A *predicate function* is *"a function that either returns TRUE or FALSE."* While a *predicate functional* *"takes a vector and a predicate function and do something useful."*^{***}

In other words, the predicate functionals take in `.x`

, which is a vector, a dataframe, or a list, and test the predicate on every element of `.x`

. For example, you can test if every element is numeric with the `is.numeric()`

predicate from R-Base, or if the mean of some elements is under 5 with this mapper: `~mean(.x) < 5`

.

Which of these functions is **NOT** a predicate?

50 XP

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