I have been asked to calculate a postfix expression , which means that for every consequtive 2 numbers there is an arithmetical operation in between , for example fot the given string “123*+” the result will be 7 –> 1+(2*3)
I tried to write the code in C but for some reason i get that the result is 0
can someone tell me why? or if my direction is wrong?
And how can i return to the user that there was an error with the given string such as uknown char or empty string if the function needs to return an int. I can’t return NULL right.?
thank you

#include <stdio.h>
#define ERROR -2147483648
int Postfix(char *str) {
    if(!str)
        return ERROR;
    int str_length;
    for ( str_length = 0;str[str_length]!=''; str_length++); // calculating the exact number of chars in the string
    int mid=str_length/2;// the length of the string will always be odd because its not possible that the number of digits equals to the number of arithmetics puncuation sice for every artihmetic
    //puncutation there are 2 digits.
    int *current, *prev, result=0;

    char *arithmetic;
    arithmetic=str+mid+2;// address of the first arithemetic sign in the array

    prev=str+mid;// address of the  second last digit in the array

    current=str+mid+1; // address of the last digit in the array

    for (int i = 0; i < mid; i++) { // the loop will run the exact number of arithmetic operations
        prev=str+mid-i;
        arithmetic=str+mid+i;
        switch(*arithmetic) {
            case '+': {
                result+=(*current)+(*prev);
                break;
            }
            case '-': {
                result+=(*current)-(*prev);
                break;
            }
            case '*': {
                result+=(*current)*(*prev);
                break;
            }
            case '/': { // need to handle the case with division by 0?
                result+=(*current)/(*prev);
                break;
            }
            default: { // need to handle the case where an arithmetic which is not (+,-,*,/) was inserted?
                result+=0;
                break;
            }

        }
        *current=result;
    }
    return result;
}
int main() {
char c[]="231*+";
int result=Postfix(&c);
    printf("%d",result);
    if(result!=ERROR)
        printf("The result is: %d",result);
    else
        printf("An error occured.");
return 0;
}