C code to eveluate Postfix Expression

/*C program to evaluate the given postfix expression*/

# include<stdio.h> 
# include<conio.h> 

typedef struct 
{ 
int a[100]; 
int top; 
}STACK; 
void push(STACK *s,int x) 
{ 
if(s->top==99) 
printf("STACK OVERFLOW\n"); 
else 
s->a[++s->top]=x; 
} 

int pop(STACK *s) 
{ 
int x; 
if(s->top<0) 
printf("STACK UNDERFLOW\n"); 
else 
{ 
x=s->a[s->top--]; 
return x; 
} 
} 
int operation(int p1,int p2,char op) 
{ 
switch(op) 
{ 
case '+':return p1+p2; 
case '*':return p1*p2; 
case '-':return p1-p2; 
case '/':return p1/p2; 
} 
} 
int evaluate(char pos[]) 
{ 
STACK s1; 
int p1,p2,result,i; 
s1.top=-1; 
for(i=0;pos[i]!='\0';i++) 
if(isdigit(pos[i])) 
push(&s1,pos[i]-'0');/*use to find the integer value of it*/ 
else 
{ 
p2=pop(&s1); 
p1=pop(&s1); 
result=operation(p1,p2,pos[i]); 
push(&s1,result); 
}/*end of for loop*/ 
return pop(&s1); 
} 
int main() 
{ 
char postfix[100]; 
//clrscr(); 
printf("Please Enter the VALID POSTFIX string\n\n Operands are SINGLE DIGIT\n\n"); 
gets(postfix); 
printf("The Result is==>%d",evaluate(postfix)); 
getch(); 
}/*end of main*/

/* Output of Postfix Expression Evaluation Program */

Output of Postfix Expression Program

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