Lesson 2 of 17 · Week 01 · Tue Sep 29

R Basics

How to work through this lesson

This is a self-guided lesson for Tuesday morning. Work through it in order:

1. Open your course folder in Positron (see Lesson 1) and create scripts/week01-basics.R.
2. Read each section below. Type every example into your script (don't copy-paste) and run it with Ctrl/Cmd+Enter. Before running a line, predict what it will print.
3. Do the five exercises at the bottom of this page. They check your answer automatically.
4. Download your proof-of-completion PDF at the end of the page.
5. Prepare the three things listed under For the afternoon.

Adapted from Software Carpentry's “Introduction to R and RStudio” episode (CC BY 4.0, The Carpentries). Text, examples and exercises have been rewritten for this course.

Finding your way around Positron

Positron's window is made of a few areas that you can resize, hide or move. By default:

• Editor (centre): where your script files open, one tab per file. This is where you write code you want to keep.
• Console (panel below the editor): where R actually runs. Its tabs also include a Terminal for command-line work.
• Explorer (left, the top icon in the thin Activity Bar): the files and folders of the folder you opened. The other Activity Bar icons open search, source control (Git) and extensions.
• Session and Plots (right-hand side bar): Variables lists every object in your R session with its type and a preview, and the Plots pane shows figures. Help pages open on the right as well.

If a pane is missing, use the View menu, or Ctrl/Cmd+Shift+P and type its name.

In the exercises on this site, the code box and its output stand in for the editor and console: the code runs in a real R session inside your browser.

R as a calculator

7 + 3 * 2       # multiplication before addition: 13
(7 + 3) * 2     # parentheses first: 20
2 ^ 10          # exponent: 1024
17 %/% 5        # integer division: 3
17 %% 5         # remainder: 2
6.02e23         # scientific notation for 6.02 x 10^23

If a line ends before R thinks the expression is finished (say, 7 +), the console shows a + prompt and waits for the rest. Finish the line or press Esc to cancel.

R also has the usual mathematical functions. You call a function by writing its name followed by parentheses holding its arguments:

sqrt(81)        # 9
log(100)        # natural log: 4.60517
log10(100)      # base-10 log: 2
exp(1)          # e: 2.718282
abs(-4.5)       # 4.5
round(3.14159, digits = 2)

Comparing things

5 == 5          # equal to (two = signs!)
5 != 3          # not equal to
4 < 9
4 >= 9

Comparisons return TRUE or FALSE: logical values. One warning: computers store most decimals approximately, so 0.1 + 0.2 == 0.3 is FALSE. To compare decimal numbers, use all.equal(), which allows a tiny tolerance, wrapped in isTRUE() so you always get a single TRUE/FALSE:

0.1 + 0.2 == 0.3                    # FALSE
isTRUE(all.equal(0.1 + 0.2, 0.3))   # TRUE

Variables and assignment

Store a value under a name with the assignment arrow <- (in Positron, Alt+- types it for you):

rain_mm <- 12.5
rain_in <- rain_mm / 25.4
rain_in
rain_mm <- rain_mm + 3     # overwrite with a new value

Assignment prints nothing. Type the name on its own line to see the value, and watch it appear in the Variables pane. The right-hand side is computed first, using the current values, and only then stored. That's why rain_mm <- rain_mm + 3 makes sense.

Rules for names: they may contain letters, digits, . and _; they must start with a letter or a dot (a dot can't be followed by a digit); they can't be reserved words like if or TRUE; and they are case-sensitive (Rain ≠ rain). Names starting with a dot are hidden: they work, but ls() doesn't show them. Pick one style and stick to it; this course uses lower_snake_case.

Comments start with #. Use them to explain why you did something; the code already says what.

A first look at vectors

1:6             # the integers 1 to 6
2 ^ (1:6)       # 2, 4, 8, 16, 32, 64
temps <- c(18.5, 21.0, 19.2)
temps * 9 / 5 + 32

Most R operations work on a whole vector at once. We'll come back to this many times.

Managing your environment

ls() lists the objects you've created. rm() deletes objects: rm(rain_in), or several at once with rm(rain_in, temps). Notice the empty parentheses in ls(): they tell R to call the function. Typing ls without them prints the function's own code.

When you pass an argument by name, use =, not <-: round(3.14159, digits = 2). Inside a function call, <- would create a variable instead of setting the argument.

Errors vs. warnings: an error stops R (nothing was computed). A warning means R did something but suspects it isn't what you wanted. Read both: the message usually points at the problem.

Packages

Base R is extended by thousands of packages. The two functions to know:

• install.packages("dplyr"): downloads and installs a package. Do this once, in the console. Note the quotes. To install several in one go: install.packages(c("dplyr", "tidyr")).
• library(dplyr): loads an installed package so you can use its functions. Put this at the top of every script that needs it.

Other helpers: installed.packages(), update.packages(), remove.packages("name").

For the afternoon

Bring these three things to the discussion & peer-review session:

1. Your proof-of-completion PDF for this lesson.
2. Your scripts/week01-basics.R, which should run from top to bottom without errors.
3. One surprise: an output or error you didn't predict, plus a sentence on what you now think is going on.

Peer review. Swap scripts with a partner and answer these, in writing, about their script:

• Does it run top to bottom, in a fresh R session, without errors? If not, where does it stop?
• Could you tell what each object holds just from its name?
• Find one comment that explains why, not just what. If there isn't one, suggest one.
• What's one thing they did that you'll borrow?

Exercises

Each exercise gives you up to 10 checked attempts. Use Run as many times as you like to experiment first, then click Check answer when you're ready. Your exercises use numbers/subsets unique to you, so answers won't match a classmate's exactly.