Why won't this while loop work?

[trueblue]

Good day,

I am trying to use this method but the literals "add", "subtract" or "divide" are not stopping the while loop from iterating. Why? Is there a better or correct way of doing this? Many thanks.

public String getOperation()
{
String operatorSelect = Dialog.request("Please input add, subtract or multiply");

while(!operatorSelect.equals("add") || !operatorSelect.equals("subtract") || !operatorSelect.equals("multiply"))
{
Dialog.alert("This is not a valid operator, please try again");
operatorSelect = Dialog.request("Please input add, subtract or multiply");
}
return operatorSelect;
}

[Fubarable]

You may want to use Venn diagrams to help you visualize this type of problem. You'll see that your while condition encompasses all possible conditions -- is always true. Now try drawing the Venn diagram using && instead of ||, and see what comes out.

[Eranga]

Simply read a bit about logical operators in Java. Then you can have a clear idea about the || and && operators, and what's the most suitable for yours.

[trueblue]

Thanks for your inputs.

The loop works fine when || is replaced with && or ^ but the statement didn't seem to read right?
OR seemed the right thing to say rather than AND....

[Fubarable]

Your first statement is equivalent to this:

int x = getUserInput();
while ((x != 2) || (x != 3)
{
  x = getUserInput();
}

Now tell me when the boolean condition in the while statement will ever be false? Do you see the fallacy here?

If the user enters 2, the second statement is true and because it uses an or operator, the whole statement is true. Same for 3, and same for any number entered. Your logic is the same. Again, learn to use Venn diagrams as most of us understand this better visually. Best of luck.

[trueblue]

Ah yes, I see what you're saying - the compound expression will always evaluate to true because if the user inputted int variable equals 2 and returns false for the first part of the compound expression then it will evaluate to true for the second part because 2 does not equal 3.
Many thanks for your help and insightfulness.

[Fubarable]

Glad it helped. Good luck with your projects.