# 34 Writing Functions

Have you noticed how we will often calculate the same statistical measures for many different variables in our data? For example, let’s say that we have some pretty standard data about some study participants that looks like this:

library(dplyr)
study <- tibble(
age       = c(32, 30, 32, 29, 24, 38, 25, 24, 48, 29, 22, 29, 24, 28, 24, 25,
25, 22, 25, 24, 25, 24, 23, 24, 31, 24, 29, 24, 22, 23, 26, 23,
24, 25, 24, 33, 27, 25, 26, 26, 26, 26, 26, 27, 24, 43, 25, 24,
27, 28, 29, 24, 26, 28, 25, 24, 26, 24, 26, 31, 24, 26, 31, 34,
26, 25, 27, NA),
age_group = c(2, 2, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1,
1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 2,
2, 1, 1, 1, NA),
gender    = c(2, 1, 1, 2, 1, 1, 1, 2, 2, 2, 1, 1, 2, 1, 1, 1, 1, 2, 2, 1, 1,
1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 2, 1, 2, 2, 1, 2, 2, 1,
1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 2, 2, 1, 1, 2, 1, 2, 1,
1, 1, 2, 1, NA),
ht_in     = c(70, 63, 62, 67, 67, 58, 64, 69, 65, 68, 63, 68, 69, 66, 67, 65,
64, 75, 67, 63, 60, 67, 64, 73, 62, 69, 67, 62, 68, 66, 66, 62,
64, 68, NA, 68, 70, 68, 68, 66, 71, 61, 62, 64, 64, 63, 67, 66,
69, 76, NA, 63, 64, 65, 65, 71, 66, 65, 65, 71, 64, 71, 60, 62,
61, 69, 66, NA),
wt_lbs    = c(216, 106, 145, 195, 143, 125, 138, 140, 158, 167, 145, 297, 146,
125, 111, 125, 130, 182, 170, 121, 98, 150, 132, 250, 137, 124,
186, 148, 134, 155, 122, 142, 110, 132, 188, 176, 188, 166, 136,
147, 178, 125, 102, 140, 139, 60, 147, 147, 141, 232, 186, 212,
110, 110, 115, 154, 140, 150, 130, NA, 171, 156, 92, 122, 102,
163, 141, NA),
bmi       = c(30.99, 18.78, 26.52, 30.54, 22.39, 26.12, 23.69, 20.67, 26.29,
25.39, 25.68, 45.15, 21.56, 20.17, 17.38, 20.8, 22.31, 22.75,
26.62, 21.43, 19.14, 23.49, 22.66, 32.98, 25.05, 18.31, 29.13,
27.07, 20.37, 25.01, 19.69, 25.97, 18.88, 20.07, NA, 26.76,
26.97, 25.24, 20.68, 23.72, 24.82, 23.62, 18.65, 24.03, 23.86,
10.63, 23.02, 23.72, 20.82, 28.24, NA, 37.55, 18.88, 18.3,
19.13, 21.48, 22.59, 24.96, 21.63, NA, 29.35, 21.76, 17.97,
22.31, 19.27, 24.07, 22.76, NA),
bmi_3cat  = c(3, 1, 2, 3, 1, 2, 1, 1, 2, 2, 2, 3, 1, 1, 1, 1, 1, 1, 2, 1, 1,
1, 1, 3, 2, 1, 2, 2, 1, 2, 1, 2, 1, 1, NA, 2, 2, 2, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 2, NA, 3, 1, 1, 1, 1, 1, 1, 1, NA, 2, 1,
1, 1, 1, 1, 1, NA)
) %>%
mutate(
age_group = factor(age_group, labels = c("Younger than 30", "30 and Older")),
gender    = factor(gender, labels = c("Female", "Male")),
bmi_3cat  = factor(bmi_3cat, labels = c("Normal", "Overweight", "Obese"))
) %>%
print()
## # A tibble: 68 × 7
##      age age_group       gender ht_in wt_lbs   bmi bmi_3cat
##    <dbl> <fct>           <fct>  <dbl>  <dbl> <dbl> <fct>
##  1    32 30 and Older    Male      70    216  31.0 Obese
##  2    30 30 and Older    Female    63    106  18.8 Normal
##  3    32 30 and Older    Female    62    145  26.5 Overweight
##  4    29 Younger than 30 Male      67    195  30.5 Obese
##  5    24 Younger than 30 Female    67    143  22.4 Normal
##  6    38 30 and Older    Female    58    125  26.1 Overweight
##  7    25 Younger than 30 Female    64    138  23.7 Normal
##  8    24 Younger than 30 Male      69    140  20.7 Normal
##  9    48 30 and Older    Male      65    158  26.3 Overweight
## 10    29 Younger than 30 Male      68    167  25.4 Overweight
## # ℹ 58 more rows

When we have data like this, it’s pretty common to calculate something like the number of missing values, mean, median, min, and max for all of the continuous variables. So, we might use the following code to calculate these measures:

study %>%
summarise(
n_miss = sum(is.na(age)),
mean   = mean(age, na.rm = TRUE),
median = median(age, na.rm = TRUE),
min    = min(age, na.rm = TRUE),
max    = max(age, na.rm = TRUE)
)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      1  26.9     26    22    48

Great! Next, we want to do the same calculations for ht_in. Of course, we don’t want to type everything in that code chunk again, so we copy and paste. And change all the instances of age to ht_in:

study %>%
summarise(
n_miss = sum(is.na(ht_in)),
mean   = mean(ht_in, na.rm = TRUE),
median = median(ht_in, na.rm = TRUE),
min    = min(ht_in, na.rm = TRUE),
max    = max(ht_in, na.rm = TRUE)
)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      3  66.0     66    58    76

