Strange -1.#IND error in VB.NET

Question

Here's a weird calculation error in VB.NET. I've simplified my problem to the following. I can write:

Console.WriteLine(-0.78125 ^ 2.5)

and get -0.53947966093944366.

But if I change it to Console.WriteLine((-0.78125 + 0) ^ 2.5), I get -1.#IND.

Or, if it try:

Dim D as Double = -0.78125
Console.WriteLine(D ^ 2.5)

I also get -1.#IND.

I can only get the calculation to return a result if I use a single literal number in the expression, but when I use a variable of any data type I get -1.#IND.

I've read the other posts that explaining "-1.#IND", but they indicate that one of the numbers in the expression is NaN, which is not the case here. Why is this happening?

Answer 1

You can figure out what's going on by trying following:

Console.WriteLine(-1 ^ 0.5)

It prints -1, but in fact it is the same as sqrt(-1) which does not have a result in Real numbers. That's weird, isn't it? So what's going on here?

^ has higher precedence than -, so -1 ^ 0.5 is actually -(1 ^ 0.5). That's why it prints -1.

You can check the precedence list here: Operator Precedence in Visual Basic

The same happens with your code. -0.78125 ^ 2.5 is actually interpreted as -(0.78125 ^ 2.5) which is valid, but if you do (-0.78125 + 0) ^ 2.5 (or even (-0.78125) ^ 2.5) it's not valid anymore, because it would require calculating square root from negative value. That's why you're getting NaN (or -1#IND).