Negative one for illegal/default values

[diablo84]

Hi:
What is the general consensus on passing of negative one (an impossible value) to a method to denote that a default value must be used or that parameter should be ignored? I have seen it done in many places, but is it considered an acceptable practice? When should/shouldn't it be used?
Example:

public void doSomething (int param1, int param2, int param3) {
    if (param1 != -1)
        _myObject.x = param1;
    if (param2 != -1)
        _myObject.y = param2;
    ...
    ...
}

[Zack]

Personally I think -1 is used because it is not an acceptable number to call an array index. Negative one is generally accepted as "infinity" in computer coding for legacy reasons, so it has kind of, more or less, become the "undefined" number. Since everything is zero- or one-based (arrays, Strings, basic counting, string lengths, etc.), negative one will hardly ever be a valid value.

You can use it wherever you see fit, really. Just try to think of it like this: "Can -1 be an acceptable value in this parameter? If yes, then it cannot be a default value. If not, then it can be."

Much luck.