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#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>
#include <string.h>
/* The C Programming Language: 2nd Edition
*
* Exercise 7-5: Rewrite the postfix calculator of Chapter 4 to use scanf
* and/or sscanf to do the input and number conversion.
*
* Notes:
* scanf wants allocated memory of some sort, meaning you're either going
* to pass it a pointer (with malloc()'d memory) or you're passing a variable
* with the unary & (address-of) operator. Failing to use scanf correctly
* _will_ create runtime problems! gcc won't tell you what happens without
* -Wall and -Wextra, but scanf segfaults by trying to access an uninitialized
* pointer.
*
* Thus there are two ways to use scanf:
*
* int foo = 0;
* scanf("%d", &foo);
* // do stuff
*
* -- or --
*
* int *foo = malloc(sizeof(int));
* scanf("%d", foo);
* // do stuff
* free(foo);
*
* For the sake of sanity in an easy exercise, I'll be using the former form
* since a few values on the stack won't hurt anything.
*/
#define MAXOP 100
#define MAXVAL 100
#define BUFSIZE 100
void push(double);
double pop(void);
int sp = 0; // Next free stack position
double val[MAXVAL]; // Value stack
char buf[BUFSIZE]; // buffer
int bufp = 0; // next free position in buf
double vars[27];
/* Reverse Polish calculator:
*
* Binary operations (+-*\)
* operand operand operator
*
* Example: 6 minus 2 in Reverse Polish Notation is "6 2 -"
*/
int main() {
char *c;
double a = 0, op2;
char s[MAXOP];
char ltr = '\0';
while (scanf("%s%c", s, <r) == 2) {
if (sscanf(s, "%lf", &a) == 1) {
push(a);
} else if (sscanf(s, "%s", buf)) {
for (c = buf; *c; c++) {
switch (*c) {
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: Cannot divide by zero.\n");
}
break;
default:
printf("Error: Unknown command %s\n", s);
break;
}
}
if (ltr == '\n') {
printf("%.8f\n", pop());
}
}
}
return 0;
}
void push(double f) {
if (sp < MAXVAL) {
val[sp++] = f;
} else {
printf("Error: Stack full. Cannot push %g\n", f);
}
}
double pop(void) {
if (sp > 0) {
return val[--sp];
} else {
printf("Error: Stack empty.\n");
return 0.0;
}
}
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