How to evaluate this statement?

Question

If I defined the absolute value of a number as ``` #define ABS(X) X >= 0 ? X : (-1) * X ``` what would ``` ABS(2) + ABS(-3) ``` evaluate to? My friend claims it evaluates to 2.

Accepted Answer

When you type:

```
ABS(2) + ABS(-3)
```

This will substitute out to:

```
2 >= 0 ? 2 : (-1) * 2 + -3 >= 0 ? -3 : (-1) * -3
```

You can break this down:

```
2 >= 0 ? 2 : -5 >= 0 ? -3 : (-1) * -3
```

Or:

```
2 >= 0 ? 2 : (-5 >= 0 ? -3 : (-1) * -3)
```

The first part (`2 >= 0`) evaluates as true, so it evaluates to `2`.

Note that you could easily solve this by writing the macro as:

```
#define ABS(X) ((X) >= 0 ? (X) : (-1) * (X))
```

This will keep the evaluation order as expected, and cause it to resolve to 5 instead of 2. That being said, using an inline function would be much cleaner, and avoid this entire scenario.

How to evaluate this statement?

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