How does the logical negation operator ! work?

Question

I am not trying to solve any particular problem, but trying to learn R and understand its logical negation operator "!" documented on page http://stat.ethz.ch/R-manual/R-devel/library/base/html/Logic.html It works for me when used in combination with =, in expressions such as: ``` 1 != 2 TRUE ``` But I can't quite comprehend standalone application of this operator. For instance, can I use it to select elements of the list which do not have specific name. Here's my attempt to do that, but it did not work: ``` vector1 <- 1:5 # just making vector of 5 numbers vector2 <- 5:1 # same vector backwards list <- list(Forward=vector1, Backwards=vector2) # producing list with two elements x = "Forward" list[!x] ``` My output is: ``` Error in !x : invalid argument type ``` Will appreciate any hints on where my logic goes wrong in this case, and what are other good uses of this operator except for != case.

Accepted Answer

First, it's probably best not to think of `!=` as `!` acting on `=`, but rather as a separate binary operator altogether.

In general, `!` should only be applied to boolean vectors. So this is probably more like what you are after:

```
vector1 <- 1:5 # just making vector of 5 numbers
vector2 <- 5:1 # same vector backwards
l <- list(Forward=vector1, Backwards=vector2) # producing list with two elements
x = "Forward"
l[!(names(l) %in% x)]
```

where `names(l) %in% x` returns a boolean vector along the names of the list `l` indicating whether they are contained in `x` or not. Finally, I avoided the use of `list` as a variable, since you can see it is a fairly common function as well.

How does the logical negation operator ! work?

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