If you never used R before…

If you never used R before…

If you’re new to R, this “pre-vignette” tutorial serves as an introduction to using evolved. It was largely inspired by the first tutorials in EEB 485 and EEB 391/516 (Population and Community Ecology & Evolutionary processes and Macroevolution), but it is extended and modified to work in our context. This document is meant to serve as a first contact with the tools that will be of major importance in our vignettes with evovled. Nothing related to this tutorial is required to use the R package, but we will be using all the tools explained here, so please make sure you have completed it before then (or to be genrlly familiar with R).

This tutorial will be somewhat long, and if this is your first time using R, it will also be a bit confusing. This is because a reasonable part of what you will see here is pretty abstract. We ask you to trust the learning process and keep reading until the end. We highly encourage you to reach out to other peers more knowledgeable in R, if needed. We know that different folks come with very different levels of comfort with R – which means you will learn a lot with your colleagues, however comfortable you may currently be with R.

What is R and how can I install it?

R is (1) an open source software, (2) a programming language, and (3) a programming environment. The first one means it is a computer program you can freely download, use, or modify to your own needs. The second means it is a language that allows humans and machines to communicate. The third means it gives you everything you need to create your own programs under a specific language. Scientists, especially in the biological sciences, widely use R to run simulations and analyze their datasets, so we will do this too.

If you’re using your own computer and have not yet installed R, you can download and install it for free by pasting this in your web browser:

http://cran.us.r-project.org/

After you download it, install it as you would do with any other program – the installer program will direct you though the major steps. When you open R, you should see something close to Figure 1.

Please download and install R before progressing to install RStudio.

Example of recently-opened R on an Apple computer. The interface looks slightly different on a Windows computer, but the main features are the same. Note that depending on the details, you may or may not see the same warnings and errors as the screenshot.

Example of recently-opened R on an Apple computer. The interface looks slightly different on a Windows computer, but the main features are the same. Note that depending on the details, you may or may not see the same warnings and errors as the screenshot.

What is RStudio and how can I install it?

RStudio is a development environment for R. As a very coarse simplification, you can think of RStudio as a complement to R that makes some tasks easier – a “shell” enveloping R that makes some communications between you and R easier. However, many would say that RStudio is not much of a helpful resource at all. If you never worked with R before, you are encouraged to try both RStudio and just “plain” R separately and make your own conclusion as to which makes you more comfortable. You are not required to use RStudio to use evolved, but you can use it if you think it will help your learning process (you are required to use R, though).

Here in this class we will use programming as a learning tool, but programming is also a skill that will develop your quantitative ability and will make you a better scientist. If you really want to train this skill and get really good at it, we highly recommend that at some point of your career, you get used to programming just with R – and you are invited to use this class as one opportunity to do that. However, if your intention is to use R only occasionally, it might be worth jumping straight to RStudio, at the cost of being limited by a series of details and constraints that RStudio has.

If you need RStudio for your personal machine, you can download and install it for free by pasting this in your web browser:

http://rstudio.org

After you download it, install as you would do with any other program – the installer program will direct you though the major steps.

If you intend to use RStudio, please download and install it before progressing to install RMarkdown. We will cover RMarkdown later in this tutorial.

Starting R and RStudio:

When you start R, you should see something like figure 1. If you start RStudio instead, it will open R inside of the RStudio environment, and should look like figure 2.

Example of recently-opened Rstudio. Note that the appearance of RStudio is customizable, so your interface might not look the same as the RStudio your instructors or your fellow students have

Example of recently-opened Rstudio. Note that the appearance of RStudio is customizable, so your interface might not look the same as the RStudio your instructors or your fellow students have

Just looking at figures 1 and 2, it is easy to see that RStudio gives a lot more information. We will briefly walk through the items shown in figure 2 to familiarize you with each quadrant in the RStudio environment. Note that in both figures we are not showing the upper part of the screen (containing the tabs “File”, “Edit”, “Code”, “View”, etc.), but sometimes you will use those options too.

  • Upper-left: The script. This is where you view, edit, and save text files, including R scripts (.R files) and RMarkdown (.Rmd) files. You will use this quadrant the most during your learning activities.

  • Lower-left: the console (a.k.a terminal, a.k.a. command line). This is where lines of script get processed and computed. Here, you can see what code was actually input and run, and what the computer’s output was. You can also paste and execute commands directly into the console, instead of through the script file. This space will also be used exhaustively.