Now, let’s do the same calculations for wt_lbs and bmi. Again, we will copy and paste, and change the variable name as needed:

study %>%
summarise(
n_miss = sum(is.na(wt_lbs)),
mean   = mean(wt_lbs, na.rm = TRUE),
median = median(wt_lbs, na.rm = TRUE),
min    = min(ht_in, na.rm = TRUE),
max    = max(wt_lbs, na.rm = TRUE)
)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      2  148.   142.    58   297
study %>%
summarise(
n_miss = sum(is.na(bmi)),
mean   = mean(bmi, na.rm = TRUE),
median = median(bmi, na.rm = TRUE),
min    = min(bmi, na.rm = TRUE),
max    = max(bmi, na.rm = TRUE)
)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      4  23.6   22.9  10.6  45.2

And, we’re done!

However, there’s a problem. Did you spot it? We accidently forgot to change ht_in to wt_lbs in the min calculation above. Therefore, our results incorrectly indicate that the minimum weight was 58 lbs. Part of the reason for making this mistake in the first place is that there is a fair amount of visual clutter in each code chunk. What I mean is that it’s hard to quickly scan each chunk and see only the elements that are changing.

Additionally, each code chunk was about 8 lines of code. Even with only 4 variables, that’s still 32 lines. I think we can improve on this code by writing our own function. That’s exactly what we will do in the code chunk below. For now, don’t worry if you don’t understand how the code works. We will dissect it later.

continuous_stats <- function(var) {
study %>%
summarise(
n_miss = sum(is.na({{ var }})),
mean   = mean({{ var }}, na.rm = TRUE),
median = median({{ var }}, na.rm = TRUE),
min    = min({{ var }}, na.rm = TRUE),
max    = max({{ var }}, na.rm = TRUE)
)
}

Now, let’s use the function we just created above to once again calculate the descriptive measures we are interested in.

continuous_stats(age)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      1  26.9     26    22    48
continuous_stats(ht_in)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      3  66.0     66    58    76
continuous_stats(wt_lbs)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      2  148.   142.    60   297
continuous_stats(bmi)
## # A tibble: 1 × 5
##   n_miss  mean median   min   max
##    <int> <dbl>  <dbl> <dbl> <dbl>
## 1      4  23.6   22.9  10.6  45.2

Pretty cool, right? We reduced 32 lines of code to 13 lines of code! Additionally, it’s very easy to quickly scan our code and see that the only thing changing from chunk-to-chunk is the name of the variable that we are passing to our function and ensure that it is actually changing. As an added bonus, because we’ve strategically given our function an informative name, the intent behind what we are trying to accomplish is clearer now – we are calculating summary statistics about our continuous variables.

I hope that this little demonstration has left you feeling like writing your own functions can be really useful, and maybe even kind of fun. I’m going to get into the nuts and bolts of how to write your own functions shortly, but first I want to briefly discuss when to write your own functions.

## 34.1 When to write functions

I’ll again quote Hadley Wickham, prolific R developer and teacher. He says, “You should consider writing a function whenever you’ve copied and pasted a block of code more than twice (i.e. you now have three copies of the same code).”9 I completely agree with this general sentiment. I’m only going to amend my advice to you slightly. Specifically, you should consider using an appropriate method for repeating operations whenever you’ve copied and pasted a block of code more than twice. In other words, writing a function is not the only option available to us when we notice ourselves copying and pasting code.

## 34.2 How to write functions

Now, the fun part – writing our own functions. I know that writing functions can seem intimidating to many people at first. However, the basics are actually pretty simple.

### 34.2.1 The function() function

It all starts with the function() function. This is how you tell R that you are about to write your own function.

If you think back to the chapter on Speaking R’s language, we talked about the analogy that is sometimes drawn between functions and factories.

To build on that analogy, thefunction() function is sort of like the factory building. Without it, there is no factory, but an empty building alone doesn’t do anything interesting:

function()
## Error: <text>:2:0: unexpected end of input
## 1: function()
##    ^

In order to build our bicycles, we need to add some workers and equipment to our empty factory building. The R function equivalent to the workers and equipment is the function body.

And just like the factory needs doors to contain our workers and equipment and keep them safe (I’m admittedly reaching a little on this one, but just go with it), our function body needs to be wrapped with curly braces.

We already talked about how the values we pass to arguments are raw material inputs that go into the factory.

In the bicycle factory example, the raw materials were steel and rubber. In the function displayed above, the raw materials are variables.

If we want to be able to call our function (i.e., use it) later, then we have to have some way to refer to it. Therefore, we will assign our function a name.

### 34.2.2 The function writing process

So, we have some idea about why writing our own functions can be a good idea. We have some idea about when to write functions (i.e., don’t repeat yourself… more than twice). And, we now know what the basic components of functions are. They are the function() function, the function body (wrapped in curly braces), the function argument(s), and the function name. But, if this is your first time being exposed to functions, then you may still be feeling like you aren’t quite sure how to get started with writing your own. So, here’s a little example of how my function writing workflow typically goes.

