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#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
/* The C Programming Language: 2nd Edition
*
* Exercise 4-4: Add commands to print the top element of the stack without
* popping, to duplicate it, and to swap the top two elements. Add a
* command to clear the stack.
*
* Answer: Achieving this is pretty simple. Make your functions, then assign
* them to "commands", which are basically any special characters that aren't
* being used in the switch statement. Pretty basic manipulation of arrays.
*
* Note that any expression that has subtraction at the end needs either a
* space or an additional newline input after it in order to calculate. This
* is a limitation of the getchar() based approach.
*/
#define MAXOP 100
#define NUMBER '0'
#define MAXVAL 100
#define BUFSIZE 100
int getop(char []);
void push(double);
double pop(void);
int getch(void);
void ungetch(int);
void stack_top(void);
double dupe_top(void);
void swap_top_two(void);
void clear_stack(void);
int sp = 0; // Next free stack position
double val[MAXVAL]; // Value stack
char buf[BUFSIZE]; // buffer for ungetch
int bufp = 0; // next free position in buf
/* Reverse Polish calculator:
*
* Binary operations (+-*\%)
* operand operand operator
*
* Example: 6 minus 2 in Reverse Polish Notation is "6 2 -"
*/
int main() {
int type;
double op2;
char s[MAXOP];
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 != 0.0) {
push(pop() / op2);
} else {
printf("Error: Cannot divide by zero.\n");
}
break;
/* Yay for modulus! */
case '%':
op2 = pop();
if (op2 != 0.0) {
push((int)pop() % (int)op2);
} else {
printf("Error: Cannot modulo by zero.\n");
}
break;
/* Top of stack */
case '?':
stack_top();
break;
/* Dupe the top of the stack */
case '#':
dupe_top();
break;
/* Swap the top two */
case '~':
swap_top_two();
break;
/* Clear the stack */
case '!':
clear_stack();
break;
case '\n':
printf("\t%.8g\n", pop());
break;
default:
printf("Error: Unknown command %s\n", s);
break;
}
}
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;
}
}
int getop(char s[]) {
int i = 0;
int c, next;
while ((s[0] = c = getch()) == ' ' || c == '\t') {
}
s[1] = '\0';
/* The final check is for negative numbers. */
if (!isdigit(c) && c != '.' && c != '-') {
return c;
}
/* The second half of this if-statement accounts for negatives */
if (c == '-') {
next = getch();
if (!isdigit(next) && next != '.') {
return c;
} else {
c = next;
}
} else {
c = getch();
}
while (isdigit(s[++i] = c)) {
c = getch();
}
if (c == '.') {
while (isdigit(s[++i] = c = getch())) {
}
}
s[i] = '\0';
if (c != EOF) {
ungetch(c);
}
return NUMBER;
}
int getch(void) {
return (bufp > 0) ? buf[--bufp] : getchar();
}
void ungetch(int c) {
if (bufp >= BUFSIZE) {
printf("ungetch: Too many characters.\n");
} else {
buf[bufp++] = c;
}
}
void stack_top(void) {
if (sp > 0) {
printf("Top of stack is %8g\n", val[sp - 1]);
} else {
printf("Error: Stack empty.\n");
}
}
double dupe_top(void) {
double temp = pop();
push(temp);
push(temp);
}
void swap_top_two(void) {
double tmp1, tmp2;
tmp1 = pop();
tmp2 = pop();
push(tmp1);
push(tmp2);
}
void clear_stack(void) {
sp = 0;
}
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