Your First R Session

Tonight you split a $87.60 dinner three ways with your friends Dev and Mara, and you want to add an 18% tip. That one ordinary task, work out the tip, total it up, divide by three, round to the cent, is enough to meet everything a first R session is made of.

By the end of this lesson you will be able to:

• Run code in R and read the result it prints back
• Store a value in an object with the <- arrow, then reuse it
• Call a function, read your first error message, and ask R for help

Prerequisites: none at all. This is the very first lesson. You only need R in front of you, either in RStudio or the editable boxes on this page, where you press Run to execute a line.

R is a calculator that talks back

The simplest thing you can do in R is type a sum and press Enter. R works it out and prints the answer straight back:

Two things to notice. The line #> [1] 4 is R replying; on this page that reply appears under the code after you press Run. The [1] is just R numbering the answer, and with a single value it always reads [1], so you can ignore it for now. The real result is the 4.

Now the actual job. Your dinner tip is 18% of the $87.60 bill, which is the bill times 0.18. The * means multiply, and anything after a # is a note to yourself that R ignores:

So the tip is about $15.77. R understands the everyday operators: + add, - subtract, * multiply, / divide, and ^ for powers, with the usual order of operations (and round brackets ( ) to force the order you want). That is genuinely all "running code" means: you hand R an expression, it evaluates it, and it shows you the value.

Give a value a name with <-

Retyping 87.60 every time you need the bill would get old fast. Instead, store it once under a name. In R you do that with the assignment arrow, <-, which you read as "gets":

bill <- 87.60 points the value on the right into the name on the left. A name with a value attached is called an object. Notice two things: assigning is silent (R printed nothing on the first line), so you type the name on its own line to see inside it; and R shows 87.6, dropping the harmless trailing zero.

Now give the other facts names too, and let R combine them. An object can be built from other objects:

Two habits to start with now. R is case-sensitive: bill, Bill, and BILL are three different names. And assigning a new value to an existing name simply overwrites it, with no warning. (You may see = used for assignment too; it works, but <- is the convention this course follows.)

Which line stores a value?

You want to put the number 3 into an object called people. Which line does that?

Hand a value to a function

You and your friends do not split $34.456 evenly; you split dollars and cents. To tidy a number, you call a function. A function is a named tool: you give it one or more arguments (its inputs) inside round brackets, and it hands back a value.

First, each person's raw share is the total divided by the number of people:

Now round it to 2 decimal places with the round() function. Its first argument is the number, and its second, digits, is how many decimals to keep:

You passed two arguments separated by a comma, by position: the number first, the digits second. You can also pass them by name, which reads more clearly and frees you from remembering the order:

Many arguments have a default, a value used when you do not supply one. For round(), digits defaults to 0, so leaving it off rounds to a whole number:

That is the whole pattern of using R: store values in objects, then call functions on them. Everything else is more objects and more functions.

Round the share yourself

Each person owes per_person dollars, which is 34.456. Call round() so it shows their share to the nearest cent (2 decimal places). Replace the blank with the full function call, then check it.

round(per_person, 2)

Reading your first error

Sooner or later R will print a line starting with Error, and that is completely normal. An error is not a wall; it is R telling you, quite precisely, what it could not do. The trick is simply to read it.

Here is the single most common beginner error. You stored the total as total (lowercase), but you ask for it with a capital T:

Total
#> Error: object 'Total' not found

Read it literally: there is no object named Total. R quotes the exact name it looked for and could not find. "object not found" almost always means one of two things: a typo or wrong capital (you meant total), or you never created it in the first place. The fix is to correct the name, or to assign it before you use it.

A second one you will hit early is an unfinished line. In the R console (in RStudio or your own session), forget the closing bracket and R shows a + prompt instead of an answer:

round(per_person, 2
+ 

That + means "I am waiting for the rest of your line." Either type the missing ) and press Enter, or press Esc to cancel and start again. When something looks stuck, an unbalanced bracket is the usual cause.

What is R telling you?

You run a line and see Error: object 'Total' not found. What does it mean?

Getting help

How did we know round() even had a digits argument? We asked R. The quickest peek is args(), which lists just a function's arguments and their defaults:

There it is in black and white: round() takes x (the number) and digits, and digits = 0 is the default we relied on earlier. For the full explanation, with examples, open a function's help page. These are console commands you run in RStudio or your own R session:

?round                   # open round()'s help page
help(round)              # exactly the same as ?round
example(round)           # run the examples from the help page
??"standard deviation"   # search the help when you do not know the function name

?name opens the help page in RStudio's Help pane (or your browser); args(name) is the fast reminder of just the arguments; and ??"some text" searches across the help when you cannot remember a function's name. Getting comfortable with ? early is one of the highest-value habits in R.

From bill to each person's share

Dev offers to leave a more generous 20% tip. Update the tip rate, recompute the total, and show each person's new share rounded to the cent. The first two lines are done; replace the blank with the final calculation. (Each person should owe 35.04.)

tip_rate <- 0.20
total    <- bill + bill * tip_rate
round(total / people, 2)
#> [1] 35.04

References

A few trustworthy places to keep going, all free:

• An Introduction to R (official manual) - the canonical first walk through R's basics, straight from the R project.
• R for Data Science (2e): Workflow basics - a friendly take on objects, names, and calling functions.
• Hands-On Programming with R - a gentle, beginner-first book that starts exactly where this lesson does.
• Getting Help with R - the official rundown of ?, ??, and where to ask when you are stuck.

Lesson 1 complete

You just ran a real R session. You used R as a calculator, stored values in objects with the <- arrow and reused them, called round() with positional, named, and default arguments, read an "object not found" error and fixed it, and asked R for help with args() and ?. Those few moves, run code, store it, call a function, are the spine of everything that follows.

Next, Lesson 2: Atomic Vectors and Data Types. So far each object has held a single value. You will see how R packs many values into one vector, meet its core types (numbers, text, and TRUE/FALSE), and learn how to tell what type any object is.