First, let’s simulate some new data for this example. Let’s say we have two data frames that contain first and last names:

people_1 <- tribble(
~id_1, ~name_first_1, ~name_last_1, ~street_1,
1,     "Easton",      NA,           "Alameda",
2,     "Elias",       "Salazar",    "Crissy Field",
3,     "Colton",      "Fox",        "San Bruno",
4,     "Cameron",     "Warren",     "Nottingham",
5,     "Carson",      "Mills",      "Jersey",
7,     "Aubrey",      "Rice",       "Buena Vista",
8,     "Ellie",       "Schmidt",    "Division",
9,     "Robert",      "Garza",      "Red Rock",
10,    "Stella",      "Daniels",    "Holland"
) %>%
print()
## # A tibble: 10 × 4
##     id_1 name_first_1 name_last_1 street_1
##    <dbl> <chr>        <chr>       <chr>
##  1     1 Easton       <NA>        Alameda
##  2     2 Elias        Salazar     Crissy Field
##  3     3 Colton       Fox         San Bruno
##  4     4 Cameron      Warren      Nottingham
##  5     5 Carson       Mills       Jersey
##  6     6 Addison      Meyer       Tingley
##  7     7 Aubrey       Rice        Buena Vista
##  8     8 Ellie        Schmidt     Division
##  9     9 Robert       Garza       Red Rock
## 10    10 Stella       Daniels     Holland
people_2 <- tribble(
~id_2, ~name_first_2, ~name_last_2, ~street_2,
1,     "Easton",      "Stone",      "Alameda",
2,     "Elas",        "Salazar",    "Field",
3,     NA,            "Fox",        NA,
4,     "Cameron",     "Waren",      "Notingham",
5,     "Carsen",      "Mills",      "Jersey",
7,     "Aubrey",      "Rice",       "Buena Vista",
8,     NA,            "Schmidt",    "Division",
9,     "Bob",         "Garza",      "Red Rock",
10,    "Stella",      NA,           "Holland"
) %>%
print()
## # A tibble: 10 × 4
##     id_2 name_first_2 name_last_2 street_2
##    <dbl> <chr>        <chr>       <chr>
##  1     1 Easton       Stone       Alameda
##  2     2 Elas         Salazar     Field
##  3     3 <NA>         Fox         <NA>
##  4     4 Cameron      Waren       Notingham
##  5     5 Carsen       Mills       Jersey
##  6     6 Adison       <NA>        <NA>
##  7     7 Aubrey       Rice        Buena Vista
##  8     8 <NA>         Schmidt     Division
##  9     9 Bob          Garza       Red Rock
## 10    10 Stella       <NA>        Holland

In this scenario, we want to see if first name, last name, and street name match at each ID between our data frames. More specifically, we want to combine the two data frames into a single data frame and create three new dummy variables that indicate whether first name, last name, and address match respectively. Let’s go ahead and combine the data frames now:

people <- people_1 %>%
bind_cols(people_2) %>%
print()
## # A tibble: 10 × 8
##     id_1 name_first_1 name_last_1 street_1      id_2 name_first_2 name_last_2 street_2
##    <dbl> <chr>        <chr>       <chr>        <dbl> <chr>        <chr>       <chr>
##  1     1 Easton       <NA>        Alameda          1 Easton       Stone       Alameda
##  2     2 Elias        Salazar     Crissy Field     2 Elas         Salazar     Field
##  3     3 Colton       Fox         San Bruno        3 <NA>         Fox         <NA>
##  4     4 Cameron      Warren      Nottingham       4 Cameron      Waren       Notingham
##  5     5 Carson       Mills       Jersey           5 Carsen       Mills       Jersey
##  7     7 Aubrey       Rice        Buena Vista      7 Aubrey       Rice        Buena Vista
##  8     8 Ellie        Schmidt     Division         8 <NA>         Schmidt     Division
##  9     9 Robert       Garza       Red Rock         9 Bob          Garza       Red Rock
## 10    10 Stella       Daniels     Holland         10 Stella       <NA>        Holland

Now, our first attempt at creating the dummy variables might look something like this:

people %>%
mutate(
name_first_match = name_first_1 == name_first_2,
name_last_match  = name_last_1 == name_last_2,
street_match     = street_1 == street_2
) %>%
# Order like columns next to each other for easier comparison
select(id_1, starts_with("name_f"), starts_with("name_l"), starts_with("s"))
## # A tibble: 10 × 10
##     id_1 name_first_1 name_first_2 name_first_match name_last_1 name_last_2 name_last_match street_1     street_2    street_match
##    <dbl> <chr>        <chr>        <lgl>            <chr>       <chr>       <lgl>           <chr>        <chr>       <lgl>
##  1     1 Easton       Easton       TRUE             <NA>        Stone       NA              Alameda      Alameda     TRUE
##  2     2 Elias        Elas         FALSE            Salazar     Salazar     TRUE            Crissy Field Field       FALSE
##  3     3 Colton       <NA>         NA               Fox         Fox         TRUE            San Bruno    <NA>        NA
##  4     4 Cameron      Cameron      TRUE             Warren      Waren       FALSE           Nottingham   Notingham   FALSE
##  5     5 Carson       Carsen       FALSE            Mills       Mills       TRUE            Jersey       Jersey      TRUE
##  6     6 Addison      Adison       FALSE            Meyer       <NA>        NA              Tingley      <NA>        NA
##  7     7 Aubrey       Aubrey       TRUE             Rice        Rice        TRUE            Buena Vista  Buena Vista TRUE
##  8     8 Ellie        <NA>         NA               Schmidt     Schmidt     TRUE            Division     Division    TRUE
##  9     9 Robert       Bob          FALSE            Garza       Garza       TRUE            Red Rock     Red Rock    TRUE
## 10    10 Stella       Stella       TRUE             Daniels     <NA>        NA              Holland      Holland     TRUE

Let’s take a moment to review the results we got. In row 1 we see that “Easton” and “Easton” match, and the value for name_first_match is TRUE. So far, so good. In row 2, we see that “Elias” and “Ela” do not match, and the value for name_first_match is FALSE. That is also the result we wanted. In row 3, we see that “Colton” and “NA” do not match; however, the value in name_first_match is NA. In this case, this is not the result we want. We have a problem. That brings us to the first step in my typical workflow.

