infix to postfix conversion in C

main.c

#include <stdio.h>
#include <stdlib.h>
#include "Yigit.h"
#include <string.h>

int oncelik(char islec) {
    switch(islec) {
    case '+':
    case '-':
        return 1;
    case '*':
    case '/':
        return 2;
    case '^':
        return 3;
    default:
        return 0;
    }
    printf("(oncelik) hatali:%c\n",islec);
    exit(-1);
}
char *in2post(char *ifade) {
    struct Yigit *PostfixStack = yaratYigit(100);
    struct Yigit *OperatorStack = yaratYigit(100);
    int elementPos = 0;

    while(ifade[elementPos] != 'x'){
        int priority = oncelik(ifade[elementPos]);
        if(priority ==0){
            ekleYigit(PostfixStack,ifade[elementPos]);
        }else{
            if(bosmuYigit(OperatorStack)==1){
                ekleYigit(OperatorStack,ifade[elementPos]);
            }else{
                int priorityTepeOperator = oncelik(tepedekiYigit(OperatorStack));
                if(priority>priorityTepeOperator){
                    ekleYigit(OperatorStack,ifade[elementPos]);
                }else{
                    while((priority<=priorityTepeOperator) && bosmuYigit(OperatorStack) != 1){
                        char tepeOperator = cikarYigit(OperatorStack);
                        ekleYigit(PostfixStack,tepeOperator);
                        priorityTepeOperator = oncelik(tepedekiYigit(OperatorStack));
                    }
                    ekleYigit(OperatorStack,ifade[elementPos]);
                }
            }
        }
        elementPos++;
    }
    while(bosmuYigit(OperatorStack)!=1){
        char tepeOperator = cikarYigit(OperatorStack);
        ekleYigit(PostfixStack,tepeOperator);
    }
    return PostfixStack->dizi;
}
int main() {
    char infix[8] = {'3', '*' , '5' , '-' , '6' , '/' , '2','x'};
    printf("%s",in2post(infix));
    return 0;
}

Yigit.h

struct Yigit {
    int tepe; /* yigitin en üstündeki elemanin indeksi */
    int boy; /* yigitin alabilecegi en çok eleman sayisi */
    char *dizi; /* yigit yaratilirken verilen boyda bir dizi */
};
struct Yigit *yaratYigit(int boy);
void ekleYigit(struct Yigit *y, char X);
char cikarYigit(struct Yigit *y);
int dolumuYigit(struct Yigit *y);
int bosmuYigit(struct Yigit *y);
char tepedekiYigit(struct Yigit *y);
void yazdirYigit(struct Yigit *y);

struct Yigit *yaratYigit(int boy) {
    struct Yigit *y= (struct Yigit *)malloc(sizeof(struct Yigit));
    y->tepe= -1; /* bos oldugunu gösterir */
    y->boy= boy; /* yigitin boy bilgisi saklanir */
    y->dizi= (char *)malloc(sizeof(char)*boy);
    return y;
}
void ekleYigit(struct Yigit *y, char X) {
    y->tepe++;
    y->dizi[ y->tepe]= X;
}
char cikarYigit(struct Yigit *y) {
    y->tepe--;
    return y->dizi[ y->tepe+1];
}
int dolumuYigit(struct Yigit *y) {
    if(y->tepe >= (y->boy-1)) return 1; /* evet dolu */
    return 0; /* hayır dolu değil */
}
int bosmuYigit(struct Yigit *y) {
    if(y->tepe == -1) return 1; /* evet boş */
    return 0; /* hayır boş değil */
}
char tepedekiYigit(struct Yigit *y) {
    return y->dizi[y->tepe];
}
void yazdirYigit(struct Yigit *y) {
    int j;
    for(j=0; j<=y->tepe; j++){
        printf("%c",y->dizi[j]);
    }
    printf("\n");
}