  • Upper-right: Four tabs:

    • Environment: for loading/saving/examining data (you may or may not use this one much, will depend on your habits)
    • History: a list of commands you’ve typed (again, use here will depend on your habits)
    • Connections: for linking to specific database types (we will basically not use this during vignettes)
    • Tutorial: a useful resource with tutorial demonstrations of R code and functions (not used at all in evolved)

  • Lower-right: Five tabs:

    • Files: a list of files in the “working directory” (more on this later in this tutorial)
    • Plots: where the plots/figures you create will appear (we will use this extensively in vignettes)
    • Packages: an area for managing installed R packages (probably you will not use it much during vignettes)
    • Help: access to official documentation for R (we will use this extensively)
    • Viewer: for viewing web content (probably not used frequently in vignettes)

If you want to use RStudio, there are a series of configurations that we highly recommend you have. You can access them by going to the upper part of the screen (the one that contains the tabs “File”, “Edit”, “Code”, “View”, etc), clicking Tools > Global Options, and then editing options. Opinions on the best configuration vary, but below you have the link to a blog post that shows a possible configuration that might be helpful during our classes: (link: https://www.r-bloggers.com/2019/05/my-rstudio-configuration/)

Use your time to get used to all this new software.

Working Routine in R

In this short tutorial we will help you to navigate through your files. You can think of this as a checklist to use at the start of any of the labs in this class. Steps 1-4 have to be done before any lab. Steps 5 and 6 will be needed depending on the lab.

We start by revising what the “Desktop” of your computer is – the standard folder most computer operating systems (i.e., Windows, MacOS, Linux…) have. It is the first “place” your computer puts you in when you turn it on, seen in the screen shot below, which is my Desktop:


If you are not comfortable with coding or R, we suggest you create a folder for storing all our classwork in the “Desktop” of your computer, like we did in the screen shot above. If you are comfortable with programming, work as you wish.

Step 1: Setting work directory

The working directory is the folder in your computer that R will navigate to during a session. Any time you import files or write files to a folder, R will automatically use the folder that is set as the working directory. To set the working directory, open R and click, in the upper bar, on Session > Set Working Directory > Choose Directory as in the figure below (notice that my .Rmd file is on the left panel and my R console, different from the RStudio default, is on the right – notice also that I minimized all other panels in my RStudio):


After clicking on Choose Directory, you should see something like the screenshot below:

Once there, you can navigate to the folder where you are storing files for this class, or a subfolder of that folder, to set it as your working directory. It’s important to keep your working folders for each lab organized, as they will include several different files types, including both R scripts (.R files) and data files (e.g., .txt, .csv, etc.). For instance if you had downloaded the files cyt-oxidase1-mt-aligned.txt and R_functions.R and added them to your working directory folder, it would look like this:

After following the above steps, you will have set the directory in which you will be working for the current R “session”. This is the folder R will use as a reference, from now until you close R or change the directory.

Step 2: Checking working directory

(I) To check what working directory you are in, type getwd() into the console or into a script file. This function will tell you which folder is currently set as the working directory by giving you a path name to the folder. If you decided to store your folder in the Desktop, you should see something in the following format (note this follows Mac/Linux formatting, Windows will write this slightly differently):

".../[username]/Desktop/[name_of_your_folder]"

The things we put inside brackets in the line above are things we expect to be different in your case, given your operational system and the names of your folders.

In my computer (in a Mac/Linux system), I get:

getwd()
[1] "/Users/matheus/Desktop/EEB516_labs/lab_05"

R can also return a “compacted” version of the exact same line, which would be:

getwd()
[1] "~/Desktop/EEB516_labs/lab_05"

This contraction in particular occurs in Linux and MacOS systems. Windows may show a similar contraction, but it can vary from version to version, and you are welcome to reach out to your GSI in case there are any problems. This contraction is a resource that helps more advanced programmers (at the cost of being slightly confusing for newcomers).

What I recommend you to do is: once you have finished step 1 and your working directory is set, run getwd() and paste its output into the setwd() function. Insert that line in your “setup” code chunk. This will help RMarkdown to compile more smoothly. It will look more or less like the following screenshot:

Another strange behavior that RMarkdown might do is to automatically change your folder after every chunk of code you run. This is extremely counterproductive, and in the end (step 7) we provide a guide on a possible way to solve this.

(II) To check what files are inside your working directory, type dir() into the console or into a script file. This will list the names of every file or subfolder within the working directory. For example, if I had two files in my working directory named cyt-oxidase1-mt-aligned.txt and R_functions.R, I would see the following output after typing dir():

[1] "cyt-oxidase1-mt-aligned.txt"
[2] "R_functions.R"

(III) To check what R objects are currently in your workspace, type ls(), which gives you a list of all objects currently loaded in your R environment. Before you start your lab, the results should always be character(0), which means there is currently nothing in your workspace – we call this a “cleaned” workspace.


