Error: expression must be a modifiable lvalue (tal * tal)

Question

So here is my problem:

#include "stdafx.h"

int kvad(int tal) {
    int sum;
    tal * tal = sum; /* The left "tal" has the error: Expression must be a modifiable lvalue*/
    return sum;
}

int kub(int alt) {
    int pro;
    alt * alt * alt = pro; /* The left "alt" has the error: Expression must be a modifiable lvalue*/
    return pro;
}

int _tmain(int argc, _TCHAR* argv[])
{
    int ggr, gda, tre, tva;
    printf("Hur många tal att multiplicera: ");
    scanf_s("%d", ggr);
    printf("\n i   i * i   i * i * i\n=== ======= ===========\n");
    for (gda = 1; gda <= ggr; gda++) {
        tva = kvad(gda);
        tre = kub(gda);
        printf("%2d%6d%10d\n", gda, tva, tre);
    }
    return 0;
}

I do not know if the last part is needed but I am not sure so I did include it anyway.

I know that there are other threads with similar problems but I can't find the solution there.

Answer 1

An lvalue is a value that can be assigned to, and if often a variable. By contrast, an rvalue is a value that cannot be assigned to. The names come from the trend that lvalues tend to appear on the left side of an assignment, whereas rvalues tend to appear on the right side of an assignment. In your code, you have:

tal * tal = sum;

Which is an error because tal * tal yields a value that can't be assigned to. The expression results in a number, but this is different from a variable because it doesn't make sense to assign a number to another number. It would be like saying 5 = variable.

Also remember that assignment is non commutative. That is, tal * tal = sum is not equivalent to sum = tal * tal. The name on the left of the = is what will be assigned the value on the right, so the name on the left must always be something that can be assigned to, that is, an lvalue.