Introduction to R: Assignment, vectors, functions

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Note: This lecture content was originally created by voluntary contributions to Data Carpentry and has been modified to align with the aims of EEB313. Data Carpentry is an organization focused on data literacy, with the objective of teaching skills to researchers to enable them to retrieve, view, manipulate, analyze, and store their and other’s data in an open and reproducible way in order to extract knowledge from data. For EEB313, we are making all our content available under the same license, The Creative Commons, so that anyone in the future can re-use or modify our course content, without infringing on copyright licensing issues.

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This saying captures the spirit, generosity, and fun involved in being a part of these open source projects.

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Lesson Preamble

Learning Objectives

• Define the following terms as they relate to R: call, function, arguments, options.
• Use comments within code blocks.
• Do simple arithmetic operations in R using values and objects.
• Call functions and use arguments to change their default options.
• Define our own functions
• Inspect the content of vectors and manipulate their content.
• Create for-loops

Lecture outline

• Setting up the R Notebook (10 min)
• Creating objects/variables in R (10 min)
• Using and writing functions (15 min)
• Vectors and data types (15 min)
• Subsetting vectors (15 min)
• Missing data (10 min)
• Loops and vectorization (20 min)

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Setting up the R Notebook

Let’s remove the template RStudio gives us, and add a title of our own.

---
title: Introduction to R
---

This header block is called the YAML header. This is where we specify whether we want to convert this file to a HTML or PDF file. This will be discussed in more detail in another class. For now, we just care about including the lecture title here. For now, let’s type out a note:

This lecture covers fundamental R usage, such as assigning values to a variable, using functions, and commenting code, and more.

Under this sentence, we will insert our first code chunk. Remember that you insert a code chunk by either clicking the “Insert” button or pressing Ctrl/Cmd + Alt + i simultaneously. To run a code chunk, you press the green arrow, or Ctrl/Cmd + Shift + Enter.

Creating objects in R

As we saw in our first class, 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.

x <- 3
x + 5

## [1] 8

You can name an object in R almost anything you want:

joel <- 3
joel + 5

## [1] 8

Challenge

So far, we have created two variables, joel and x. What is the sum of these variables?

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., joel is different from Joel). There are some names that cannot be used because they are they are reserved for fundamental functions in R (?Reserved lists these words). 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 or use tab completion to see if the name is already in use.

It’s also best to avoid dots (.) within a variable name as in my.dataset. Historically, there are many functions in R with dots in their name, but since dots have a special meaning in R, it’s better to not use them and instead use underscores (_).

It is also recommended to use nouns for variable 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 variables, etc.). Using a consistent coding style¹ makes your code clearer to read for your future self and your collaborators. RStudio will format code for you if you highlight a section of code and press Ctrl/Cmd + Shift + a.

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:

weight_kg <- 55    # doesn't print anything
(weight_kg <- 55)  # but putting parentheses around the call prints the value of `weight_kg`

## [1] 55

weight_kg          # and so does typing the name of the object

## [1] 55

The variable weight_kg is stored in the computer’s memory where R can access it, and we can start doing arithmetic with it efficiently. For instance, we may want to convert this weight into pounds (weight in pounds is 2.2 times the weight in kg):

2.2 * weight_kg

## [1] 121

We can also change a variable’s value by assigning it a new one:

weight_kg <- 57.5
2.2 * weight_kg

## [1] 126.5

This means that assigning a value to one variable does not change the values of other variables. For example, let’s store the animal’s weight in pounds in a new variable, weight_lb:

weight_lb <- 2.2 * weight_kg

and then change weight_kg to 100.

weight_kg <- 100

Challenge

What do you think is the current content of the object weight_lb? 126.5 or 220?

weight_lb

Comments

A comment is piece of text in your code that will not be executed with the rest of the code, but its purpose is to be a note to the person reading the code. Traditionally, comments have been the only way to make notes alongside your code, but since we are using the R Notebook format, we can also write notes in Markdown. Therefore, comments are not as important for us, but they are still the best method of leaving short notes and explanations for specific lines of code. The comment character in R is #, anything to the right of a # in a script will be ignored by R.

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/Cmd + 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/Cmd + Shift + C.

weight_lb <- 2.2 * weight_kg # Actually, 1 kg = 2.204623 lbs

Challenge

What are the values after each statement in the following?

mass <- 47.5
age  <- 122
mass <- mass * 2.0      # mass?
age  <- age - 20        # age?
mass_index <- mass/age  # mass_index?

Functions and their arguments

Functions can be thought of as recipes. You give a few ingredients as input to a function, and it will generate an output based on these ingredients. Just as with baking, both the ingredients and the actual recipe will influence what comes out of the recipe in the end: will it be a cake or a loaf of bread? In R, the inputs to a function are not called ingredients, but rather arguments, and the output is called the return value of the function. A function does not technically have to return a value, but often does so. Functions are used to automate more complicated sets of commands and many of them are already predefined in R. 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:

sqrt(9)

## [1] 3

Which is the same as assigning the value to a variable and then passing that variable to the function:

a <- 9
b <- sqrt(a)
b

## [1] 3

Here, the value of a is given to the sqrt() function, the sqrt() function calculates the square root, and returns the value which is then assigned to variable b. 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, as we will see later on.

