How can I put sin cos function into this RPN calculator - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <ctype.h>
#define MAXVAL 100
#define MAXOP 100
#define NUMBER '0'
#define BUFSIZE 100
#define sinn 'sin'
int sp = 0;
double val[MAXVAL];
char buf[BUFSIZE];
int bufp = 0;
double aatof(char []);
void push(double);
double pop(void);
int ggetop(char []);
int ggetch(void);
void unggetch(int);
int main()
{
int type;
double op2;
int op3;
char s[MAXOP];
while ((type = ggetop(s)) != EOF)
{
switch (type)
{
case NUMBER:
push(aatof(s));
break;
case '+':
push(pop() + pop());
break;
case '*':
push(pop() * pop());
break;
case '-': // negative number
if (isdigit(ggetop(s))){
push(0-aatof(s));
break;}
case '--': // minus
op2 = pop();
push(pop() - op2);
break;
case '/':
op2 = pop();
if (op2 != 0.0)
push(pop() / op2);
else
printf("error: zero divisor\n");
break;
case '%':
op3 = (int)pop();
if (op3 != 0)
push((int)pop() % op3);
else
printf("error: zero divisor\n");
break;
case 'sin':
push(sin(pop()));
break;
case '\n':
printf("\t%.8g\n", pop());
break;
default:
printf("error: unknown command %s\n", s);
break;
}
}
return 0;
}
int ggetop(char s[])
{
int i, c;
while ((s[0] = c = ggetch()) == ' ' || c == '\t')
;
s[1] = '\0';
if (!isdigit(c) && c != '.')
i = 0;
if (isdigit(c))
while (isdigit(s[++i] = c = ggetch()))
;
if (c == '.')
while (isdigit(s[++i] = c = ggetch()))
;
s[i] = '\0';
if (c != EOF)
unggetch(c);
return NUMBER;
}
int ggetch(void)
{
return (bufp > 0) ? buf[--bufp] : getchar();
}
void unggetch(int c)
{
if (bufp >= BUFSIZE)
printf("unggetch: too many characters\n");
else
buf[bufp++] = c;
}
double aatof(char s[])
{
double val, power;
int i, sign;
for (i = 0; isspace(s[i]); i++)
;
sign = (s[i] == '-') ? -1 : 1;
if (s[i] == '+' || s[i] == '-')
i++;
for (val = 0.0; isdigit(s[i]); i++)
val = 10.0 * val + (s[i] - '0');
if (s[i] == '.')
i++;
for (power = 1.0; isdigit(s[i]); i++)
{
val = 10.0 * val + (s[i] - '0');
power *= 10.0;
}
return sign * val / power;
}
void push(double f)
{
if (sp < MAXVAL)
val[sp++] = f;
else
printf("error: stack full, can't push %g\n", f);
}
double pop(void)
{
if (sp > 0)
return val[--sp];
else
{
printf("error: stack empty\n");
return 0.0;
}
}
I am not sure how can I put the sin cos pow function into different cases. I've tried to use isalpha() in ggetop(char s[]) and turn letters into integer but it doesn't seem to work. what I want to do is process the letter and give back the ASCII number, and add them into type. I wonder how to process the letters altogether at once and not individually.
if(isalpha(c)){
s[++i] = c = ggetch();
return c;}
are there other ways to add sin cos and pow functions into the programs?