#### 34.2.2.1 I spot a need for a function

In some cases, the need I spot is purely repetitive code – like the example at the beginning of this chapter. In other cases, like this one, a built-in R function is not giving me the result I want.

Here is the basic problem in this particular case:

1 == 1
## [1] TRUE
1 == 2
## [1] FALSE
1 == NA
## [1] NA
NA == 2
## [1] NA
NA == NA
## [1] NA

The equality operator (==) always returns NA when one, or both, of the values being tested is NA. Often, that is exactly the result we want. In this case, however, it is not. Fortunately, we can get the result we want by writing our own function. That brings us to step 2 in the workflow.

#### 34.2.2.2 I make the code work for one specific case

Don’t try to solve the entire problem for every case right out of the gate. Instead, solve one problem for a specific case, and then build on that win! Let’s start by trying to figure out how to get the result we want for name_first_match in row 3 of our example data.

"Colton" == NA
## [1] NA

This is essentially what we already had above. But, we want to change our result from NA to FALSE. Let’s start by saving the result to an object that we can manipulate:

result <- "Colton" == NA
result
## [1] NA

So, now the value returned by the equality comparison is saved to an object named result. Let’s go ahead and use a conditional operation to change the value of result to FALSE when it is initially NA, and leave it alone otherwise:

result <- "Colton" == NA
result <- if_else(is.na(result), FALSE, result)
result
## [1] FALSE

Alright! This worked! At least, it worked for this case. That brings us to step 3 in the workflow.

#### 34.2.2.3 I make the solution into a “function”

How do I do that? Well, first I start with a skeleton of the function components we discussed above. They are the function() function, the function body (wrapped in curly braces), and the function name. At the moment, we don’t have any arguments. I’ll explain why soon.

is_match <- function() {

}

Then, I literally copy the solution from above and paste it into the function body, making sure to indent the code. Next, we need to run the code chunk to create the function. After doing so, you should see the function appear in your global environment. Keep in mind, this creates the function so that we can use it later, but the function isn’t immediately run.

is_match <- function() {
result <- "Colton" == NA
result <- if_else(is.na(result), FALSE, result)
result
}

Now, let’s test out our shiny new function. To run the function, we can simply type the function name, with the parentheses, and run the code chunk.

is_match()
## [1] FALSE

And, it works! When we ask R to run a function we are really asking R to run the code in the body of the function. In this case, we know that the code in the body of the function results in the value FALSE because this results in FALSE:

result <- "Colton" == NA
result <- if_else(is.na(result), FALSE, result)
result
## [1] FALSE

And all we did was stick that code in the function body. Said another way, this:

result <- "Colton" == NA
result <- if_else(is.na(result), FALSE, result)
result

and this:

is_match()

mean essentially the same thing to R now. I hope that makes sense. Stick with me even if it still isn’t quite clear. We’ll get more practice soon.

At this point, you may be wondering about the function arguments, and why there aren’t any. Well, we can try passing a value to our is_match() function. How about we pass the name “Easton” from the first row of our example data above:

is_match(name = "Easton")
## Error in is_match(name = "Easton"): unused argument (name = "Easton")

But, we get an error. R doesn’t know what the name argument is or what to do with the values we are passing to it. That’s because we never said anything about any arguments when we created the is_match() function. We left the parentheses where the function arguments go empty.

is_match <- function() {
result <- "Colton" == NA
result <- if_else(is.na(result), FALSE, result)
result
}

Let’s create is_match() again, but this time, let’s add an argument:

is_match <- function(name) {
result <- "Colton" == NA
result <- if_else(is.na(result), FALSE, result)
result
}
is_match(name = "Easton")
## [1] FALSE

Hmmm, let’s add another argument and see what happens:

is_match <- function(name_1, name_2) {
result <- "Colton" == NA
result <- if_else(is.na(result), FALSE, result)
result
}
is_match(name_1 = "Easton", name_2 = "Easton")
## [1] FALSE

It looks as though the arguments we are adding don’t have any effect on our returned value. That’s because they don’t. I oversimplified how function arguments work just a little bit in our factory analogy earlier. When we add arguments to function our definition (i.e., when we create the function) it’s really more like adding a loading dock to our factory. It’s a place where our factory can receive raw materials. However, there still needs to be equipment inside the factory that can use those raw materials. If we drop off a load of rubber at our bicycle factory, but there’s no machine inside our bicycle factory that uses rubber, then we wouldn’t expect dropping off the rubber to have any effect on the outputs coming out of the factory.

We have similar situation above. We dropped the name “Easton” off at our is_match() function, but nothing inside our is_match() function can use the name “Easton”. There’s no machinery to plug that name into. That brings us to step 4 in the workflow.

#### 34.2.2.4 I start generalizing the function

As it stands right now, our is_match() function can’t accept any new names. The only result we will ever get from the current version of our is_match() function is the result of testing the equality between the values “Colton” and NA, and then converting that value to FALSE. This isn’t a problem if the only values we care about comparing are “Colton” and NA, but of course, that isn’t the case. We need a way to make our function work for other values too. Said another way, we need to make our function more general.

