Why Pointer assignment shows lvalue error when assignments look appropriate?

Question

I am given a piece of code for which we have to guess output.

My Output: 60

#include <stdio.h>

int main()
{
    int d[] = {20,30,40,50,60};
    int *u,k;
    u = d;
    k = *((++u)++);
    k += k;
    (++u) += k;

    printf("%d",*(++u));

    return 0;
}

Expected: k = *((++u)++) will be equal to 30 as it will iterate once(++u) and then will be iterated but not assigned. So we are in d[1].

(++u) += k here u will iterate to next position, add k to it and then assign the result to further next element of u.

Actual result:

main.c: In function ‘main’:
main.c:16:16: error: lvalue required as increment operand
     k = *((++u)++);
                ^
main.c:18:11: error: lvalue required as left operand of assignment
     (++u) += k;

And this has confused me further in concepts of pointers. Please help.

Answer 1

As the compiler has told you, the program is not valid C.

In C, pre-increment results in an rvalue expression, which you may not assign to or increment.

It's not a logical problem; it's a language problem. You should split that complex formula into multiple code statements.

That's all there is to it.

(In C++ it's an lvalue though and you can do both those things.)

Answer 2

In C, ++a is not an l-value.

Informally this means that you can't have it on the left hand side of an assignment.

It also means that you can't increment it.

So (++a)++ is invalid code.

(Note that it is valid C++).