Exercise 4.6 - RPN Calculator with variables¶

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

#define MAXOP 100
#define NUMBER '0'
#define BUFSIZE 100
#define MAXVAL 100

char buf[BUFSIZE];
int bufp = 0;
int sp = 0;
double val[MAXVAL];

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

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

/* reverse polish calculator */

int main(void) {
    int type, var = 0;
    double op2, v;
    char s[MAXOP];
    double variable[26];

    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 '=':
            pop();
            if (var >= 'A' && var <= 'Z')
                variable[var - 'A'] = pop();
            else
                printf("error: novariablename\n");
            break;

        case '\n':
            v = pop();
            printf("\t%.8g\n", v);
            break;
        default:
            if (type >= 'A' && type <= 'Z')
                push(variable[type - 'A']);
            else if (type == 'v')
                push(v);
            else
                printf("error: unknown command %s\n", s);
            break;
        }
        var = type;
    }
    return 0;
}

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;
    }
}

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()))
            ;

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

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;
}