What does "!" mean in scala?

Question

I am looking at a piece of code with the following: `graph.vertices.filter(!_._2._1)` I understand that `_` are wildcard characters in `scala` but I do not know what the `!` is supposed to do. What does `!` mean in scala?

Accepted Answer

Scala doesn't have operators at the syntax level. All operations are methods.

For example, there is no add operator in the syntax, but numbers have a `+` method:

```
2.+(3)   // result is 5
```

When you write `2 + 3`, that's actually syntax sugar for the expression above.

Any type can define a `unary_!` method, which is what `!something` gets desugared to. Booleans implement it, with the obvious meaning of logical negation ("not") that the exclamation mark has in other languages with C heritage.

In your question, the expression is an abbreviated form of the following call:

```
graph.vertices.filter { t => !(t._2._1) }
```

where `t` is a tuple-of-tuples, for which the first element of the second element has a type that implements `unary_!` and (as required by `.filter`) returns a `Boolean`. I would bet the money in my pocket that the element itself is a `Boolean`, in which case `!` just means "not."

What does "!" mean in scala?

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