Output of ++*p++

Question

Can anyone explain me the output.

#include<stdio.h>
int main() {
    int a[]={10,20,30};
    int *p=a;
    ++*p++;
    printf("%d  %d  %d  %d",*p,a[0],a[1],a[2]);
}

output is 20 11 20 30

Postfix incrementation has a higher precedence, so value of second index should have been incremented. Why is the value of first index incremented?

Answer 1

Due to operator precedence,

++*p++ is same as ++(*(p++)).

That is equivalent to:

int* p1 = p++; // p1 points to a[0], p points to a[1]
++(*p1);       // Increments a[0]. It is now 11.

That explains the output.

Answer 2

This is because the postfix operator returns the value before the increment. So the pointer is well incremented but the prefix operator still applies to the original pointer value.