infix to postfix

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

#define TYPE int
#define STACKSIZE 100

typedef struct {
    int top;
    TYPE items[STACKSIZE];
} Stack;

void initialize(Stack *s) {s->top=0;}
void push(Stack *s,TYPE value) {s->items[s->top++]=value;}
TYPE pop(Stack *s) {return s->items[--s->top];}
TYPE peek(Stack *s) {return s->items[s->top-1];}
int isfull(Stack *s) {return s->top<STACKSIZE?0:1 ;}
int isempty(Stack *s) {return s->top==0?1:0;}

void error() {printf("error");exit(-1);}

int isoperator(char e)
{
 if(e == '+' || e == '-' || e == '*' || e == '/' || e == '%') return 1;
 return 0;
}

int priority(char e)
{
 int pri = 0;
 if(e == '*' || e == '/' || e =='%') pri = 2;
 else  if(e == '+' || e == '-') pri = 1;
 return pri;
}

void infix2postfix(char* infix, char * postfix){
    char *i,*p;
    Stack X; char n1;
    initialize(&X);
    i = infix;
    p = postfix; //&postfix[0];
    while(*i){
        while(*i == ' ' || *i == '\t')
            i++;
        if( isdigit(*i) || isalpha(*i) ) {
            while( isdigit(*i) || isalpha(*i)){
                *p = *i;
                p++;
                i++;
            }
            *p++ = ' ';
        }
        if( *i == '(' )
            push(&X,*i++);
        if( *i == ')'){
            n1 = pop(&X);
            while( n1 != '(' ){
                *p++ = n1;
                *p++ = ' ';
                n1 = pop(&X);
            }
            i++;
        }
        if( isoperator(*i) ){
            if(isempty(&X))
                push(&X,*i);
            else{
                n1 = pop(&X);
                while(priority(n1) >= priority(*i)){
                    *p++ = n1;
                    *p++ = ' ';
                    n1 = pop(&X);
                }
                push(&X,n1);
                push(&X,*i);
            }
            i++;
        }
    }
    while(!isempty(&X)){
        n1 = pop(&X);
        *p++ = n1;
        *p++ = ' ';
    }
    *p = '\0';
}





int main()
{
    Stack s; initialize(&s);
    // Input: (((8 + 1) - (7 - 4)) / (11 - 9))
    // Output: 8 1 + 7 4 - - 11 9 - /
    char  inStr[100];
    strcpy(inStr,"(((8 + 1) - (7 - 4)) / (11 - 9))");
    char post[100];
    infix2postfix(inStr,post);
    printf("%s<-->%s\n",inStr,post);
    strcpy(inStr,"a+b/c");
    infix2postfix(inStr,post);
    printf("%s<-->%s\n",inStr,post);
    strcpy(inStr,"a/b+c");
    infix2postfix(inStr,post);
    printf("%s<-->%s\n",inStr,post);
    return 0;
}