As you may have guessed already, that will require us creating an argument to receive input values and a place to use those input values in the function body. Let’s start by adding a first_name argument:

is_match <- function(first_name) {
result <- first_name == NA
result <- if_else(is.na(result), FALSE, result)
result
}
is_match(first_name = "Easton")
## [1] FALSE

👆Here’s what we did above:

• We once again created our is_match() function. However, this time we created it with a single argument – first_name. We didn’t have to name the argument first_name. We could have named it anything that we can name any other variable in R. But, first_name seemed like a reasonable choice since the value we want to pass to this argument is a person’s first name. The first_name argument will receive the first name values that we want to pass to this function.

• We replaced the constant value “Colton” in the function body with the variable first_name. It isn’t a coincidence that the name of the variable first_name matches the name of the argument first_name. R will take whatever value we give to the first_name argument and pass it to the variable with a matching name inside the function body. Then, R will run the code inside the function body as though the variable is the value we passed to it.

So, when we type:

is_match(first_name = "Easton")
## [1] FALSE

R sees:

result <- "Easton" == NA
result <- if_else(is.na(result), FALSE, result)
result
## [1] FALSE

It looks like our is_match() function is still going to return a value of FALSE no matter what value we pass to the first_name function. That’s because no matter what value we pass to result <- first_name == NA, result will equal NA. Then, result <- if_else(is.na(result), FALSE, result) will change the value of result to FALSE. So, we still need to make our function more general. As you may have guessed, we can do that by adding a second argument:

is_match <- function(first_name, first_name) {
result <- first_name == first_name
result <- if_else(is.na(result), FALSE, result)
result
}
## Error: repeated formal argument 'first_name' on line 1

Uh, oh! We got an error. This error is telling us that each function argument must have a unique name. Let’s try again:

is_match <- function(first_name_1, first_name_2) {
result <- first_name_1 == first_name_2
result <- if_else(is.na(result), FALSE, result)
result
}
is_match(first_name_1 = "Easton", first_name_2 = "Colton")
## [1] FALSE

Is this working or is our function still just returning FALSE no matter what we pass to the arguments? Let’s try to pass “Easton” to first_name_1 and first_name_2 and see what happens:

is_match(first_name_1 = "Easton", first_name_2 = "Easton")
## [1] TRUE

We got a TRUE! That’s exactly the result we wanted! Let’s do one final check. Let’s see what happens when we pass NA to our is_match() function:

is_match(first_name_1 = "Easton", first_name_2 = NA)
## [1] FALSE

Perfect! It looks like our function is finally ready to help us solve the problem we identified way back at step one. But, while we are talking about generalizing our function, shouldn’t we go ahead and use more general names for our function arguments? I mean, we were only using first names when we were developing our function, but we are going to use our function to compare last names and street names as well. In fact, our function will compare any two values and tell us whether or not they are a match. So, let’s go ahead and change the argument names to value_1 and value_2:

is_match <- function(value_1, value_2) {
result <- value_1 == value_2  # Don't forget to change the variable names here!!
result <- if_else(is.na(result), FALSE, result)
result
}

Now, we are ready to put our function to work testing whether or not the first name, last name, and street name match at each ID between our data frames:

people %>%
mutate(
name_first_match = is_match(name_first_1, name_first_2),
name_last_match  = is_match(name_last_1, name_last_2),
street_match     = is_match(street_1, street_2)
) %>%
# Order like columns next to each other for easier comparison
select(id_1, starts_with("name_f"), starts_with("name_l"), starts_with("s"))
## # A tibble: 10 × 10
##     id_1 name_first_1 name_first_2 name_first_match name_last_1 name_last_2 name_last_match street_1     street_2    street_match
##    <dbl> <chr>        <chr>        <lgl>            <chr>       <chr>       <lgl>           <chr>        <chr>       <lgl>
##  1     1 Easton       Easton       TRUE             <NA>        Stone       FALSE           Alameda      Alameda     TRUE
##  2     2 Elias        Elas         FALSE            Salazar     Salazar     TRUE            Crissy Field Field       FALSE
##  3     3 Colton       <NA>         FALSE            Fox         Fox         TRUE            San Bruno    <NA>        FALSE
##  4     4 Cameron      Cameron      TRUE             Warren      Waren       FALSE           Nottingham   Notingham   FALSE
##  5     5 Carson       Carsen       FALSE            Mills       Mills       TRUE            Jersey       Jersey      TRUE
##  6     6 Addison      Adison       FALSE            Meyer       <NA>        FALSE           Tingley      <NA>        FALSE
##  7     7 Aubrey       Aubrey       TRUE             Rice        Rice        TRUE            Buena Vista  Buena Vista TRUE
##  8     8 Ellie        <NA>         FALSE            Schmidt     Schmidt     TRUE            Division     Division    TRUE
##  9     9 Robert       Bob          FALSE            Garza       Garza       TRUE            Red Rock     Red Rock    TRUE
## 10    10 Stella       Stella       TRUE             Daniels     <NA>        FALSE           Holland      Holland     TRUE

Works like a charm! Notice, however, that we still have a lot of repetition in the code above. Unfortunately, we still don’t have all the tools we need to remove it. But, we will soon.

At this point in the chapter, my hope is that you are developing a feel for how to write your own functions and why that might be useful. With R, it’s possible to write functions that are very complicated. But, I hope the examples above show you that functions don’t have to be complicated to be useful. In that spirit, I’m hesitant to dive too much deeper into the details and technicalities of function writing at this point. However, there are a few details that I feel like I should at least mention so that you aren’t caught off guard by them as you begin to write your own functions. I will touch on each below, and then wrap up this chapter with resources for those of you who wish to dive deeper.

