# Sorting the UpsideDown

### Davide Bellone ・1 min read

Recently I've noticed a funny (ehm...) thing.

## The guilty

*It isn't true that the inverse of a negative number is always a positive number*. Or, equally, that `(x < 0) -> (-x > 0)`

.

You could say "Hey, -(-5) == 5". Yes, that's true.

We can test it this way:

```
[Test]
public void TestInverse()
{
int x = -5;
int y = -x;
Assert.IsTrue(y > 0);
}
```

But what if we consider **edge cases**?

```
[Test]
public void TestInverse_EdgeCase()
{
int x = int.MinValue;
int y = -x;
Assert.IsTrue(y > 0);
}
```

It will fail. Miserably.

## The reason

The reason is simple: **the sign occupies space**.

In fact, the range of int is *-2,147,483,648* to *2,147,483,647*. The inverse of *-2,147,483,648* would cause overflow, and returns the same value.

## The lesson

Why am I pointing at this?

Imagine you are implementing a `CompareTo(x, y)`

method, you know, the usual one that returns `0`

if the values are considered equal, -1 if x < y and 1 if x > y.

You could use this method to sort an array.

Now you want to sort that array descending. What to do?

This edge case explains why it is a terrible idea to use `CompareTo(-x, -y)`

. Results can be unexpected.

The best solution is to simply switch the parameters: `CompareTo(y, x)`

.