For loops

It is possible to make R repeat certain lines of code using loops. The ability to run a line of code multiple times is the first large step on your road to making your code more structured and reusable.

Imagine we have three strings in a vector that we want to print. We could start by calling cat three times to create a program like:

loop.R

my_words <- c("Jean", "Golding", "Institute")
cat(my_words[1])
cat(my_words[2])
cat(my_words[3])

Jean

Golding

Institute

This printed the output we want. But you may feel that repeating the same call to cat is wasteful code, particularly if we want to repeat the same operation for many elements. If we can manage to write that line only once then we could save ourselves some typing and potentially make the code easier to read!

FOR EACH word IN my_words
    DO SOMETHING WITH word

We can write a for loop in R which will perform a task once for each word in our vector:

loop.R

my_words <- c("Jean", "Golding", "Institute")

for (word in my_words) {
    cat(word, " ")
}

Jean  Golding  Institute

Even in this tiny example, we have ve taken a script that was four lines of code and have reduced it to three lines, and more interestingly the same loop will work no matter how many items there are in the vector my_words.

This maps to real life where you may want, for example, to pay for each item on your shopping list. Another way of saying that could be “for each item on my shopping list, add its price to my total”, or as you would write that in R:

total <- 0
for (item in shopping_list) {
    total <- total + item
}

Exercise

Edit loop.R to have a different number of words in the vector.

Does it work if you put numbers (i.e. integers or floats) in there as well?
What happens if the vector my_words is empty? (hint: empty vectors look like vec <- vector())

Answer

Adding more items to the list makes the loop go around more times

my_words <- c("Hello", "R", "Goodbye", "R")

for (word in my_words) {
    cat(word, " ")
}

Hello  R  Goodbye  R

A vector cannot have a mixture of data types, so all elements are converted to strings.

my_words <- c("Hello", "R", as.numeric(404), "Goodbye", "R", 42)

for (word in my_words) {
    cat(word, "-", class(word), "\n")
}

Hello - character 
R - character 
404 - character 
Goodbye - character 
R - character 
42 - character

Looping over an empty list does not print anything

my_words <- vector()

for (word in my_words) {
    cat(word, " ")
}

for statement syntax

A for statement has an scaffolding, the parts of the line which must always be the same and tell R that you’re trying to make a loop, composed by the word for, the parentheses (()), the word in and the curly braces ({}) at the end of the line. These must always be there and in that order:

for  ( word in my_words )  {
    cat(word)
}

Once the scaffolding is in place, you can place between it the things that you care about. The first thing to think about is the variable that you want to loop over. In our case we want to loop over the vector my_words because we want to print every item in that vector:

for  ( word in my_words )  {
    cat(word)
}

Now we need to decide what name we want to give temporarily to each item as we get to it. As with any variable naming, it is important that we choose a good name which describe a single object from the vector. For example, if we’re looping over all students in a class then we could call the variable student or if we’re looping over a vector of ages then we could call the variable age. Here, since we’re looping over a vector of generic words, we name our variable word:

for  ( word in my_words ) {
    cat(word)
}

Finally, if we want the loop to actually do something then we need to give the loop a body. The body is the lines of code that are going to be repeated. They can be any R code but it is only within the body of the loop that we can refer to the loop variable word. As with if, it is recommended to use indentation to make it easier to identify what is in the body of the loop and what is not.

for  ( word in my_words ) {
    cat(word)
}

If we want to write more code after the end of a loop, we have to make sure that we have closed the curly braces. So this code will print:

my_words <- c("Hello", "R")

for (word in my_words) {
    cat(word, " ")
}

Hello  R

cat("Goodbye ")

Goodbye

On the contrary, the below code will print Goodbye in each iteration. This is because it was inside the body of the loop.

my_words <- c("Hello", "R")

for (word in my_words) {
    cat(word, " ")
cat("Goodbye ")
}

Hello  Goodbye R  Goodbye

Loop over a range of numbers

There’s a built in operator in R : which provides you with numbers (integers) in a range. When given two numbers, e.g. 1:10, it will give you integers, starting from the first one and going up (or down) to the number you gave in second place. We can use this directly into our loop as the object to loop over and it will print:

for (number in 1:10) {
    cat(number, " ")
}

1  2  3  4  5  6  7  8  9  10

We can also loop over a range of numbers in descending order.

for (number in 5:-5) {
    cat(number, " ")
}

5  4  3  2  1  0  -1  -2  -3  -4  -5

Exercise

Given the matrix below, write a loop that prints for columns 1 and 3, rows 1 to 4..

my_matrix <- t(matrix(1:25, nrow = 5, ncol = 5))

print(my_matrix)

     [,1] [,2] [,3] [,4] [,5]
[1,]    1    2    3    4    5
[2,]    6    7    8    9   10
[3,]   11   12   13   14   15
[4,]   16   17   18   19   20
[5,]   21   22   23   24   25

Answer

my_matrix <- t(matrix(1:25, nrow = 5, ncol = 5))

for (i in 1:4) {
    for (j in c(1,3)) {
        cat(my_matrix[i,j], " ")
    }
    cat("\n")
}