## 34.3 Giving your function arguments default values

I’ve been introducing new functions to you all throughout the book so far. Each time I do, I try to discuss some, or all, of the function’s arguments – including the default values that are passed to the arguments. I imagine that most of you have developed some sort of intuitive understanding of just what it meant for the argument to have a default value. However, this seems like an appropriate point in the book to talk about default arguments a little more explicitly and show you how to add them to the functions you write.

Let’s say that we want to write a function that will increase the value of a number, or set of numbers, incrementally. We may start with something like this:

increment <- function(x) {
x + 1
}

👆Here’s what we did above:

• We created our own function that will increase the value of a number, or set of numbers, incrementally. Specifically, when we pass a number to the x argument the value of that number plus one will be returned.

Let’s go ahead and use our function now:

increment(2)
## [1] 3

👆Here’s what we did above:

• We passed the value 2 to the x argument of our increment() function. The x argument then passed the value 2 to the x variable in the function body. Said another way, R replaced the x variable in the function body with the value 2. Then, R executed the code in the function body. In this case, the code in the function body added the values 2 and 1 together. Finally, the function returned the value 3.

Believe it or not, our simple little increment() function is a full-fledged R function. It is just as legitimate as any other R function we’ve used in this book. But, let’s go ahead and add a little more to its functionality. For example, maybe we want to be able to increment by values other than just one. How might we do that?

Hopefully, your first thought was to replace the constant value 1 in the function body with a variable that can have any number passed to it. That’s exactly what we will do next:

increment <- function(x, by) {
x + by
}

👆Here’s what we did above:

• We created our own function that will increase the value of a number, or set of numbers, incrementally. Specifically, when we pass a number to the x argument the value of that number will be incremented by the value passed to the by argument.

What value should increment() return if we pass 2 to the x argument and 2 to the by argument?

increment(2, 2)
## [1] 4

Hopefully, that’s what you were expecting. But, now what happens if we don’t pass any value to the by argument?

increment(2)
## Error in increment(2): argument "by" is missing, with no default

We get an error saying that there wasn’t any value passed to the by argument, and the by argument doesn’t have a default value. But, we are really lazy, and it takes a lot of work to pass a value to the by argument every time we use the increment() function. Plus, we almost always only want to increment our numbers by one. In this case, our best course of action is to set the default value of by to 1. Fortunately for us, doing so is really easy!

increment <- function(x, by = 1) {
x + by
}

👆Here’s what we did above:

• We created our own function that will increase the value of a number, or set of numbers, incrementally. Specifically, when we pass a number to the x argument the value of that number will be incremented by the value passed to the by argument. The default value passed to the by argument is 1. Said another way, R will pretend that we passed the value 1 to the by argument if we don’t explicitly pass a number other than 1 to the by argument.

• All we had to do to give by a default value was type = followed by the value (i.e., 1) when we created the function.

Now let’s try out our latest version of increment():

# Default value
increment(2)
## [1] 3
# Passing the value 1
increment(2, 1)
## [1] 3
# Passing a value other than 1
increment(2, 2)
## [1] 4
# Passing a vector of numbers to the x argument
increment(c(1, 2, 3), 2)
## [1] 3 4 5

## 34.4 The values your functions return

When we run our functions, they typically execute each line of code in the function body, one after another, starting with the first line and ending at the last line. Therefore, the value that your function returns (i.e., the thing that comes out of the factory) is typically dictated by the last line of code in your function body.

To show you what I mean, let’s take another look at our is_match() function:

is_match <- function(value_1, value_2) {
result <- value_1 == value_2                     # Do this first
result <- if_else(is.na(result), FALSE, result)  # Then this
result                                           # Then this
}

Why did we type that third line of code? Afterall, that line of code isn’t doing anything. Well, let’s see what happens if we take it out:

is_match <- function(value_1, value_2) {
result <- value_1 == value_2
result <- if_else(is.na(result), FALSE, result)
}
is_match("Easton", "Easton")

It appears as though nothing happened! Did our function break?

Let’s think about what typically happens when we use R’s built-in functions. When we don’t assign the value returned by the function to an object, then the returned value is printed to the screen:

sum(1, 1)
## [1] 2

But, when we do assign the value returned by the function to an object, nothing is printed to the screen:

x <- sum(1, 1)

The same thing is happening in our function above. The last line of our function body is assigning a value (i.e., TRUE or FALSE) to the variable result. Just like x <- sum(1, 1) didn’t print to the screen, result <- if_else(is.na(result), FALSE, result) doesn’t print to the screen when we run is_match("Easton", "Easton") using this version of is_match().

However, we can see in the example below that result of the operations being executed inside the function body can still be assigned to an object in our global environment, and we can print the contents of that object to screen:

x <- is_match("Easton", "Easton")
x
## [1] TRUE

If all of that seems confusing, here is the bottom line. In general, it’s a best practice for your function to print its return value to the screen. You can do this in one of three ways:

1️⃣ The value that results from the code in the last line of the function body isn’t assigned to anything. We saw an example of this above with our increment() function:

increment <- function(x, by = 1) {
x + by # Last line doesn't assign the value to an object
}
increment(2)
## [1] 3

2️⃣ If you assign values to objects inside your function, then type the name of the object that contains the value you want your function to return on the last line of the function body. We saw an example of this with our is_match() function. We can also amend our increment() function follow this pattern:

increment <- function(x, by = 1) {
out <- x + by # Now we assign the value to an object
out           # Type object name on last line of the function body
}
increment(2)
## [1] 3

3️⃣ Use the return() function.

