Chapter 3 Re-using your code: loops and functions

In the previous section we discussed how to use variables, do simple computations, and print the results. This is all useful knowledge. But it is not enough. What if we want to do similar calculations over and over again, each time using slightly different values? We can, obviously, write similar code over and over again, but that is a little stupid thing to do: first, it is tedious; second–each new line of code is a source of additional errors; and finally–if you want to change the way you perform the calculations then you have to change all these lines. Again, it is tedious and error-prone.

Here we’ll discuss two widely used constructs–for loops and functions. Both of these let you to easily re-use your existing code, but in different ways. First, we consider loops and thereafter functions. Functions will be introduced in an abstract sense, followed by an overview of built-in R functions and packages, and finally we explain how to write your own functions.

3.1 For-loops: repeating code

For loops is perhaps the most intuitive way of re-using your code. Let’s start with a simple example. Let’s say you want to print the squares of numbers from one to five in a form

1^2 = 1
2^2 = 4
...

One such line can be printed with cat() command (see Section 2.6) as

i <- 2
i2 <- i^2
cat(i, "^2 = ", i2, "\n", sep="")

## 2^2 = 4

First we use variable i to hold the value, square of which we want to compute. Thereafter we compute the result, variable i2, equal to \(2^2\). And finally we print using cat() where we provide it with five arguments: first the number, square of which we want to compute (i), thereafter the constant text where nothing changes (^2 =), and third, the result (i^2). This is followed by new line ("\n") and finally sep="" tells not to add extra spaces between the items to be printed.

Now obviously we can repeat the lines above for 5 times. But fortunately, there is an easier way to do it:

for(i in 1:5) {
   i2 <- i^2
   cat(i, "^2 = ", i2, "\n", sep="")
}
cat("done\n")

## 1^2 = 1
## 2^2 = 4
## 3^2 = 9
## 4^2 = 16
## 5^2 = 25

## done

This is for loop. It is fairly obvious what it does: it “loops over” numbers from “1” to “5”, each time setting the variable “i” equal to that number. And thereafter it computes i2 = i^2 and prints both i and i2.

Note the necessary changes we made here:

the second (computing) and third line (printing) are exactly the same as above. No changes here.
the original first like, i <- 2 is gone, and replaced by the for loop header for(i in 1:5). You can imagine it is a series of similar assignments, i <- 1, i <- 2, i <- 3, and so on.
the most important lines of code–for loop body, are inside curly braces { and }. The braces should start immediately after the for loop header. Everything that is inside of the braces will be repeated five times, what is outside of braces (printing “done”) is only done once.

Note also that inside of the for-loop body, the variable “i” takes values 1, 2, 3, …, one after the other. It is still the same variable, just the values change inside of the loop.

Exercise 3.1 The expression 1:5 is a shorthand for seq(1, 5). seq() is a function that creates sequences. Look up it’s help by ?seq (on the console), and use it to modify the loop above in a way that it prints not numbers 1,2,3,4,5; but odd numbers 1,3,5,9 (and their squares) instead.

See the solution

Exercise 3.2 Use for-loop to print the following:

7*10 = 70
7*9 = 63
7*8 = 56 
...
7*0 = 7

See the solution

Exercise 3.3 Use for-loop to print 10 caret signs like

^^^^^^^^^^

Do this by printing just one caret sign at time.

See the solution

Exercise 3.4 Use two for-loops inside each other (nested loops) to print “asivärk”:

^
^^
^^^
^^^^
^^^^^
^^^^^^
^^^^^^^
^^^^^^^^
^^^^^^^^^
^^^^^^^^^^

(The number of signs in row runs from 1 to 10)

See the solution

Exercise 3.5 Use nested for loops to print a wedge:

-vvvvvvvvvv-
--vvvvvvvv--
---vvvvvv---
----vvvv----
-----vv-----

In the first row, there are 10 tilde symbols ~.

