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How to Create and Manipulate Variables and Vectors in R

In this article, we provide a step-by-step guide on how to create and manipulate variables and vectors in R.
© Wellcome Genome Campus Advanced Courses and Scientific Conferences
When working under your current R session, the entities that R creates and uses are called “objects”.
There is a wide range of objects that can be created and that can contain variables, arrays of numbers, character strings, functions….
They can be classified into different types: Vectors, Lists, Data frames, Matrices, Factors, or Functions. In this course we will show you how to create and manipulate Vectors, Lists, and Data frames. For the sake of time and course content we will not cover the rest of the object types, but we encourage you to find more information on these in the following link: https://cran.r-project.org/doc/manuals/r-release/R-intro.html Let’s see together how Variables and Vectors can be created and manipulated.

What is a Variable and what is a Vector

Definition of a variable
Variables are objects in R that you can use to store values. It can consist of a single value, basic or complex arithmetic operations, or even be more complex such as a column in a data matrix or a data frame. We will see these complex forms in the following steps of this course.
Definition of a vector
A vector is substantially a list of variables, and the simplest data structure in R. A vector consists of a collection of numbers, arithmetic expressions, logical values or character strings for example. However, each vector must have all components of the same mode, that are called numeric, logical, character, complex, raw.

How to create and manipulate Variables

Step 1. We recommend you to work in the same working sub-directory that you created previously for analyses conducted with R. If the sub-directory is not created yet or mistakenly removed, please do it again, and launch R
$ mkdir exerciseR$ cd exerciseR
$ R
 
Step 2. If you forgot before launching R, there is another option you can use to make sure to set the correct working directory using the “setwd()” command, and then check your position using the “getwd()” command (this will also be helpful in RStudio). You can use “getwd()” in R as you used “pwd” in Unix
 
Launch R
 
$ R
 
Go to your working directory
 
> setwd("/Users/imac/Desktop/exerciseR")
> getwd()
[1] "/Users/imac/Desktop/exerciseR"
 
Step 3. Let’s create a simple variable called x. We need to assign elements to this variable. The assignment to a variable can be done in 2 different but equivalent ways, using either the “<-“ or “=” operators. You can retrieve the value of x simply by typing x
 
> x <- 3 * 4 + 2 * 5 + 3
> x = 3 * 4 + 2 * 5 + 3
> x
[1] 25
 
Step 4. Let’s create another variable called y that can either contain a new value or for example contain a basic or more complex operation on the first variable x
 
> y <- x^4 - 4*x + 5
> x
[1] 390530
 
Note 1. Naming a Variable is not trivial and must be done appropriately:
 
 
    • Variable names can contain letters, numbers, underscores and periods
 
    • Variable names cannot start with a number or an underscore
 
    • Variable names cannot contain spaces at all
 
 
> x.length <- 3*2
> x.length
[1] 6
> _x.length <- 3*2
Error : unexpected input in "_"
> 3x.length <- 3*2
Error : unexpected symbol in "3x.length"
 
Note 2. Long Variable names are allowed but must be formatted using:
 
 
    • Periods to separate words: x.y.z
 
    • Underscores to separate words: x_y_z
 
    • Camel Case to separate words: XxYyZz
 
 
> x.length <- 3*2
> x.length
[1] 6
> x_length <- 3*2
> x_length
[1] 6
> xLength <- 3*2
> xLength
[1] 6
 

How to create and manipulate Vectors

 
Step 1. A vector can be created using an in-built function in R called c(). Elements must be comma-separated.
 
