Exercise 5.10 - expr, evaluate rpn from command line

Question

Write the program expr, which evaluates a reverse Polish expression from the command line, where each operator or operand is a separate argument.

/* Write a program exprcmd, which evaluates a reverse polish expression,
   from the command line,where each operator or operand is a seperate argument.
    
    For eg:
        expr  2 3 4 + *
            evaluates to 
            2 * ( 3 + 4)
*/

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

#define MAXOP 100   /* maximum size of operand or operator */
#define NUMBER '0'  /* signal that a number was found */

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


/* reverse polish calculator, uses command line */

int main(int argc,char *argv[])
{
    char s[MAXOP];
    double op2;

    while(--argc > 0)
    {
        ungets(" "); /* push end of argument */
        ungets(*++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 divisor \n");
                    break;
            default:
                    printf("error: unknown command %s \n",s);
                    argc=1;
                    break;
        }
    }
    printf("\t %8g\n",pop());

    return 0;
}

#include<ctype.h>

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

/* getop: get next operator 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;

    i = 0;

    if( isdigit(c)) /* collect integer part */
        while(isdigit(s[++i] = c = getch()))
            ;
    if( c == '.') /* collect from 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;
}

#define MAXVAL 100 /* maximum depth of value of 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 the stack */

double pop(void)
{
    if ( sp > 0)
        return val[--sp];
    else
    {
        printf("error: stack empty \n");
        return 0.0;
    }
}

#include<string.h>
/* ungets: push string back onto the input */

void ungets(char s[])
{
    int len=strlen(s);
    void ungetch(int);

    while(len > 0)
        ungetch(s[--len]);
}

Run this

Explanation

This program reads the input to our rpn calculator from the command line itself. 2 3 4 + * and then goes about doing the RPN caculator work on it.

The main function has a signature now, that is int main(int argc, char *argv[]), that is it takes the command line args, argc for count of the args and argv is the array which stores the arguments.

So, 2, 3, 4, +, * will be stored in the array agrv as strings. In this program, we go about by getting each argument from argv and then giving that as the input to our RPN calculator, like the program in sec_4.3. If we find an operand, using the push function, we push it to the stack and when we find a operator in the input, we pop() the two operands out of the stack and do the operation.

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