Operator and Pointer precedence-CodePudding

Below is the problem I found on the internet.

int main()
{

    int a[4] = { 10, 21, 32, 43};
    int *p = a   3;
      *--p;
      *p--;


    p[2]  = p[1];


    for (int i = 0; i < 4;   i)
        printf("%d - %d\t", a[i]);

        return 0;
}

//What will be the output?

answer : 10 21 34 77

I understood a lot of things, but the only thing I'm stuck on:

What is the difference between ( *--p) and ( *p--) ?

Shouldn't these two give the same result? Because (*p--) and (*--p) give the same result in the compiler. The compiler I use is Code::Blocks

CodePudding user response：

Because (*p--) and (*--p) give the same result in the compiler.

No, they do not. *p-- decrements p but applies * to the value of p before the decrement. *--p applies * to the value of p after the decrement.

What is the difference between ( *--p) and ( *p--) ?

*--p decrements p and increments the object it points to after the decrement.

*p-- decrements p but increments the object it points to before the decrement.

CodePudding user response：

What is the difference between ( *--p) and ( *p--) ?

The difference is that --p decrements p and resolves to the new value of p, whereas p-- decrements p and resolves to the old value of p.

* works identically on both - performing indirection on p, incrementing the value p points to, and resolving to this new value.

#include <stdio.h>

int main(void)
{   
    int a = 10;         
    int b = 10;         
                       
    /* This prints 9, and `a` is now 9 */
    printf("%d\n", --a);
    /* This prints 10, and `b` is now 9 */
    printf("%d\n", b--);
    
    /* This prints 9 and 9 */
    printf("%d %d\n", a, b);
}

Shouldn't these two give the same result? Because (*p--) and (*--p) give the same result in the compiler.

The order here matters, using (*--p) before (*p--) would resolve to the same element twice. Using (*p--) before (*--p) resolves to different elements.

#include <stdio.h>

int main(void)
{
    int a[8] = { 1, 2, 3, 4, 5, 6, 7, 8 };
    int *p = a   4;
    
    /* 4 then 4 */
    printf("%d ", *--p);
    printf("%d\n", *p--);
    
    p = a   4;
    /* 5 then 3 */
    printf("%d ", *p--);
    printf("%d\n", *--p);
        
}