Lesson 3 Introduction to R
teaching: 50
exercises: 30
adapted from: https://datacarpentry.org/r-socialsci/01-intro-to-r/index.html
questions:
- What data types are available in R?
- What is an object?
- How can values be initially assigned to variables of different data types?
- What arithmetic and logical operators can be used?
objectives:
- Define the following terms as they relate to R: object, assign, call, function, arguments, options.
- Assign values to objects in R.
- Learn how to name objects.
- Use comments to inform script.
- Solve simple arithmetic operations in R.
- Call functions and use arguments to change their default options.
- Inspect the content of vectors and manipulate their content.
3.1 Creating objects in R
You can get output from R simply by typing math in the console:
3 + 5## [1] 812 / 7## [1] 1.714286However, 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. For historical reasons, you can also use = for assignments. There are slight differences depending on which syntax you use, and people have their own preferences. Famous R programmer Yihui Xie insists on =, but many code style guides recommend <-. Use whichever one feels more comfortable to you! (Typically, we recommend being consistent and just using one throughout all your code.)
In RStudio, typing Alt + - (push Alt at the same time as the - key) will write <- in a single keystroke in a PC, while typing Option + - (push Option at the same time as the - key) does the same in a Mac.
Objects can be given any name such as x, current_temperature, or subject_id. You want your object names to be explicit and not too long. They 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). If in doubt, check the help to see if the name is already in use. It’s also best to 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. It is also recommended to use nouns for object names, and verbs for function names. It’s important to 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. In R, three popular style guides are Google’s, Jean Fan’s and the tidyverse’s. The tidyverse’s is very comprehensive and may seem overwhelming at first. You can install the lintr package to automatically check for issues in the styling of your code.
3.1 Objects vs. variables
What are known as
objectsinRare known asvariablesin many other programming languages. Depending on the context,objectandvariablecan 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 parenthesis around the call prints the value of `area_hectares`## [1] 1area_hectares # and so does typing the name of the object## [1] 1Now 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] 2.47We can also change an object’s value by assigning it a new one:
area_hectares <- 2.5
2.47 * area_hectares## [1] 6.175This 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_hectaresand then change area_hectares to 50.
area_hectares <- 503.1 Exercise
What do you think is the current content of the object
area_acres? 123.5 or 6.175? > ## Solution > > The value ofarea_acresis still 6.175 because you have not re-run the linearea_acres <- 2.47 * area_hectaressince changing the value ofarea_hectares.
3.3 Vectors and data types
A vector is a very common and basic data type in R. A vector is composed of a series of values, which can be either numbers or characters. We can assign a series of values to a vector using the c() function. For example we can create a vector of household members for the households we’ve interviewed and assign it to a new object hh_members:
hh_members <- c(3, 7, 10, 6)
hh_members## [1] 3 7 10 6A vector can also contain characters. For example, we can have a vector of the building material used to construct our interview respondents’ walls (respondent_wall_type):
respondent_wall_type <- c("muddaub", "burntbricks", "sunbricks")
respondent_wall_type## [1] "muddaub" "burntbricks" "sunbricks"The quotes around “muddaub”, etc. are essential here. Without the quotes R will assume there are objects called muddaub, burntbricks and sunbricks. As these objects don’t exist in R’s memory, there will be an error message.
There are many functions that allow you to inspect the content of a vector. length() tells you how many elements are in a particular vector:
length(hh_members)## [1] 4length(respondent_wall_type)## [1] 3An important feature of a vector, is that all of the elements are the same type of data. The function class() indicates the class (the type of element) of an object:
class(hh_members)## [1] "numeric"class(respondent_wall_type)## [1] "character"The function str() provides an overview of the structure of an object and its elements. It is a useful function when working with large and complex objects:
str(hh_members)## num [1:4] 3 7 10 6str(respondent_wall_type)## chr [1:3] "muddaub" "burntbricks" "sunbricks"You can use the c() function to add other elements to your vector:
possessions <- c("bicycle", "radio", "television")
possessions <- c(possessions, "mobile_phone") # add to the end of the vector
possessions <- c("car", possessions) # add to the beginning of the vector
possessions## [1] "car" "bicycle" "radio" "television" "mobile_phone"In the first line, we take the original vector possessions, add the value "mobile_phone" to the end of it, and save the result back into possessions. Then we add the value "car" to the beginning, again saving the result back into possessions.
We can do this over and over again to grow a vector, or assemble a dataset. As we program, this may be useful to add results that we are collecting or calculating.