Hint: first create the triangle

~~~~~~~~~~
~~~~~~~~
~~~~~~
~~~~
~~

and thereafter add code to produce --s. Check out how to achieve this by using the Asivärk example above, and the seq() function to make sequences like 2, 4, 6, 8, …

See the solution

TBD: use accumulator

For-loop is one of the simplest ways to re-use the code. It is also a very old programming construct and has been around since 1950-s, see e.g. the Wikipedia page for it. There are other types of loops and constructs that are similar to loops (see Section 16.1). But now we’ll turn to functions, a different way to re-use your (and the others’) code.

3.2 What are Functions?

For-loops are good for executing a set of instructions several times in a row. But they are not good if we want to run the same lines, but not like a single chunk but a few times here and a few times there. This is where functions come to play.

You can think of a function as a named sequence of instructions: you bind a number of code lines together and attach a label to those. They are in many ways similar to variables–they are “boxes” with a label, but instead of numbers and strings, they contain lines of code. Now when you need to execute those lines, you just “call the label” (or in technical parlance, call the function). And you can do it one or more times (or even not at all) at different places in your program. It is similar how you can use your variables in different places of your code.

This is a way of encapsulating multiple instructions into a single “unit” that can be used in a variety of different contexts. So rather than repeatedly writing down all the individual instructions for “make a sandwich” every time you’re hungry, you can define a make_sandwich() function once and then just call (execute) that function when you want to perform those steps. This is the beauty of functions: as in for-loop, you can write your code once, and execute it multiple times.

But note that functions are not a substitute for for-loop, nor the way around. If you want to make 10 sandwiches, you have either to write make_sandwich 10 times, or put it into a for-loop. Loops are for repeating instructions, functions is for collecting multiple instructions under a single label (into a function).

Typically, functions also accept inputs (called arguments) so you can make the things slightly differently from time to time. For instance, sometimes you may want to call make_sandwich(cheese) while another time make_sandwich(chicken). Functions often also create something, called return value, e.g. a sandwich in the example above.

Let’s repeat the concepts of arguments and return value with an example. For instance, function max() that finds the largest value among numbers. Let’s look at the following function call:

max(1,2,3)  # returns 3

We provide arguments (inputs, sometimes called parameters), here numbers 1, 2, and 3, in parenthesis. We say that these arguments are passed to the function (passed like a ball). We say that a function then returns a value, number “3” in this example, which we can either print or assign to a variable and use later.

Finally, the functions may also have side effects. An example case is the cat() function that just prints it’s arguments.
For instance, in case of the following line of code

cat("The answer is", 1+1, "\n")

we call function cat() with three arguments: "The answer is", 1 + 1 and "\n" (the line break symbol). As cat()’s side effect, we see a sentence The answer is 2 popping up on the screen. This is what we are interested here–printing. We do not care about its return value (in fact, it does not return anything).

Exercise 3.6

Consider the function seq() (see ?seq) that creates sequences.

Does the function have return value? What is it?
Does it have side effects? What are these, if any?

See the solution

3.3 How to Use Functions

R functions are referred to by name (technically, they are values like any other variable, just not atomic values). As in many programming languages, we call a function by writing the name of the function followed immediately (no space) by parentheses (). Sometimes this is enough, for instance

Sys.Date()

gives us the current date and that’s it.

But often we want to customize what the functions do. This can be done with arguments (inputs) inside the parenthesis. Functions can accept multiple arguments, in that case they are separated by comma (,). Thus computer functions look just like multi-variable mathematical functions, although usually with fancier names than f().

Here are a few examples about customizing what the functions do:

## We want to compute square root, and we specify which number
## we want it to be computed of (25)
sqrt(25)

## [1] 5

## We want to count characters, and we specify which
## string we want these to be counted of
nchar("Hello world")  # note: space is a character too

## [1] 11

## We want to find the smallest number, and we specify which
## numbers the computer must find the minimum of
min(1, 6/8, 4/3)  # 0.75 = 6/8

## [1] 0.75

In order to indicate that something is a function, not an ordinary variable, we include empty parentheses () when referring to a function by name. This does not mean that the function takes no arguments, it is just a useful shorthand for indicating that something is a function.

Note: You always need to supply the parenthesis if you want to call the function (force it to do what it is supposed to do). If you leave the parenthesis out, you get the function definition printed on screen instead. So cat() is actually a function call while cat is the function. You can see that it is a function if you just print it as print(cat). However, we ignore this distinction here.