increment <- function(x, by = 1) {
out <- x + by
return(out)
}
increment(2)
## [1] 3

So, which method should you use? Well, for all but the simplest functions (like the one above) method 1 is not considered good coding practice. Method 3 may seem like it’s the most explicit; however, it’s actually considered best practice to use the return() function only when you want your function to return its value before R reaches the last line of the function body. For example, let’s add another line of code to our function body that adds another 1 to the value of out:

increment <- function(x, by = 1) {
out <- x + by
out <- out + 1 # Adding an extra 1
return(out)    # Return still in the last line
}
increment(2)
## [1] 4

Now, let’s move return(out) to the second line of the function body – above the line of code that adds an additional 1 to the value of out:

increment <- function(x, by = 1) {
out <- x + by
return(out)    # Return in the second line above adding an extra 1
out <- out + 1 # Adding an extra 1
}
increment(2)
## [1] 3

In the example above, the last 1 wasn’t added to the value of out because we used the return() function. Said another way, increment() returned the value of out “early”, and the last line of the function body was never executed.

In the example above, using the return() function in the way that we did obviously makes no sense. It was just meant to illustrate what the return() function can do. The return() function doesn’t actually become useful until we start writing more complex functions. But, because the return() function has the special ability to end the execution of the function body early, it’s considered a best practice to only use it for that purpose.

Therefore, in most situations, you will want to use method 2 (i.e., object name on last line) when writing your own functions.

One final note before we move on to the next section. Notice that we never used the print() function on the last line of our code. This was intentional. Using print() will give you the result you expect when you don’t assign the value that your function returns to an object in your global environment:

increment <- function(x, by = 1) {
out <- x + by
print(out)
}
increment(2)
## [1] 3

But, it will not give you the result you want if you do assign the value that your function returns to an object in your global environment:

increment <- function(x, by = 1) {
out <- x + by
print(out)
}
x <- increment(2)
## [1] 3
x
## [1] 3

## 34.5 Lexical scoping and functions

If you have been following along with the code above on your computer, you may have noticed that the objects we create inside our functions do not appear in our global environment. If you haven’t been following along, you may want to jump on your computer really quickly for this section (or just take my word for it).

The reason the objects we created inside our functions do not appear in our global environment is that R actually has multiple environments were objects can live. Additionally, R uses something called lexical scoping rules to look for the objects you refer to in your R code. The vast majority of the time, we won’t need to concern ourselves much with any of these other environments or the lexical scoping rules. However, function writing does require us to have some minimal understanding of these concepts. At the very least, you should be aware of the following when writing your own functions:

1️⃣ Objects we create inside of functions don’t live in our global environment and we can’t do anything with them outside of the function we created them in.

In the example below, we create an object named out inside of the increment() function:

increment <- function(x, by = 1) {
out <- x + by # Assign the value to the out object inside the function
out
}

We then use the function:

x <- increment(2)
x
## [1] 3

However, the out object is not available to us:

out
## Error in eval(expr, envir, enclos): object 'out' not found

2️⃣ If the function we write can’t find the object it’s looking for inside the function body, then it will try to find it in the global environment.

For example, let’s create a new function named add that adds the values of x and y together in its function body. Notice, however, that there is no y argument to pass a value to, and that y is never assigned a value inside of the add() function:

add <- function(x) {
x + y
}

When we call the function:

add(2)
## Error in add(2): object 'y' not found

We get an error. R can’t find the object y. Now let’s create a y object in our global environment:

y <- 100

And call the add() function again:

add(2)
## [1] 102

As you can see, R wasn’t able to find a value for y inside of the function body so it looked outside of the function in the global environment. This is definitely something to be aware of, but usually isn’t an actual problem.

For starters, I can’t think of a good reason to add a variable to your function body without assigning it a value inside the function body or matching it to a function argument. In other words, there’s generally no good reason to have variables that serve no purpose floating around inside your functions.

If you do assign it a value inside the function, then R will not look outside of the function for a value:

add <- function(x) {
y <- 1
x + y
}
y <- 100
add(2)
## [1] 3

Likewise, if you create the function with a matching argument, then R will not look outside of the function for a value:

add <- function(x, y) {
x + y
}
y <- 100
add(2)
## Error in add(2): argument "y" is missing, with no default

Again, this aspect of the lexical scoping rules is something to be aware of, but generally isn’t a problem in practice.

## 34.6 Tidy evaluation

Now that you have all the basics of function writing under your belt, let’s take look at what happens when we try to write functions that use tidyverse package functions in the function body.

For this section, let’s return to our study data we used for the first example in this chapter. As a reminder, here’s what the data looks like:

study
## # A tibble: 68 × 7
##      age age_group       gender ht_in wt_lbs   bmi bmi_3cat
##    <dbl> <fct>           <fct>  <dbl>  <dbl> <dbl> <fct>
##  1    32 30 and Older    Male      70    216  31.0 Obese
##  2    30 30 and Older    Female    63    106  18.8 Normal
##  3    32 30 and Older    Female    62    145  26.5 Overweight
##  4    29 Younger than 30 Male      67    195  30.5 Obese
##  5    24 Younger than 30 Female    67    143  22.4 Normal
##  6    38 30 and Older    Female    58    125  26.1 Overweight
##  7    25 Younger than 30 Female    64    138  23.7 Normal
##  8    24 Younger than 30 Male      69    140  20.7 Normal
##  9    48 30 and Older    Male      65    158  26.3 Overweight
## 10    29 Younger than 30 Male      68    167  25.4 Overweight
## # ℹ 58 more rows

We already calculated the number of missing values, mean, median, min, and max for all of the continuous variables. So, let’s go ahead and calculate the number and percent of observations for each level of our categorical variables.

