Question

I am getting an "invalid operands to binary" error for the lines "pp->element == value" and "pp->left->element > value".

int insert(NodeT **pp, Element value)
{

    if (pp->element == value)
    {
        return 0;
    }

    else if (pp->left->element > value)
    {
        pp->left = allocateNode(value);
    }
    
    else
    {
        pp->right = allocateNode(value);
    }
}

Answer 1

According to my knowledge,as the code is not completely given,I can say that either of the reasons:

1.)

When comparing two values, they must be of the same type. This is because pointer arithmetic in C is based on the size of the pointers, so for example if you subtract char* from int* you have a 1-byte object and a 4-byte object, and so there is no way for the compiler to know whether the result should be in 1-byte units or 4-byte units.

I realize your two values are both single bytes, but it's still not allowed in standard C (which makes no assumptions that objects of different types would be stored in the same area, etc.).

2.)In pp->element the "element" would not be of type Nodet or else it is not a pointer.