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.

_"I am given a piece of code for which we have to guess output."_ By whom? In what context? Are you sure you were supposed to write C? — Lightness Races in Orbit, Apr 15 '19 at 14:12
The issues you face are not related to pointers in any way. They are only related to pre-increment operator. You don't need to worry about your knowledge about pointers. — Gerhardh, Apr 15 '19 at 14:48
@LightnessRacesinOrbit Yes I was supposed to write in C and this question is from a test — pankaj, Apr 15 '19 at 17:25

score 8 · Accepted Answer · answered Apr 15 '19 at 14:08

8

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.)

answered Apr 15 '19 at 14:08

Lightness Races in Orbit

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It would be rude not to upvote this. Funny how we both write the bit about C++ at the end. – Bathsheba Apr 15 '19 at 14:09
1

@Bathsheba Anyways `*((++u)++);` looks like an expression to avoid, unless this is some learning exercise.. – sjsam Apr 15 '19 at 14:15
@Bathsheba Yeah, it should be avoided. But it was a learning exercise. – pankaj Apr 16 '19 at 07:05

score 6 · Answer 2 · answered Apr 15 '19 at 14:08

6

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++).

answered Apr 15 '19 at 14:08

Bathsheba

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It would be rude not to upvote this. Funny how we both write the bit about C++ at the end. – Lightness Races in Orbit Apr 15 '19 at 14:10

Why Pointer assignment shows lvalue error when assignments look appropriate?

2 Answers2