> c(10, 20, 30)
[1] 10 20 30
 
Step 2. A vector can be of different modes: numeric (and arithmetic), logical, or can consist of characters
 
> c(1.1, 2.2, 3.5) # numeric
[1] 1.1 2.2 3.5
>
> c(FALSE, TRUE, FALSE) # logical
[1] FALSE TRUE FALSE
>
> c("Darth Vader", "Luke Skywalker", "Han Solo") # character
[1] "Darth Vader" "Luke Skywalker" "Han Solo"
 
Note. Please note that when the value is a character data type, quotations must be used around each value, such as in “Han Solo”
 
Step 3. A vector can be assigned to a variable name, using 3 methods: either using the “<-“ or “=” operators or the assign function. You will very rarely see the last method which is to revert the order of assignment
 
> assign("x", c(10, 20, 30))
> x
[1] 10 20 30
>
> x <- c(10, 20, 30)
> x
[1] 10 20 30
>
> x = c(10, 20, 30)
> x
[1] 10 20 30
>
> c(10, 20, 30) -> x
> x
[1] 10 20 30
 
Step 4. In R, an object must be defined by properties of its fundamental components, such as the mode, that can be retrieved by the function “mode()” and the length by the function “length()”. An empty vector can be created and may still have a mode
 
> v <- numeric()
> w <- character()
 
> mode(x)
[1] "numeric"
> mode(v)
[1] "numeric"
> mode(w)
[1] "character"
>> length(x)
[1] 3
 
Step 5. Basic operations with numeric vectors
 
> x <- c(10, 20, 30)
> x
[1] 10 20 30
> 1/x
[1] 0.10000000 0.05000000 0.03333333
 
Step 6. A vector can be used in arithmetic expressions and/or as a combination of existing vectors
 
> x <- c(10, 20, 30)
> y <- x*3+4
> y
[1] 34 64 94
> z <- c(x, 0, 0, 0, x)
> z
[1] 10 20 30 0 0 0 10 20 30
> w <- 2*x + y + z
> w
[1] 64 124 184 54 104 154 64 124 184
 
Note how the addition is sequential in this last case (value 1 of x is multiplied by 2, then added to value 1 of y then added to value 1 of z, etc…)
 
Step 7. A vector can use built-in functions in R, such as mean() to calculate the mean of a certain object (here x), var() to calculate its variance, and sort() to sort the content here of object z.
 
> mean(x)
[1] 20
> var(x)
[1] 100
> sort(z)
[1] 0 0 0 10 10 20 20 30 30
 
Step 8. R uses built-in functions and operators to generate regular sequences. Here are examples of how to use rep() to repeat items (arguments needed are the value to repeat and the number of repeats) and seq() (arguments needed are the start, the end, and the interval) to create a sequence of items.
 
> a <- c(1:10)
> a[1] 1 2 3 4 5 6 7 8 9 10
> b <- rep(a, times=2)
> b[1] 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10
> b <- rep(a, each=2)
> b[1] 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10
> c <- seq(-2, 2, by=.5)
> c
[1] -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
 
Step 9. The content of a vector can be compared to another using basic operators
 
> x==x
[1] TRUE TRUE TRUE
> x==y
[1] FALSE FALSE FALSE
> x!=y
[1] TRUE TRUE TRUE
 
Step 10. The content of a Vector can be easily queried and modified. For this it is possible to use Index Vectors to subset some elements of an existing vector, using square brackets
 
> x
[1] 10 20 30
> x[3]
[1] 30
> x[3] <- 50
> x
[1] 10 20 50
> length(x)
[1] 3
 
Step 11. For Index Vectors of character strings, a “names” attribute can help identify components and query the data.
 
> dairy <- c(10, 20, 1, 40)
> names(dairy) <- c("milk", "butter", "cream", "yogurt")
> breakfast <- dairy[c("milk","yogurt")]
> breakfastmilk yogurt10 40
This might be useful to remember when you will manipulate data frames.

Discussion

Now try it yourself and discuss in the comment area below:
Question 1. Did you manage to create and manipulate Variables?
Question 2. Did you manage to create and manipulate Vectors?

Exercise

Let’s try it !
Question 1. Could you create 3 vectors:
a vector x containing the numbers 3, 10 and 30
a vector m containing the content of x repeated twice
a vector n containing two copies of x separated by a 0
Question 2. Is the content of m equal to the content of n?
Question 3. Note that you should also obtain a warning message because the 2 vectors are not of the same length. How can you check the length of both vectors?
© Wellcome Genome Campus Advanced Courses and Scientific Conferences
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