# Exercise 4.6 - RPN Calculator with variables¶

## Question¶

Add commands for handling variables. (It’s easy to provide twenty-six variables with single-letter names.) Add a variable for the most recently printed value.

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

#define MAXOP 100
#define NUMBER '0'
#define BUFSIZE 100
#define MAXVAL 100

char buf[BUFSIZE];
int bufp = 0;
int sp = 0;
double val[MAXVAL];

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

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

/* reverse polish calculator */

int main(void) {
int type, var = 0;
double op2, v;
char s[MAXOP];
double variable[26];

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:zero divisor\n");
break;
case '=':
pop();
if (var >= 'A' && var <= 'Z')
variable[var - 'A'] = pop();
else
printf("error: novariablename\n");
break;

case '\n':
v = pop();
printf("\t%.8g\n", v);
break;
default:
if (type >= 'A' && type <= 'Z')
push(variable[type - 'A']);
else if (type == 'v')
push(v);
else
printf("error: unknown command %s\n", s);
break;
}
var = type;
}
return 0;
}

void push(double f) {
if (sp < MAXVAL)
val[sp++] = f;
else
printf("error:stack full, cant 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, c;

while ((s[0] = c = getch()) == ' ' || c == '\t')
;
s[1] = '\0';
if (!isdigit(c) && c != '.')
return c;

i = 0;
if (isdigit(c))
while (isdigit(s[++i] = 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;
}
```

## Explanation¶

This adds variables to our RPN calculator. An example execution goes like this.

```10 A = 20 B = A B +
30
v
30
```

The RPN notation for assigning to variables is like this 10 A =. When an = sign is encountered the previous value is popped and the value that is stored in var variable (that is the previous one is taken) and then it’s value is assigned to the next popped variable. Thus two variables A and B are set in the above expression.

Then A B + acts as if we are acting on two numbers. A special variable v is used to assign to the last printed value.