Exercise 5.10. Write the program expr, which evaluates a reverse Polish expression from the command line.

Exercise 5.10. Write the program expr, which evaluates a reverse Polish expression from the command line, where each operator or operand is a separate argument.
For example, expr 2 3 4 + *
                            evaluates 2 * (3+4)

/* expr */

#include <stdio.h>

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

#include <math.h>

#define MAXOP 100 //max size of operand or operator

#define NUMBER '0' //signal that a number was found

int getop(char[]);

void ungetch(char[]);

void push(double);

double pop(void);

int main(int argc, char *argv[])

{

    double op2;

    char s[MAXOP];

    

    while(--argc > 0)

    {

        ungetch(" ");

        ungetch(*++argv);

        switch(getop(s))

        {

            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 devisior\n");

                break;

            default:

                printf("error: unknown command %s\n", s);

                argc = 1;

                break;

        }

    }

    printf("\t%.8g\n", pop());

    return 0;

}

#define MAXVAL 100 //maximum depth of val stack

double val[MAXVAL]; //value stack

int sp = 0; //next free stack position

/* push: push f into 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(char []);

/* getop: get next character ot 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 nember

        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';

    return NUMBER;

}

#define BUFSIZE 100

char buf[BUFSIZE];  //buffer for ungetch;

int bufp = 0; //next free position in bud

int getch(void) // get a (possibly pushed-back) character  

{

   return (bufp > 0) ? buf[--bufp] : getchar();

}

void ungetch(char argv[]) // push character back on input

{

    if(bufp >= BUFSIZE)

        printf("ungetch: too many characnters\n");

    else

        buf[bufp++] = *argv;

}

Result: