# Functions that eat functions

## Master R

We human eat different kind of foods. Similarly, functions can eat different kind of things. We’ve seen functions that eat a number, a data frame, and a vector of numbers. Today, I’m going to show you how to create functions that eat functions. By that, I mean functions that take functions as input. For example, look at the following function.

It uses a function as input (func) and outputs another function (function(x, f)). Notice the output function has two parameters: one is a numeric vector and the other is yet another function!

We can use arg_func to implement a function that takes a vector of numbers and a function as inputs, and returns the elements of the vector where the input function returns the biggest value.

For example, when applying the function $f(x)=x^2$ to the vector -10:5, it should obtain the biggest value at x=-10. And when appling it to -5:5, the maximum value should happen at x=-5 and x=5. Let’s see if arg_max() gives us the same results.

Indeed, arg_max() gives us the correct results. We can also implement arg_min() in the same fashion.

Similarly, we can implement arg_mean() and arg_median().