Thread: Incrementing Structure Member Array

Will someone please tell me why this

Code:

*++p->vars = NULL;

gives me this error

error: lvalue required as increment operand

vars is an array of pointers to char and p is a pointer to a structure. vars is a member of that structure.

Thank you.

Originally Posted by hk_mp5kpdw

To start off with, study you operator precedence table.

-> has the highest precedence so that should give me the address of my pointer array.

++ and * have the same precedence and associate from right to left so it should increment my array address then take the object in that address and assign NULL to it.

That's my understanding of it and is exactly what I want to do. What am I missing?

You say vars is an array of pointers to char. So:

Code:

char* vars[];

We now that

Code:

*vars == vars[0]

So what you have is:

Code:

(++(p->vars))[0] = NULL;

So, yeah you are wrong, you need to assign a character

Originally Posted by C_ntua

You say vars is an array of pointers to char. So:

Code:

char* vars[];

We now that

Code:

*vars == vars[0]

So what you have is:

Code:

(++(p->vars))[0] = NULL;

So, yeah you are wrong, you need to assign a character

Actually vars == &vars[0] so ++vars would be the same as &vars[1] and so *++vars == vars[1].

Went back and reread section 5.3 of K&R and found my answer.

The correspondence between indexing and pointer arithmetic is very close. By definition, the value of a variable or expression of type array is the address of element zero of the array. Thus after the assignment

pa = &a[0];
pa and a have identical values. Since the name of an array is a synonym for the location of the initial element, the assignment pa=&a[0] can also be written as

pa = a;
Rather more surprising, at first sight, is the fact that a reference to a[i] can also be written as *(a+i). In evaluating a[i], C converts it to *(a+i) immediately; the two forms are equivalent. Applying the operator & to both parts of this equivalence, it follows that &a[i] and a+i are also identical: a+i is the address of the i-th element beyond a. As the other side of this coin, if pa is a pointer, expressions might use it with a subscript; pa[i] is identical to *(pa+i). In short, an array-and-index expression is equivalent to one written as a pointer and offset.

There is one difference between an array name and a pointer that must be kept in mind. A pointer is a variable, so pa=a and pa++ are legal. But an array name is not a variable; constructions like a=pa and a++ are illegal.

Thanks for the replies everyone.