Link Search Menu Expand Document
CMU Libraries Open Data Science Workshops

Creating objects in R

You can get output from R simply by typing math in the console:

3+5 
[1] 8 
12 / 7 
[1] 1.714286

However, to do useful and interesting things, we need to assign values to objects. To create an object, we need to give it a name followed by the assignment operator <-, and the value we want to give it:

area_hectares <-1.0

<- is the assignment operator. It assigns values on the right to objects on the left. So, after executing x <-3, the value of x is 3. The arrow can be read as 3 goes into x.

Note: You can also use = for assignments, but not in every context. Because of the slight differences in syntax, it is good practice to always use <- for assignments. More generally we prefer the <- syntax over = because it increases the readability of the code.

Objects can be given any name such as x, current_temperature, or subject_id. However, there are some general rules and best practices concerning naming.

Naming Rules

  • Cannot start with a number (2x is not valid, but x2 is).
  • R is case sensitive (e.g., age is different from Age).
  • There are some names that cannot be used because they are the names of fundamental functions in R (e.g., if, else, for, see here for a complete list). In general, even if it’s allowed, it’s best to not use other function names (e.g., c, T, mean, data, df, weights).

Naming Best Practices

  • Object names should be explicit and not too long.
  • Use nouns for object names, and verbs for function names.
  • Avoid dots (.) within an object name as in my.dataset. There are many functions in R with dots in their names for historical reasons, but because dots have a special meaning in R (for methods) and other programming languages, it’s best to avoid them.
  • Be consistent in the styling of your code (where you put spaces, how you name objects, etc.). Using a consistent coding style makes your code clearer to read for your future self and your collaborators.

Objects vs. variables

What are known as objects in R are known as variables in many other programming languages. Depending on the context, object and variable can have drastically different meanings. However, in this lesson, the two words are used synonymously. For more information see: https://cran.r-project.org/doc/manuals/r-release/R-lang.html#Objects.

When assigning a value to an object, R does not print anything. You can force R to print the value by using parentheses or by typing the object name:

area_hectares <- 1.0 # doesn't print anything 
(area_hectares <-1.0) # putting parentheses around the call prints the value of `area_hectares` 
[1] 1 
area_hectares # and so does typing the name of the object 
[1] 1

Now that R has area_hectares in memory, we can do arithmetic with it. For instance, we may want to convert this area into acres (area in acres is 2.47 times the area in hectares):

2.47 * area_hectares 
[1] 2.47

We can also change an objectÕs value by assigning it a new one:

area_hectares <-2.5 
2.47 * area_hectares 
[1] 6.175

This means that assigning a value to one object does not change the values of other objects. For example, let’s store the plot’s area in acres in a new object, area_acres:

area_acres <-2.47 * area_hectares

and then change area_hectares to 50.

area_hectares <-50

Exercise

What do you think is the current content of the object area_acres? 123.5 or 6.175?