calculator-chapt4.c

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

#define MAXOP 100
#define NUMBER  0
#define TRUE 1
#define FALSE 0

/* This programme is a basic calculator.

Extra cases have been added to:
1. Show the top item of the stack without permanently popping it.
2. Swap the top two items on the stack.
3. Duplicate the top item on the stack.
4. Clear the stack.

I have used functions for each of the new cases rather than have the
code inline in order to limit the physical size of the switch block.

In anticipation of the following exercise the following characters have
been used for the operations (in the same order as above): ? ~ # !
rather than use alphabetic characters.

It is actually rather difficult to be original in this exercise.

This is exercise 4-4 from Kernighan & Ritchie, page 79.

 */

int Getop(char s[]);
void push(double val);
double pop(void);
void showtop(void);
void clear();

int main(void)
{
    int type;
    double op2, temp, i1, i2;
    char s[MAXOP];
    int flag = TRUE;

    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)
                    push(pop() / op2);
                else
                    printf("\nError: division by zero!");
                break;
            case '%':
                op2 = pop();
                if(op2)
                    push(fmod(pop(), op2));
                else
                    printf("\nError: division by zero!");
                break;
            case '?':
                showtop();
                break;
            case '#':
                temp = pop();
                push(temp);
                push(temp);
                break;
            case '~':
                i1 = pop();
                i2 = pop();

                push(i1);
                push(i2);
                break;
            case '!':
                clear();
            case '\n':
                printf("\n\t%.8g\n", pop());
                break;
            default:
                printf("\nError: unknown command %s.\n", s);
                break;
        }
    }
    return EXIT_SUCCESS;
}

#define MAXVAL 100

int sp = 0;          /* Next free stack position. */
double val[MAXVAL];  /* value stack. */

void push(double f)
{
    if(sp < MAXVAL)
        val[sp++] = f;
    else
         printf("\nError: stack full can't push %g\n", f);
}

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


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

/* Getop: get next operator or numeric operand. */
int getop(char s[])
{
    int i = 0;
    int c;
    int next;

    while((s[0] = c = getch()) == ' ' || c == '\t')
        ;
    s[1] = '\0';

    /* Not a number but may contain a unary minus. */
    if(!isdigit(c) && c != '.' && c != '-')
        return c;               

    if(c == '-')
    {
        next = getch();
        if(!isdigit(next) && next != '.')
        {
           return c;
        }
        c = next;
    }
    else
        c = getch();
 
    while(isdigit(s[++i] = c))
            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];
int bufp = 0;

/* Getch: get a ( possibly pushed back) character. */
int getch(void)
{
     return (bufp > 0) ? buf[--bufp]: getchar();
}

/* unGetch: push character back on input. */
void ungetch(int c)
{
    if(bufp >= BUFSIZE)
        printf("\nUnGetch: too many characters\n");
    else
        buf[bufp++] = c;
}

void showtop()
{
    if(sp > 0)
    {
        printf("top %f\n", val[sp-1]);
    }
    else
        printf("Empty stack\n");
}

void clear()
{
    sp = 0;
}