why lvalue required as increment operand error? [duplicate]

Answer 1

Postfix increment binds tighter than prefix increment so what you would want would be something like:

a = b + (++c)++;

This is not legal C, though, as the the result of prefix increment (like the result of postfix increment in your example) is not an lvalue. This means that it's just a value; it doesn't refer to a particular object like 'c' any more so trying to change it makes no sense. It would have no visible effect as no object would be updated.

Personally I think that doing it in two statements is clearer in any case.

a = b + c + 1;
c += 2;

Answer 2

LVALUE means, that there isn't a variable the operation is supposed to be performed on.

C files are basically nothing but text files, which require a particular formatting, so the compiler can understand it.

Writing something like ++Variable++ is complete nonsense for the compiler.

You can basically imagine ++c as:

Var += 1;
return Var;

while c++ is:

int Buf = Var;
Var += 1;
return Buf;

To 'repair' your code:

void main() {
    int a=1,b=5,c=3;
    a = b + (++c);  //Equals 5 + 4
    printf("a=%d   b= %d   c= %d \n",a,b, ++c);  //a = 9, b = 5, c = 5
}

This way, you'll get the result you wanted, without the compiler complaining.

Please remember, that when using ++c or c++ in a combined operation, the order DOES matter. When using ++c, the higher value will be used in the operation, when using c++, it will operate with the old value.

That means:

int a, c = 5;
a = 5 + ++c;  //a = 11

while

int a, c = 5;
a = 5 + c++;  //a = 10

Because in the latter case, c will only be '6' AFTER it is added to 5 and stored in a.