Reverse Polish Calculator

#

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

#define MAXOP 100
#define NUMBER '\0'
#define BUFSIZE 100
#define MAXVAL 100
int sp = 0;
double val[MAXVAL];

char buf[BUFSIZE];
int bufp = 0;
void push(double f) {
    if (sp < MAXVAL) {
        val[sp++] = f;
    }
    else {
        printf("error: stack full can't push");
    }
}


double pop(void) {
    if (sp > 0) {
        return val[--sp];

    }
    else {
        printf("error: stack empty can't pop");
        return 0.0;
    }

}


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;
    }
}


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

    while ((s[0] = c = getch()) == ' ' || c == '\t') {

    }
    s[1] = '\0';
    if (!isdigit(c) && c != '.') {
        return c;
    }
    i = 0;
    if (!isdigit(c)) {
        while (isdigit(s[++i] = c = getch())) {
        }
    }
    if (c == '.') {
        while (isdigit(s[++i] = c = getch())) {

        }
    }
}

int main(void) {

    int type;
    double op2;
    int sign = 1;
    char s[MAXOP];
    while ((type = getop(s)) != EOF) {
        if (type == '-') {
            sign = -1;
            continue;
        }
        switch (type) {

        case NUMBER:
            push(atof(s) * sign);
            break;
        case '+':
            push(pop() + pop());
            break;
        case '-':
            op2 = pop();
            push(pop() - op2);
            break;
        case '/':
            op2 = pop();
            if (op2 != 0.0) {
                push(pop() / op2);
                break;
            }
            else {
                printf("Error: zero divisor\n");
                break;
            }
        case '\n':
            printf("\t%.8g\n", pop());
            break;
        case '%':
            op2 = pop();
            push(fmod(pop(), op2));
            break;
        case 'P':
            printf("%c", buf[bufp - 1]);
            break;
        case 'D':
            push(buf[bufp - 1]);
            break;
        case 'S':
            int c = buf[bufp - 1];
            buf[bufp - 1] = buf[bufp - 2];
            buf[bufp - 2] = c;
            break;
        case 'V':
            for (int i = 0;i < BUFSIZE - 1;i++) {
                buf[i] = 0;
            }
            break;
        case 'B':
            push(sin(pop()));
            break;
        case 'N':
            op2 = pop();
            push(pow(pop(), op2));
            break;
        case 'L':
            break;
        default:
            printf("error: unknown command %s\n", s);
            break;
        }
        sign = 1;
    }

}