How to use lists in R

In the [last post](http://ift.tt/18yhf3k), I went over the basics of lists, including constructing, manipulating, and converting lists to other classes. Knowing the basics, in this post, we'll use the **apply()** functions to see just how powerful working with lists can be. I've done two posts on apply for dataframes and matrics, [here](http://ift.tt/1i25x6o) and [here](http://ift.tt/1A8JbS4), so give those a read if you need a refresher. Intro to apply-based functions for lists There are a variety of apply functions that can be used depending on what you want to do. The table below shows the function, what it inputs, and what it outputs: For example, if we have a list and you want to produce a vector (of the same length), we use **sapply()**. If we have a vector and want to produce a list of the same length, we use **lapply()**. Let's try an example. The syntax of lapply is: lapply(INPUT, function(x) (Some function here)) where INPUT, as we see from the table above, must be a vector or a list, and function(x) is any kind of function that takes **each element of the INPUT** and applies the function to it. The function can be something that already exists in R, or it can be a new function that you've written up. For example, let's construct a list of 3 vectors like so: mylist=5} #apply that function to the list sapply(mylist, span.fun) Creating a list using lapply You don't need to have a list already created to use lapply() - in fact, lapply can be used to _make_ a list. This is because the key about **lapply()** is that it *returns* a list of the same length as whatever you input. For example, let's initialize a list to have 2 empty matrices that are size 2x3. We'll use lapply(): our input is just a vector containing 1 and 2, and the function we specify uses the matrix() function to construct a 2x3 matrix of empty cells for each element of this vector, so it returns a list of two such matrices. If instead of empty matrices you wanted to fill these matrices with random numbers, you could do so too. Check out both possibilities below. #initialize list to to 2 empty matrices of 2 by 3 list2

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