If you call any of these functions interactively, R will display the returned value (the output) in the console. However, the computer is not able to “read” what is written in the console—that’s for humans to view! If you want the computer to be able to remember a returned value, you need store it in a variable:

# store min value in smallest.number variable
smallest_number <- min(1, 6/8, 4/3)

# we can then use the variable as normal, such as for a comparison
min_is_big <- smallest_number > 1  # FALSE

Often it is not necessary to even store the returned values, we can use the function calls directly in the computations

3.14 + sqrt(5)  # add two numbers, the other is the function return value

## [1] 5.376068

We can nest multiple function calls, i.e. give function calls as arguments to other functions. In the following example, the returned value of the “inner” function sqrt() is immediately used as an argument for the middle function min(). Its return value, in turn, is fed to the outer function print(). Because that value is used immediately, we don’t have to assign it a separate variable name. It is known as an anonymous variable.

## function calls inside function calls works too!
print(min(1.5, sqrt(3)))

## [1] 1.5

note also that in the last example, we are solely interested in the side effect of the print() function. It also returns it’s argument (here 1.5, as this is the min(1.5, sqrt(3)) here) but we do not store it in a variable.

3.4 Positional and named arguments

R functions take two types of arguments: positional arguments and named arguments. This is because the function has to know how to treat each of it’s arguments. For instance, we can round number \(e = 2.718282\) to 3 digits by round(2.718282, 3). But in order to do this, the round() function must know that 2.718282 is the number and 3 is the requested number of digits, and not the other way around. It understands this because it requires the number to be the first argument, and digits the second argument. This approach works well in case of known small number of inputs. However, this is not an option for functions with variable number of arguments, such as cat(). cat() just prints out all of it’s (potentially a large number of) inputs, except a limited number of special named arguments. One of these is sep, the string to be placed between the other pieces of output (by default just a space is printed). Note the difference in output between

cat(1, 2, "-", "\n")

## 1 2 -

cat(1, 2, sep="-", "\n")

## 1-2-

In the first case cat() prints 1, 2, "-", and the line break "\n", all separated by a space. In the second case the name sep="-" ensures that "-" is not printed out but instead treated as the separator between 1, 2 and "\n".

3.5 Built-in R Functions

As you have likely noticed, R comes with a variety of functions that are built into the language. In the above example, we used the print() function to print a value to the console, the min() function to find the smallest number among the arguments, and the sqrt() function to take the square root of a number. Here is a few examples you can experiment with.

Function Name	Description	Example
`sum(a,b,...)`	Calculates the sum of all input values	`sum(1, 5)` returns `6`
`round(x,digits)`	Rounds the first argument to the given number of digits	`round(3.1415, 3)` returns `3.142`
`toupper(str)`	Returns the characters in uppercase	`toupper("hi there")` returns `"HI THERE"`
`paste(a,b,...)`	Concatenate (combine) characters into one value	`paste("hi", "there")` returns `"hi there"`
`nchar(str)`	Counts the number of characters in a string	`nchar("hi there")` returns `8` (space is a character!)
`c(a,b,...)`	Concatenate (combine) multiple items into a vector (see chapter 7)	`c(1, 2)` returns `1, 2`
`seq(a,b)`	Return a sequence of numbers from a to b	`seq(1, 5)` returns `1, 2, 3, 4, 5`

To learn more about any individual function, look them up in the R documentation by using ?FunctionName account as described in Section 2.7.

Being able to program in a language is to some extent just knowing what functions are available in that language and how to use those. So you should look around and become familiar with these functions. But you do not need to memorize them! Instead, figure out how to learn to use them when you need it.

3.6 Packages: even more functions

Although R comes with lots of built-in functions, you can always use more. Packages are additional sets of R functions (they also tend to contain data, variables and certain other things) that are written and published by the R community. Because many R users encounter the same data analysis challenges, programmers are able to use these libraries and thus benefit from the work of others (this is the amazing thing about the open-source community—people solve problems and then make those solutions available to others). Popular packages include dplyr for manipulating data, ggplot2 for visualizations, and data.table for handling large datasets.