The above steps should have helped you set everything and check what files and objects are available to you. Below are some troubleshooting tips in case things went awry in the previous steps.

IF your result from getwd() is different from what it should be, try step 1 (setting your work directory) again.

IF your result from dir() is different from what it should be, you could either be in the wrong folder (so go back to step 1) or you could not have all the files in the correct folder – correct this by moving files in your computer.

IF ls() returns any other thing than "character(0)" you should clean your workspace in R by typing rm(list=ls()) in the console, but be careful as this will erase everything you have previously created within R (although things can be preserved if you previously have saved them in a file external to R such as a .txt or .csv file).

Having a clean workspace is very important to working properly in R – many problems arise from forgetting to start from a completely clean workspace.

Step 3: Saving files

Make sure all files are being saved correctly. This is important to not lose your work if R crashes, class time ends, or you have to stop for any other reason. Go to the upper bar and click on File > Save As as in the figure below.

A window will pop up in the folder you are working in. You can save your .Rmd file there, as you would do with any other file.

Now, if you run dir() again, you should see your saved file. As I saved my file with the name lab_05_answers.Rmd, I got:

[1] "cyt-oxidase1-mt-aligned.txt"
[2] "lab_05_answers.Rmd"         
[3] "R_functions.R"

Step 4: Compiling/knitting your RMarkdown

Now is the time to make sure that your RMarkdown compiles. You can compile it by clicking just above the script window in RStudio on Knit:

Now, if you run dir() again, you should see your new compiled .html document in the working folder:

[1] "cyt-oxidase1-mt-aligned.txt"
[2] "lab_05_answers.html"         
[3] "lab_05_answers.Rmd"         
[4] "R_functions.R"

Note: I strongly recommend you always knit/compile your .Rmd after finishing each question. There is nothing more disappointing than finishing an RMarkdown and then not being able to compile it. So knitting along the way will eliminate this possibility.

Step 5: Sourcing R scripts

You might need to source files – which is just reading an .R file, importing any functions or data from that script file to your current script file. All your files should be in the folder where you are working, so if you followed previous steps, you just have to type: source("name_of_file.R") (“name_of_file” is merely a placeholder).

If there are no errors after typing this in, the script you just sourced ran until the end with no issue. If your sourced file crashed or printed an error, something is wrong, so please revise all steps above and if things continue to not work, ask your GSI.

Step 6: Loading external data

You might need to load external files, such as .csv files. We will probably tell you, in each tutorial, which function to use. But the general format would be:

object <- function_that_reads_file("file_name")

For instance, if we had a spreadsheet called data.csv to read in and we have correctly followed all the steps above, we would run:

data <- read.csv("data.csv")

Too check that the data was read correctly, we could do something like this:

head(data)

Step 7: Debbuging one specific strange behavior some students found

A strange behavior RMarkdown might have is to automatically change your working directory folder after every chunk of code you run. This is extremely counterproductive, but you can fix it by going to the upper bar and clicking Tools > Global Options as in the figure below:

Then go to RMarkdown > Evaluate chunks in directory and set the option to “Document”, as in the figure below:

The configurations you see in the screenshot above are used by Matheus – feel free to try them as well if you want.

If RStudio keeps changing the directory after you have done the steps above, reach out to your GSI. Other ways to try to solve any problem you may encounter is to look it up on Google, as R and RStudio have a very big, helpful, and kind online community that usually helps with all kinds of problems – all of us regularly troubleshoot things by looking them up online.

What is RMarkdown and how can I install it (optional)?

RMarkdown is a programming language and environment that generates reporting documents. It makes it easy to mix a decently formatted document (and nice-looking equations) with R code, and this is why you want to use RMarkdown when studying ecology and evolutionary biology.

If you know LaTeX, you should be familiar with RMarkdown’s syntax (i.e., the way functions are written and used in a programming language) as they resemble each other. If you don’t know what LaTeX means, no problem – you will have time to familiarize yourself with this kind of tool.

Downloading RStudio automatically downloads RMarkdown too. Otherwise, you can download RMarkdown directly from the R console, by typing.

install.packages('rmarkdown')
# Using the code above, you will install RMarkdown from The Comprehensive R 
# Archive Network (a.k.a. CRAN).

Please check that you have the knitr package installed in your computer, as RMarkdown needs it.

You can create a new RMarkdown file in RStudio by going to the upper tabs, then clicking on File > New > R Markdown.