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 either 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.

To access help about sqrt, we are first going to learn about tab-completion. Type s and press Tab.

s<tab>q

You can see that R gives you suggestions of what functions and variables are available that start with the letter s, and thanks to RStudio they are formatted in this nice list. There are many suggestions here, so let’s be a bit more specific and append a q, to find what we want. If we press enter or tab again, R will insert the selected option.

You can see that R inserts a pair of parentheses together with the name of the function. This is how the function syntax looks for R and many other programming languages, and it means that within these parentheses, we will specify all the arguments (the ingredients) that we want to pass to this function.

If we press tab again, R will helpfully display all the available parameters for this function that we can pass an argument to. The word parameter is used to describe the name that the argument can be passed to. More on that later.

sqrt(<tab>

There are many things in this list, but only one of them is marked in purple. Purple here means that this list item is a parameter we can use for the function, while yellow means that it is a variable that we defined earlier.²

To read the full help about sqrt, we can use the question mark, or type it directly into the help document browser.

?sqrt

As you can see, sqrt() takes only one argument, x, which needs to be a numerical vector. Don’t worry too much about the fact that it says vector here; we will talk more about that later. Briefly, a numerical vector is one or more numbers. In R, every number is a vector, so you don’t have to do anything special to create a vector. More on vectors later.

Let’s try a function that can take multiple arguments: round().

round(<tab>
?round

If we try round with a value:

round(3.14159)

## [1] 3

Here, we’ve called round() with just one argument, 3.14159, and it has returned the value 3. That’s because the default is to round to the nearest whole number, or integer. If we want more digits we can pass an argument to the digits parameter, to specify how many decimals we want to round to.

round(3.14159, digits = 2)

## [1] 3.14

So, above we pass the argument 2, to the parameter digits. Knowing this nomenclature is not essential for doing your own data analysis, but it will be very helpful when you are reading through help documents online and in RStudio.

We can leave out the word digits since we know it comes as the second parameter, after x.

round(3.14159, 2)

## [1] 3.14

As you notice, we have been leaving out x from the beginning. If you provide the names for both the arguments, we can switch their order:

round(digits = 2, x = 3.14159)

## [1] 3.14

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.

Writing functions

In this class, you will be working a lot with functions, especially those that someone else has already written. When you type sum, c(), or mean(), you are using a function that has been made previously and built into R. To remove some of the magic around these functions, we will go through how to make a basic function of our own. Let’s start with a simple example where we add two numbers together:

add_two_numbers <- function(num1, num2) {
    return(num1 + num2)
}
add_two_numbers(4, 5)

## [1] 9

As you can see, running this function on two numbers returns their sum. We could also assign to a variable in the function and return the function.

add_two_numbers <- function(num1, num2) {
    my_sum <- num1 + num2
    return(my_sum)
}
add_two_numbers(4, 5)

## [1] 9

Challenge

Can you write a function that calculates the mean of 3 numbers?

Vectors and data types

A vector is the most common and basic data type in R, and is pretty much the workhorse of R. A vector is composed by 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, which stands for “concatenate (combine/connect one after another) values into a vector” For example we can create a vector of animal weights and assign it to a new object weight_g:

weight_g <- c(50, 60, 65, 82) # Concatenate/Combine values into a vector
weight_g

## [1] 50 60 65 82

You can also use the built-in command seq, to create a sequence of numbers without typing all of them in manually.

seq(0, 30) # This is the same as just `0:30`

##  [1]  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
## [26] 25 26 27 28 29 30

seq(0, 30, 3) # Every third number

##  [1]  0  3  6  9 12 15 18 21 24 27 30

A vector can also contain characters:

animals <- c('mouse', 'rat', 'dog')
animals

## [1] "mouse" "rat"   "dog"

The quotes around “mouse”, “rat”, etc. are essential here and can be either single or double quotes. Without the quotes R will assume there are objects called mouse, rat and dog. 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(weight_g)

## [1] 4

length(animals)

## [1] 3

An 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(weight_g)

## [1] "numeric"

class(animals)

## [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(weight_g)

##  num [1:4] 50 60 65 82

str(animals)

##  chr [1:3] "mouse" "rat" "dog"

You can use the c() function to add other elements to your vector:

weight_g <- c(weight_g, 90) # add to the end of the vector
weight_g <- c(30, weight_g) # add to the beginning of the vector
weight_g

## [1] 30 50 60 65 82 90

In the first line, we take the original vector weight_g, add the value 90 to the end of it, and save the result back into weight_g. Then we add the value 30 to the beginning, again saving the result back into weight_g.

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 it is a linear vector of a single type, e.g. all numbers. 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" for TRUE and FALSE (the boolean data type)
• "integer" for integer numbers (e.g., 2L, the L indicates 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

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). In this class, we will focus on data frames, which is most commonly used one for data analyses.

Challenge

• 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? Find out by using class to test these examples.