An atomic vector is the simplest R data type and is a linear vector of a single type. Above, we saw 2 of the 6 main atomic vector types that R uses: "character" and "numeric" (or "double"). These are the basic building blocks that all R objects are built from. The other 4 atomic vector types are:
"logical"forTRUEandFALSE(the boolean data type)"integer"for integer numbers (e.g.,2L, theLindicates to R that it’s an integer)"complex"to represent complex numbers with real and imaginary parts (e.g.,1 + 4i) and that’s all we’re going to say about them"raw"for bitstreams that we won’t discuss further
You can check the type of your vector using the typeof() function and inputting your vector as the argument.
Vectors are one of the many data structures that R uses. Other important ones are lists (list), matrices (matrix), data frames (data.frame), factors (factor) and arrays (array).
3.3 Exercise
We’ve seen that atomic vectors can be of type character, numeric (or double), integer, and logical. But what happens if we try to mix these types in a single vector?
3.3 Solution
R implicitly converts them to all be the same type.
What will happen in each of these examples? (hint: use
class()to check the data type of your objects):
r num_char <- c(1, 2, 3, "a") num_logical <- c(1, 2, 3, TRUE) char_logical <- c("a", "b", "c", TRUE) tricky <- c(1, 2, 3, "4")Why do you think it happens?
3.3 Solution
Vectors can be of only one data type. R tries to convert (coerce) the content of this vector to find a “common denominator” that doesn’t lose any information.
How many values in
combined_logicalare"TRUE"(as a character) in the following example:num_logical <- c(1, 2, 3, TRUE) char_logical <- c("a", "b", "c", TRUE) combined_logical <- c(num_logical, char_logical)
3.3 Solution
Only one. There is no memory of past data types, and the coercion happens the first time the vector is evaluated. Therefore, the
TRUEinnum_logicalgets converted into a1before it gets converted into"1"incombined_logical.
You’ve probably noticed that objects of different types get converted into a single, shared type within a vector. In R, we call converting objects from one class into another class coercion. These conversions happen according to a hierarchy, whereby some types get preferentially coerced into other types. Can you draw a diagram that represents the hierarchy of how these data types are coerced?
3.2 Comments
All programming languages allow the programmer to include comments in their code. To do this in R we use the
#character. Anything to the right of the#sign and up to the end of the line is treated as a comment and is ignored by R. You can start lines with comments or include them after any code on the line.RStudio makes it easy to comment or uncomment a paragraph: after selecting the lines you want to comment, press at the same time on your keyboard Ctrl + Shift + C. If you only want to comment out one line, you can put the cursor at any location of that line (i.e. no need to select the whole line), then press Ctrl + Shift + C.
3.2.1 Functions and their arguments
Functions are “canned scripts” that automate more complicated sets of commands including operations, assignments, etc. Many functions are predefined, or can be made available by importing R packages (more on that later). A function usually gets one or more inputs called arguments. Functions often (but not always) return a value. A typical example would be the function
sqrt(). The input (the argument) must be a number, and the return value (in fact, the output) is the square root of that number. Executing a function (‘running it’) is called calling the function. An example of a function call is:Here, the value of
areais given to thesqrt()function, thesqrt()function calculates the square root, and returns the value. If we want to use that value later, we can assign it to the objectb. This function is very simple, because it takes just one argument.The return ‘value’ of a function need not be numerical (like that of
sqrt()), and it also does not need to be a single item: it can be a set of things, or even a dataset. We’ll see that when we read data files into R.Arguments can be anything, not only numbers or filenames, but also other objects. Exactly what each argument means differs per function, and must be looked up in the documentation (see below). Some functions take arguments which may be specified by the user, or, if left out, take on a default value: these are called options. Options are typically used to alter the way the function operates, such as whether it ignores ‘bad values’, or what symbol to use in a plot. However, if you want something specific, you can specify a value of your choice which will be used instead of the default.
Let’s try a function that can take multiple arguments:
round().Here, we’ve called
round()with just one argument,3.14159, and it has returned the value3. That’s because the default is to round to the nearest whole number. If we want more digits we can see how to do that by getting information about theroundfunction. We can useargs(round)or look at the help for this function using?round.We see that if we want a different number of digits, we can type
digits = 2or however many we want.If you provide the arguments in the exact same order as they are defined you don’t have to name them:
This is considered fairly risky practice, because it relies on you knowing the exact order of the arguments.
If you do name the arguments, you can switch their order:
It’s good practice to put the non-optional arguments (like the number you’re rounding) first in your function call, and to specify the names of all optional arguments. If you don’t, someone reading your code might have to look up the definition of a function with unfamiliar arguments to understand what you’re doing.