Objects (Assignment)

Why Do We Need Objects?

Reading

R4DS 2.3: Object Names

Advanced Reading

Advanced R 2.2: Binding Basics

So far, every value we’ve used has disappeared as soon as R printed it, we haven’t saved anything. To do anything useful with R, we need a way to store a value so we can use it again later. We do this by creating objects that hold data.

Creating Objects

We ‘assign’ values to objects via the ‘assignment’ operator, <-.

The following code assigns the values 7, 2.5, and "foo" to the objects x, y, and z, respectively.

x <- 7
y <- 2.5
z <- "foo"

Do: run the code above.

See: notice the lack of printed output. These three commands create three new objects in your environment (x, y, z) that store the assigned data values (7, 2.5, "foo"). However, we haven’t yet asked R to do anything with those stored values, so we don’t see any printed output.

Explain: assignment is a silent action: it changes what R remembers, but it doesn’t print anything by itself. This trips up almost everyone the first time, so it’s worth internalizing now: assigning a value and displaying a value are two separate steps.

Viewing an Object’s Value

To view the contents of an object, we can evaluate the object’s name without assignment.

Run the following code to print the values saved as x and y.

Practice: Predict, Then Run

Given x <- 7 and y <- 2.5 (already run above), predict what each line below will print before running it.

x + y
x
x - 1
x
x + y
[1] 9.5
x
[1] 7
x - 1
[1] 6
x
[1] 7

Notice that x still equals 7 after the last line, even though we just computed x - 1. Doing arithmetic with an object does not change the value stored in that object, R only updates an object’s value when you explicitly re-assign it with <-.

Copies, Not Renames

We can also create new objects by assigning them the values of existing objects.

The following code creates a new object, w, that takes the value of the existing object z. When we print the value of w, you can see that it has the same value as z.

w <- z
w
[1] "foo"
z
[1] "foo"

When we create w above, we are not replacing z with w: we’re making a copy of z and calling that copy w. So, both w and z are still available. In almost all cases, R will copy objects during assignment. This behavior is good to keep in mind when you’re working with larger datasets: you can quickly flood your memory with unnecessary copies of your data, if you’re not careful.

Quick Check

  • After running w <- z, does changing w also change z? Why or why not?
  • What is printed when you run x <- 7 by itself? What about when you run just x on the next line?

Test both questions in the editor if you’re not sure.

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