C programming; lvalue required [duplicate]

Question

Possible Duplicate:
Lvalue required error

I am having an error in my C program

main () {
    int arr[] = {1, 2, 3, 4};
    printf("%d", *arr);
    arr++;
    printf("%d", *arr);
}

When I compile this code I get lvalue required error. for the line with arr++. Any Help!

Answer 1

The operand of the pre- and postfix versions of ++ and -- must be a modifiable lvalue. Unfortunately, array expressions such as arr are non-modifiable lvalues, hence your error message.

If you want to walk the array using a pointer expression, you'll have to declare a second pointer and set it to point to the first element of the array.

int *p = arr; // or &arr[0] - in this context they evaluate to the same thing

Answer 2

arr is a constant, you can't change its value. You can add a

int *p = arr;

And then do a

p++;

Answer 3

Your problem is that arr is an array and arrays are not lvalues. You need a pointer.

int arr[] = {1, 2, 3, 4};
int *p = &arr;
printf("%d", *p);
p++;
printf("%d", *p);

Answer 4

lvalue generally refers to the value on the left of an assignment (=) operator. Since arr++ is shorthand for arr = arr + 1, that's what it's referring to.

Basically, arr as an array, and there is no support for changing the value of an array. I think you want a pointer to the array. Pointers can be incremented in the way your code attempts to.

Answer 5

arr is constant, you can not change it's value.

Keep it simple, access the array this way:

int main (int argc, char *argv[]) {
    int arr[] = {1, 2, 3, 4};
    printf("arr[0] == %d\n", arr[0]);
    printf("arr[1] == %d\n", arr[1]);
    printf("arr[2] == %d\n", arr[2]);
    printf("arr[3] == %d", arr[3]);

    return 0;
}

I recommend this documentation for lvalues and rvalues.

And also, this c-faq about "Arrays are not pointers".