Exercise 4-6: Calculator: variables

Add commands for handling variables. (It's easy to provide twenty-six variables with single-letter names.) Add a variable for the most recently printed value.

Each capital letter A to Z is a variable, which starts at 0; the letter pushes its value. = pops a variable's value and the value below it, stores that value in the variable and pushes it back, so 3 A = sets A to 3 and leaves 3 on the stack. v pushes the value most recently printed by a newline, which starts at 0.

The book's reverse Polish calculator:

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

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

/* reverse Polish calculator */
int 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;
    }
}

/* 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 != '.')
        return c;      /* not a number */
    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;
}