In the introduction you were told that R will read your script, starting at the top and running each line of code until it reaches the bottom. While largely true, 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 reusable.

Imagine we have two strings that we want to print. We could start by making a variable containing one of the words and then printing it:

word <- "Hello"

print(word)

To print our second word, we could copy and paste those two lines to create a program which can print both words:

word <- "Hello"

print(word)

word <- "R"

print(word)

This will print to the screen

[1] "Hello"
[1] "R"

as we want. But we can see that this code is wasteful as the two print lines are identical to each other. They both print whatever data is contained in the variable word. If we can manage to write that line only once then we could save ourselves some typing!

Let’s start by making a container for our words, an R list makes sense:

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

we can now write a loop which will perform a task once for each word in our list:

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

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

Copy that code into a file called loop.R and run it in the terminal with Rscript loop.R. You should see that it prints the same output as our previous example.

We’ve taken a script that was four lines of code and have reduced it to three lines. That might not seem like much of a reduction but the loop we wrote will work no matter how many items there are in the list my_words.

Most loops in R work by doing some set of actions for each item in the list. For this reason, this sort of loop is sometimes called a for-each loop.

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

for (item in shopping_list){
    buy(item)
}

EXERCISE

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

Does it work if you put integers or floats in there as well?

What happens if the list my_words is empty?

Answer

Loop syntax breakdown

Before we move on to other things we can do with loops, let’s first make sure that we understand what’s happening on those three lines of R code that make up the loop.

The first line is where most of the magic is happening and I like to break it down into five sections, three of fixed scaffolding and two where you as a programmer can have input.

The scaffolding is the parts of the line which must always be the same and which R uses to know that you’re trying to make a loop. They’re pointed out here as the word for, the word in and the curly bracket ({) at the end of the line. These must always be there and in that order:

 ↓         ↓          ↓
for (word in my_words){
    print(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 object that you want to loop over. In our case we want to loop over the list my_words because we want to perform some action on every item in that list (we want to print the item):

                ↓
for (word in my_words){
    print(word)
}

Now we have decided what object we are looping over, 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 describes a single object from the list. 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 list of ages then we could call the variable age. Here, since we’re looping over a list of generic words, we name our variable word:

      ↓
for (word in my_words){
    print(word)
}

That’s all that’s required to tell R that we’re making a loop, but 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:

for (word in my_words){
    print(word)       # body of the loop between curly brackets
}

Finally, we get to a peculiarity of R in that it uses curly brackets to decide what is in the body of the loop and what is not. Remember that it will only repeat the code in the body. All code in the body must be contained within the curly brackets.

                 open curly bracket
                      ↓
for (word in my_words){
    print(word)
}
↑
close curly bracket

If we want to write code after the end of a loop, we have to make sure that it is after the close curly bracket. So this code:

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

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

print("...Goodbye")

will print:

[1] "Hello"
[1] "R"
[1] "...Goodbye"

but this code:

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

for (word in my_words){
    print(word)
    print("...Goodbye")
}

will print:

[1] "Hello"
[1] "...Goodbye"
[1] "R"
[1] "...Goodbye"

See how the ...Goodbye was repeated in the second example, this is because it was inside the body of the loop since it was between the curly brackets.

Note that it is convention to indent the lines within a body of a loop using four spaces. This makes it easier to see that these lines are in the body of the loop. You don’t have to do this, but you will make it easier for people reading your code to understand what it does if you do.

Also note that the placement of the curly brackets is also convention. You can place them on any line you want, as long as the open curly bracket is before the body of the loop, and the close curly bracket is after the body. It is convention to place the open curly bracket on the same line as for, and the close curly bracket on its own line directly after the last line of the body of the loop.

What can we loop over

A lot of the power of loops comes from being able to put a lot of different things in the place of my_words.

Most simply, instead of putting a variable name there, you can put a list directly:

for (word in c("Hello", "R")){
    print(word)
}

As well as lists we can put anything which R considers iterable. For now we haven’t come across many of those but as we keep learning we’ll discover many more. One example is the seq function, that can be used to iterate over a fixed set of numbers:

for (i in seq(1,10)){
    print(i)
}


for (i in seq(10,1,-1)){
    print(i)
}

will print the numbers from 1 to 10, and then from 10 to 1.

EXERCISE

Experiment with loop.R and make it loop over both lists and sequences.

Answer

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