Most of the R packages do not ship with the R software by default, and need to be installed (once) and then loaded into your interpreter’s environment (each time you wish to use them). While this may seem cumbersome, it is a necessary trade-off between speed and size. R software would be huge and slow if it would include all available packages.

Luckily, it is quite simple to install and load R packages from within R. To do so, you’ll need to use the built-in R functions install.packages and library.⁷ Below is an example of installing and loading the stringr package (which contains many handy functions for working with character strings):

## Do this ONLY ONCE on your computer!
install.packages("stringr")

You should install each package only once per computer. As installation may be slow and and resource-demanding, you should not do it repeatedly inside your script!. Even more, if your script is also run by other users on their computers, you should get their explicit consent before installing additional software for them! The easiest remedy is to solely rely on manual installation.

Example 3.1

Installation in scripts usually fails with error message when you try to compile the report (or knit rmarkdown, see Section 10.4). This is not because it is not possible, but because you haven’t told R where to download the package. It is more foolproof to install packages manually.

Exactly the same syntax—install.packages("stringr")—is also used for re-installing it. You may want to re-install it if a newer version comes out, of if you upgrade your R and receive warnings about the package being built under a previous version of R.

After installation, the easiest way to get access to the provided functions is by loading the package:

## Load the package--
## make stringr() functions available in this R session
library(stringr)

Depending on the package and your setup, you may see various messages popping up on your console. These are typically harmless, unless you spot an error there.

library(stringr) makes all functions in the stringr package available for R (see the documentation for a list of functions included with the stringr library). For instance, if we want to pad the word “justice” from left with tildes to create a width-10 string, we can do

str_pad("justice", 10, "left", "~")

## [1] "~~~justice"

We can use str_pad function without any additional hassle because library() command made it available.

This is an easy and popular approach. However, what happens if more than one package call a function by the same name? For instance, many packages implement function filter(). In this case the more recent package will mask the function as defined by the previous package. You will also see related warnings when you load the library. In case you want to use a masked function you can write something like package::function() in order to call it. For instance, we can do the example above with

stringr::str_pad("justice", 10, "left", "~")

## [1] "~~~justice"

This approach—specifying namespace in front of the function—ensures we access the function in the right package. If we call all functions in this way, we don’t even have to load the package with library() command. This is the preferred approach if you only need to call functions of a large library only a few times.

3.7 Writing Functions

Now when you are familiar with how to use the other peoples’ functions, it is time to write your own. Any time you have a task that you may repeat throughout a script—or sometimes when you just want to organize your code better—it’s a good practice to write a function to perform that task. This will limit repetition and reduce the likelihood of errors, as well as make things easier to read and understand (and thus identify flaws in your analysis).

Functions are in many ways just a few lines of “canned” code. Exactly as numbers and strings, they are normally stored in the “labeled boxes in memory”–assigned to variables.

3.7.1 Basics of defining functions

Let’s explain how to write functions through an example. We create a function fullName that takes first name and last name as strings, and returns full name, i.e. a single string made of the these names.

fullName <- function(first, last) {
  full <- paste(first, last)
  return(full)
}

This is not particularly interesting function–it just concatenates two strings–but it helps us to explain all the major building blocks of functions.

Function definition contains several important pieces:

We assign the function to a variable (here fullName). As in case of other variables, this means we store this chunk of code into memory, and label that memory area fullName.
but what we store into memory is not numbers or characters, but R code. This code is put into a function using the syntax function(....) to indicate that you are creating a function (and not a number or character string).
the rest of the code follows the function(...) declaration inside curly braces { ... }.
We often want to feed the function with certain inputs, here the first and last name. These inputs must be referred to somehow inside of your function. You list these in the parentheses in the function(....) declaration. These are called formal arguments. Formal arguments will contain the actual values when calling the function (called actual arguments). For example, when we call fullName("Alice", "Kim"), the value of the first actual argument ("Alice") will be assigned to the first formal argument (first), and the value of the second actual argument ("Kim") will be assigned to the second formal argument (last). Inside the function’s body, both of these formal arguments behave exactly as ordinary variables with values “Alice” and “Kim” respectively.