You can create .html files from code by clicking on the “Knit” button just above the script window (not shown in figure 2, since the file open there is a script file, .R, not an RMarkdown file, .Rmd).

More on RMarkdown

RMarkdown is the result of a philosophy called “literate programming”, first idealized by Donald Knuth during the ’70s. Doctor Knuth believed that programmers should think of programs not only as lists of commands, but also as works of literature. One of his quotes about this approach to coding is “Instead of imagining that our main task is to instruct a computer what to do, let us concentrate rather on explaining to human beings what we want a computer to do”. This has very clear and positive consequences both for teaching and for science, so those who feel comfortable with RMarkdown should follow Dr. Knuth’s ideas when studying using evolved, and use RMarkdown as an intrinsic part of your learning process generally.

Our experience is that it is worthwhile to use RMarkdown to create notes for your scientific practices, such as data analyses, model making/testing, etc. We understand, however, that the learning curve of R (and maybe RStudio) + RMarkdown might be a little to steep. And this is why you can also just give yourself a little time to learn each tool.

This is our recommended routine to work with evolved:

  1. Each lab tutorial is available as vignette in evovled. We recommend skimming the text before starting the tutorial.

  2. The rest of your studies will be reading the tutorial and answering the questions in it. We highly recommend everyone to read the vignette completely before starting to answer the questions.

  3. You can use RMarkdown to knit your own answers as PDF files (and this will, with time, prove to be very useful for you in science), but depending on your operational system this might be a bit of a headache. You can also use Microsoft Word or Google Docs, as long as it is usful to you.

RMarkdown advantages

In addition to plotting and integrating comments with code, RMarkdown also allows you to easily write equations with beautiful notation. Below, we will write down some of the equations we will see in evolved. No need to understand what they mean (biologically speaking) now – I just want to show you the RMarkdown equation-writing feature.

You are likely reading a PDF/html file, but a major point is that this file is built so you can visualize both “sides” of writing a PDF from a RMarkdown file: the “public-facing” side is the PDF file you are currently reading, but the “internal” side is the code that generates the PDF. The .Rmd file that generated this PDF document is posted on MJ’s github so that you can see how each of those two files communicate (and how Markdown works “under the hood”).

The point of the following text example is for you to have an idea of how nicely-written equations, code, and text can be merged into a single, organized document through RMarkdown.

Download and open the .Rmd file which has the same name as this PDF (posted on MJ’s github to learn the syntax associated with everything we are doing below.

An example of why RMarkdown is so neat:

The effective population size when there is time-varying population size (i.e., bottlenecks and expansions) is:

\(N_e = \frac{1}{\frac{1}{t}\sum_{i=1}^{t} \frac{1}{N_i}}\)

The probability of identity-by-descent of two alleles is:

\(f_t = \frac{1}{2N} + (1-\frac{1}{2N})f_{t-1}\)

And the rate of heterozygosity loss is:

\(\frac{H_t}{H_0} = (e^{-\frac{1}{2N}})^t\)

Which is also equal to \(e^{-\frac{t}{2N}}\). Of course, this assumes a diploid organism, so the census size of this population is \(N\).

RMarkdown can also very easily merge R plots and text. For instance if we want to graph equation 3, we could simply write:

#create a numeric vector representing time:
t=seq(from=0, to=200, by=0.001)

#insert population size:
N=20

Ht_Ho=exp(-1/(2*N))^t

plot(x=t, y=Ht_Ho, type="l", lwd=4, 
     ylab="Rate of heterozygosity loss",
     xlab="Time", main="Plot of equation (3)")

RMarkdown practice (part I)

Please take a look of the RMarkdown used to make this file (in MJ’s github and try to locate the code responsible for each part of what you have just read, putting special attention to codes, plots, and equations.

RMarkdown practice (part II)

Now let’s do something which is more hands-on. Create a brand new .Rmd file and then insert text in it by pasting some of the contents of this tutorial to the new file. To run a “test” of one piece of your pasted code, click on a line of your R code in the document and then choose Code > Run Selected Line(s) to run that line in the R console. Alternatively, you can also click the Run button in the top left pane, but the quickest way to do this is the Command+Return on a Mac keyboard, or Control+Return on a Windows keyboard. Test what works best on Linux, if you use it.

Select Run Region > Run Current Chunk to run an entire “chunk” of R code. Save your changes in your .Rmd, and knit to HTML. In the future, note that RStudio automatically saves your .Rmd file whenever you knit it to an HTML or PDF file - but recall that for the purposes of this class we will only accept PDF submissions.