Simple Reverse Polish Notation calculator in C

rpn.c

/*

RPN Calcuator in C
by Spitemim, 2022/04/12

To compile:
$ cc rpn.c -lreadline -o rpn

*, +, /, and - are operators and work as expected
Use ~ for negative numbers (~500, ~5.5, etc.)
Use @ to clear the stack

*/

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

#include <readline/readline.h>
#include <readline/history.h>

#define STACK_MAX 1024

double stack[STACK_MAX] = {0.0};
int top = -1;

void
push(double d) {
    if (top + 1 == STACK_MAX) {
        puts("\tStack overflow!");
        return;
    }

    stack[++top] = d;
}

double
pop(void)
{
    if (top < 0) {
        puts("\tStack underflow!");
        return 0.0;
    }

    return stack[top--];
}

void
showstack(void)
{
    int i;

    printf("\t[ ");

    for (i = 0; i <= top; ++i) {
        printf("%g ", stack[i]);
    }

    /*printf("] - TOP: %d\n", top);*/
    printf("]\n");
}

void
interpret(char *buf)
{
    int nc;
    double d;

    do {
        if (isdigit(*buf)) {
            /* read one float into d */
            sscanf(buf, "%lf%n", &d, &nc);
            buf += nc;
            push(d);
        } else if (*buf == '~') {
            /* if a number doesn't immediately follow the ~, ignore */

            ++buf;

            if (isdigit(*buf)) {
                sscanf(buf, "%lf%n", &d, &nc);
                buf += nc;
                push(-d);
            }
        } else {
            /* operators */

            switch (*buf) {
            case '+':
                push(pop() + pop());
                break;
            case '*':
                push(pop() * pop());
                break;
            case '-':
                d = pop();
                push(pop() - d);
                break;
            case '/':
                d = pop();
                push(pop() / d);
                break;
            case '@':
                top = -1;
                break;
            }
        }
    } while (*buf++);
}

int
main(void)
{
    char *buf;

    puts("Reverse Polish Notation Calculator");
    puts(" +, -, *, / work as expected");
    puts(" ~ for negative numbers, @ to clear stack\n");

    for (;;) {
        buf = readline("> ");
        add_history(buf);

        if (strlen(buf) != 0) {
            interpret(buf);
            showstack();
        }

        free(buf);
    }

    free(buf);
    return 0;
}