We could have made the formal argument names anything we wanted (name_first, xyz, etc.). This is all well, as long as we also use the same names in the lines of code that makes the function body.

The formal argument names are only valid within the function’s body (inside of the curly braces). Variables first, last, and full only exist within this particular function. They are “forgotten” as soon as the function is done its job. If you try to access those outside of the function, you get and error like Error: object 'first' not found.
Body: The body of the function is a block of code between curly braces { } (a “block” is represented by curly braces surrounding code statements). The opening brace { must follow the function(....) declaration, the closing } will complete the function–whatever follows the closing brace is not part of the function any more. The opening { is often put immediately after the arguments list, and the closing } on its own line.

The function body specifies all the instructions (lines of code) that your function will perform. A function can contain as many lines of code as you want—you’ll usually want more than 1 to make it worth writing, but if you have more than fits to your screen, then you might want to break it up into separate functions. You can use the formal arguments here exactly as you use any other variables. You can also create new variables, call other functions, you can even declare functions inside functions… basically any code that you would write outside of a function can be written inside as well! One very useful trick is to print the values of local variables–this helps you to understand what is going on inside the function and spot the problems.

But remember that all the variables you create in the function body are local variables. These are only visible from within the function and “will be forgotten” as soon as the function is done–when you return from the function. However, variables defined outside of the function are visible from within too.
Return value is what your function produces. You can specify this by calling the return() function and passing it the value that you wish your function to return. It is typically the last line of the function. Note that even though we returned a variable called full, that variable was local to the function and so doesn’t exist outside of it; thus we have to store the returned value into another variable if we want to use it later (for instance, as name <- fullName("Alice", "Kim")).

After we have defined the function, we can call (execute) it. We call a function we defined exactly in the same way we call built-in functions. For instance, we may call it as

fullName("Alice", "Kim")

## [1] "Alice Kim"

When we do so, R will take the actual arguments (here "Alice" and "Kim") and assign these to the formal arguments (here first and last). Then it executes each line of code in the function body one at a time. Here the body only contains a single line, paste(first, last), and when the actual arguments are substituted in place of formal arguments, it becomes paste("Alice", "Kim"). This, in turn produces "Alice Kim". When it gets to the return() call, it will end the function and return the given value. We can either assign it to a variable, or print it right away as above.

Example 3.2 Let’s create a function that converts length given in feet to meters:

feet2m <- function(feet) {
   meters <- feet * 0.3048
   return(meters)
}

Now we can compute

feet2m(500)

## [1] 152.4

i.e. elevation gain of 500ft on a trail is the same as having to climb 152.4 meters.

Exercise 3.7 The supermassive black hole in M87 is 55 millions of light-years away. Create a function ly2km that converts light-years to kilometers. Use this function to compute distance to to the black hole in kilometers.

See the solution

3.7.2 More details about functions

Now when you are familiar with the basics of functions, we discuss a few more easy and widely used details.

3.7.2.1 Return statement is optional

return() statement is usually not necessary in R as R implicitly returns the last value it computes (last expression it evaluated) anyway.⁸ So we may shorten the definition fullName() function into

fullName <- function(first, last) {
  full <- paste(first, last)
}

or even not store the concatenated names into full:

fullName <- function(first, last) {
   paste(first, last)
}

The last evaluation was concatenating the first and last name, and hence the full name will be automatically returned.

Example 3.3 The feet2m() function we created above can be writtng in a shorter form as

feet2m <- function(feet) {
   feet * 0.3048
}

This is exactly equivalent to the definition above, which one do you prefer is the question of coding style.

Exercise 3.8 Write a function decade that converts years to decade. Do not use return statement.

Here is an example of a few years and the corresponding decades:

year	decade
2024	2020
1931	1930
1969	1960
1970	1970

Show that it works when you pass it these years.

Hint: use integer division %/%, see Section 2.5.1.