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')

• This happens because vectors can be of only one data type. Instead of throwing an error and saying that you are trying to mix different types in the same vector, R tries to convert (coerce) the content of this vector to find a “common denominator”. A logical can be turn into 1 or 0, and a number can be turned into a string/character representation. It would be difficult to do it the other way around: would 5 be TRUE or FALSE? What number would ‘t’ be?

• 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?

Subsetting vectors

If we want to extract one or several values from a vector, we must provide one or several indices in square brackets. For instance:

animals <- c("mouse", "rat", "dog", "cat")
animals[2]

## [1] "rat"

animals[c(3, 2)]

## [1] "dog" "rat"

We can also repeat the indices to create an object with more elements than the original one:

more_animals <- animals[c(1, 2, 3, 2, 1, 4)]
more_animals

## [1] "mouse" "rat"   "dog"   "rat"   "mouse" "cat"

R indices start at 1. Programming languages like Fortran, MATLAB, Julia, and R start counting at 1, because that’s what human beings typically do. Languages in the C family (including C++, Java, Perl, and Python) count from 0 because that was historically simpler for computers and can allow for more elegant code.

Conditional subsetting

Another common way of subsetting is by using a logical vector. TRUE will select the element with the same index, while FALSE will not:

weight_g <- c(21, 34, 39, 54, 55)
weight_g[c(TRUE, FALSE, TRUE, TRUE, FALSE)]

## [1] 21 39 54

Typically, these logical vectors are not typed by hand, but are the output of other functions or logical tests. For instance, if you wanted to select only the values above 50:

weight_g > 50    # will return logicals with TRUE for the indices that meet the condition

## [1] FALSE FALSE FALSE  TRUE  TRUE

## so we can use this to select only the values above 50
weight_g[weight_g > 50]

## [1] 54 55

We will consider conditions in more detail in the next few lectures.

Missing data

As R was designed to analyze datasets, it includes the concept of missing data (which is uncommon in other programming languages). Missing data are represented in vectors as NA.

When doing operations on numbers, most functions will return NA if the data you are working with include missing values. This feature makes it harder to overlook the cases where you are dealing with missing data. You can add the argument na.rm = TRUE to calculate the result while ignoring the missing values.

heights <- c(2, 4, 4, NA, 6)
mean(heights)

## [1] NA

max(heights)

## [1] NA

mean(heights, na.rm = TRUE)

## [1] 4

max(heights, na.rm = TRUE)

## [1] 6

## Extract those elements which are not missing values.
heights[!is.na(heights)]

## [1] 2 4 4 6

## Returns the object with incomplete cases removed. The returned object is an atomic vector of type `"numeric"` (or `"double"`).
na.omit(heights)

## [1] 2 4 4 6
## attr(,"na.action")
## [1] 4
## attr(,"class")
## [1] "omit"

## Extract those elements which are complete cases. The returned object is an atomic vector of type `"numeric"` (or `"double"`).
heights[complete.cases(heights)]

## [1] 2 4 4 6

Recall that you can use the class() function to find the type of your atomic vector.

Challenge

1. Using this vector of length measurements, create a new vector with the NAs removed.

lengths <- c(10, 24, NA, 18, NA, 20)

2. Use the function median() to calculate the median of the lengths vector.

Loops and vectorization

Loops, specifically for-loops, are essential to programming in general. However, in R, you should avoid them as often as possible because there are more efficient ways of doing things that you should use instead. It is still important that you understand the concept of loops and you might also use them in some of your own functions if there is no vectorized way of going about what you want to do.

You can think of a for-loop as: “for each number contained in a list/vector, perform this operation” and the syntax basically says the same thing:

v <- c(2, 4, 6)
for (num in v) {
    print(num)
}

## [1] 2
## [1] 4
## [1] 6

Instead of printing out every number to the console, we could also add numbers cumulatively, to calculate the sum of all the numbers in the vector:

# To increment `w` each time, we must first create the variable,
# which we do by setting `w <- 0`, referred to as initializing.
# This also ensures that `w` is zero at the start of the loop and
# doesn't retain the value from last time we ran this code.
w <- 0
for (num in v) {
    w <- w + num
}
w

## [1] 12

If we put what we just did inside a function, we have essentially recreated the sum function in R.

my_sum <- function(input_vector) {
    vector_sum <- 0
    for (num in input_vector){
        vector_sum <- vector_sum + num
    }
    return(vector_sum)
}

my_sum(v)

## [1] 12

Although this gives us the same output as the built-in function sum, the built-in function has many more optimizations so it is much faster than our function. In R, it is always faster to try to find a way of doing things without writing a loop yourself. When you are reading about R, you might see suggestions that you should try to vectorize your code to make it faster. What people are referring to, is that you should not write for loops in R and instead use the ready-made functions that are much more efficient in working with vectors and essentially performs operations on entire vector at once instead of one number at a time. If anyone is interested in more details about how this works, please ask after class, but conceptually loops operate on one element at a time while vectorized code operates on all elements of a vector at once.

In our next lecture, we’ll dive into working with real data using all that we’ve learned today.

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1. Refer to the class resources page for which style to adhere to.↩

2. There are a few other symbols as well, all of which can be viewed at the end of this post about RStudio code completion.↩

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