Push(double);
double Pop(void);

/* Reverse Polish calculator */
main()
{
	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("error: zero divisor\n");
			}
			break;
		case '\n':
			printf("\t%.8g\n", Pop());
			break;
		default:
			printf("error: unknown command %s\n", s);
			break;
		}
	}
	return 0;
}

#define MAXVAL 100 /* Maximum depth of val stack */

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

/* Push: Push f onto value stack */
void Push(double f)
{
	if (sp < MAXVAL)
	{
		val[sp++] = f;
	}
	else
	{
		printf("error: 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("error: stack empty\n");
		return 0.0;
	}
}

#include <ctype.h>

int GetCh(void);
void UngetCh(int);

/* GetOp: Get next character or numeric operand */
int GetOp(char s[])
{
	int i, c;
	while ((s[0] = c = GetCh()) == ' ' || c == '\t')
		;
	s[1] = '\0';
	if (!isdigit(c) && c != '.') /* Not a number */
	{
		return c;
	}
	i = 0;
	if (isdigit(c)) /* Collect integer part */
	{
		while (isdigit(s[++i] = c = GetCh()))
			;
	}
	if (c == '.') /* Collect fraction part */
	{
		while (isdigit(s[++i] = c = GetCh()))
			;
	}
	s[i] = '\0';
	if (c != EOF)
	{
		UngetCh(c);
	}
	return NUMBER;
}

#define BUFSIZE 100

char buf[BUFSIZE]; /* Buffer for UngetCh */
int bufp = 0; /* Next free position in buf */

int GetCh(void) /* Get a (possibly pushed-back) character */
{
	return (bufp > 0) ? buf[--bufp] : getchar();
}
void UngetCh(int c) /* Push character back on input */
{
	if (bufp >= BUFSIZE)
	{
		printf("UngetCh: too many characters\n");
	}
	else
	{
		buf[bufp++] = c;
	}
}

// Output:
/*
1 2 + 3 4 - * 5 6 + /
		-0.27272727

*/