See the solution

3.7.3 Default argument values

Consider the fullName() function again. Now assume we have a lot of peole who’s family name is “Kim”. We can get their full names as

fullName("Alice", "Kim")

## [1] "Alice Kim"

fullName("Eun-Gyeong", "Kim")

## [1] "Eun-Gyeong Kim"

fullName("Ji-Won", "Kim")

## [1] "Ji-Won Kim"

and so on. In that case we may want to write the function slightly differently. We can make family name optional–if we do not supply it, the function will take “Kim”, but if we want another names besides “Kim”, we can supply it. This can be achieved through default argument value in the form last = "Kim":

fullName <- function(first, last = "Kim") {
                           # take 'Kim' if not supplied
   paste(first, last)
}

Now we can write

fullName("Eun-Gyeong")  # will take 'Kim' as last name

## [1] "Eun-Gyeong Kim"

fullName("Mongkut", "Sisuk")  # will take 'Sisuk' as last name

## [1] "Mongkut Sisuk"

As you see, can use default values to make the code somewhat simpler. In this example, the default argument is not very valuable: it is easy to just write “Kim” as needed. But deault arguments are widely used in a different context–for functions that take a large number of arguments. Imagine a function plot(). This might take a large number of arguments: what to plot, how big the plot should be, which color to use, whether to save it to a file, what should be the file format… It is just too much work to supply all these arguments if you just want to make a quick plot. This is where the defaults help–the default arguments ensure you get a nice plot, and if you are not happy with what you get, you can adjust it by changing some of the default values.

Exercise 3.9 Write a function date() that takes 3 arguments: day, month, and year, and returns the date as yyyy-mm-dd. If year is not submitted, it will assume it is 2024. It should work like this:

date(30, 3, 2012)
  "2012-3-30"
date(30, 3)
  "2024-3-30"

See the solution

3.7.4 Returning values versus producing output

Unfortunately, the difference between functions that produce output (i.e. print on screen, see more in Producing output) and that return a value (that will be automatically printed on screen) may be less than obvious. For instance, consider two functions that compute minutes in day:

minutesDay1 <- function() {
   mid <- 24*60
   cat(mid, "\n")  # print the numbers
}
minutesDay2 <- function() {
   mid <- 24*60
   return(mid)  # return the number
}

When we call the former function on console, we get

minutesDay1()

## 1440

and when we call the latter, we get

minutesDay2()

## [1] 1440

The output produced by both functions appears almost the same. However, behind the scene (well, behind the screen 😄) there are important differences:

The first function produces output, i.e. it always prints the number 1440. The second function does not print anything. The line [1] 1440 we see when we call minutesDay2(), that line is created by R console that automatically prints the last result. So in the second case 1440 is not something that the function prints, but the result, the return value that the R Console prints.
The difference may not matter when we run it like in the example above. But if we want to assign the result to a variable, then what happens is not the same any more:

mid1 <- minutesDay1()

## 1440

mid2 <- minutesDay2()

minutesDay1() still prints 1440 but minutesDay2() is now silent. This is because the result is not automatically printed if it is assigned to a variable.

The second function returns the value that can be assigned to a variable (mid2 in this example). The first function automatically returns whatever its last statement, cat(mid, "\n") returns.
This happens to be the special empty symbol NULL:

mid1

## NULL

mid2

## [1] 1440

We can see mid1 variable is empty while mid2 contains the expected value.

So the first example prints the result but does not return it, and the second function does not print it but returns the value. One can also create a function that does both (or neither). But which approach is better?

Obviously, it depends on what you are doing. In practice, it is common to have two types of functions–one type only prints and does not compute anything, and the other type only computes and returns a value, but leaves printing to the dedicated output functions.

Exercise 3.10 Create a function that takes a single argument, name, and returns a string: “Hi <name>, isn’t it a nice day today?” where <name> should be replaced by the argument name.

Demonstrate that the function prints the message when called on R console, but does not print anything if its result is assigned to a variable.

Hint: check out paste() and paste0() functions to concatenate strings.

See the solution

Resources

R Function Cheatsheet

When using RStudio, one can also load packages on the packages’ pane by clicking the respective checkboxes. There is also an install button for package installation. However, the former actually runs the library command and the later install.packages command. You will see these commands on console when you click the buttons there.↩︎
It is, however, necessary in certain other languages.↩︎