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#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
/* The C Programming Language: 2nd Edition
*
* 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).
*/
#define STACKSIZE 100
#define NUMBER '0'
double stack[STACKSIZE];
double *stp = stack; // stack pointer
void push(double);
double pop(void);
/* This solution only offers basic arithmetic as proof of concept. See
* Chapter 4's exercises on how to extend this to a fully featured
* calculator. Note that for most shells, you'll need to escape the
* multiplication symbol (*) or it will expand to every file in the current
* directory; NOT what you want in this situation. :)
*
* So if you want 6 × 8 for example, you'll need to type `6 8 \*`
*/
int main(int argc, char *argv[]) {
int argp; // current argument position
double op2;
for (argp = 1; argp < argc; argp++) {
int argtype = opt_type(argv[argp]);
switch(argtype) {
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: divide by zero\n");
}
break;
case NUMBER:
push(atof(argv[argp]));
break;
}
}
printf("%f\n", pop());
}
void push(double num) {
if (stp >= stack + STACKSIZE) {
printf("Cannot push: stack full.\n");
return;
}
*(++stp) = num;
}
double pop(void) {
if (stp == stack) {
printf("Cannot pop: stack empty.\n");
return 0.0;
}
return *(stp--);
}
int opt_type(char op[]) {
int i = 0;
switch(op[i]) {
case '*':
return '*';
case '+':
return '+';
case '/':
return '/';
case '-':
if (op[i+1] == '\0') {
return '-';
}
break;
}
while (isdigit(op[i])) {
i++;
}
if (op[i] == '\0') {
return NUMBER;
}
}
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