We know that we have 3 categorical variables (i.e., age_group, gender, and bmi_3cat), and we know that we want to perform the same calculation on all of them. So, we decide to write our own function. Following the workflow we discussed earlier, our next step is to make the code work for one specific case:

study %>%
count(age_group) %>%
mutate(percent = n / sum(n) * 100)
## # A tibble: 3 × 3
##   age_group           n percent
##   <fct>           <int>   <dbl>
## 1 Younger than 30    56   82.4
## 2 30 and Older       11   16.2
## 3 <NA>                1    1.47

Great! Thanks to dplyr, we have the result we were looking for! The next step in the workflow is to make our solution into a function. Let’s copy and paste our solution into a function skeleton like we did before:

cat_stats <- function(var) {
study %>%
count(age_group) %>%
mutate(percent = n / sum(n) * 100)
}
cat_stats()
## # A tibble: 3 × 3
##   age_group           n percent
##   <fct>           <int>   <dbl>
## 1 Younger than 30    56   82.4
## 2 30 and Older       11   16.2
## 3 <NA>                1    1.47

So far, so good! Now, let’s replace age_group with var in the function body to generalize our function:

cat_stats <- function(var) {
study %>%
count(var) %>%
mutate(percent = n / sum(n) * 100)
}
cat_stats(age_group)
## Error in count():
## ! Must group by variables found in .data.
## ✖ Column var is not found.

Unfortunately, this doesn’t work. As I said in the introduction to this part of the book, non-standard evaluation prevents us from using dplyr and other tidyverse packages inside of our functions in the same way that we might use other functions. Fortunately, the fix for this is pretty easy. All we need to do is embrace (i.e., wrap) the var variable with double curly braces:

cat_stats <- function(var) {
study %>%
count({{ var }}) %>%
mutate(percent = n / sum(n) * 100)
}
cat_stats(age_group)
## # A tibble: 3 × 3
##   age_group           n percent
##   <fct>           <int>   <dbl>
## 1 Younger than 30    56   82.4
## 2 30 and Older       11   16.2
## 3 <NA>                1    1.47

Now, we can use our new function on the rest of our categorical variables:

cat_stats(gender)
## # A tibble: 3 × 3
##   gender     n percent
##   <fct>  <int>   <dbl>
## 1 Female    43   63.2
## 2 Male      24   35.3
## 3 <NA>       1    1.47
cat_stats(bmi_3cat)
## # A tibble: 4 × 3
##   bmi_3cat       n percent
##   <fct>      <int>   <dbl>
## 1 Normal        43   63.2
## 2 Overweight    16   23.5
## 3 Obese          5    7.35
## 4 <NA>           4    5.88

This is working beautifully! However, we should probably make one final adjustment to our cat_stats() function. Let’s say that we had another data frame with categorical variable we wanted to analyze:

other_study <- tibble(
id = 1:10,
age_group = c(rep("Younger", 9), "Older"),
) %>%
print()
## # A tibble: 10 × 2
##       id age_group
##    <int> <chr>
##  1     1 Younger
##  2     2 Younger
##  3     3 Younger
##  4     4 Younger
##  5     5 Younger
##  6     6 Younger
##  7     7 Younger
##  8     8 Younger
##  9     9 Younger
## 10    10 Older

Now, let’s pass age_group to our cat_stats() function again:

cat_stats(age_group)
## # A tibble: 3 × 3
##   age_group           n percent
##   <fct>           <int>   <dbl>
## 1 Younger than 30    56   82.4
## 2 30 and Older       11   16.2
## 3 <NA>                1    1.47

Is that the result you expected? I hope not! That’s the same result we got from the original study data. Have you figured out why this happened? Take another look at our function definition:

cat_stats <- function(var) {
study %>%
count({{ var }}) %>%
mutate(percent = n / sum(n) * 100)
}

We have the study data frame hard coded into the first line of the function body. In the same way we need a matching argument-variable pair to pass multiple different columns into our function, we need a matching argument-variable pair to pass multiple different data frames into our function. We start by adding an argument to accept the data frame:

cat_stats <- function(data, var) {
study %>%
count({{ var }}) %>%
mutate(percent = n / sum(n) * 100)
}

Again, we could name this argument almost anything, but data seems like a reasonable choice. Then, we replace study with data in the function body to generalize our function:

cat_stats <- function(data, var) {
data %>%
count({{ var }}) %>%
mutate(percent = n / sum(n) * 100)
}

And now we can use our cat_stats() function on any data frame – including the other_study data frame we created above:

cat_stats(other_study, age_group)
## # A tibble: 2 × 3
##   age_group     n percent
##   <chr>     <int>   <dbl>
## 1 Older         1      10
## 2 Younger       9      90

We can even use it with a pipe:

other_study %>%
cat_stats(age_group)
## # A tibble: 2 × 3
##   age_group     n percent
##   <chr>     <int>   <dbl>
## 1 Older         1      10
## 2 Younger       9      90

Some of you may be wondering why we didn’t have to wrap data with double curly braces in the code above. Remember, we only have to use the curly braces with column names because of non-standard evaluation. More specifically, because of one aspect of non-standard evaluation called data masking. Data masking is what lets us refer to a column in a data frame without using dollar sign or bracket notation. For example, age_group doesn’t exist in our global environment as a standalone object:

age_group
## Error in eval(expr, envir, enclos): object 'age_group' not found

It only exists as a part of (i.e. a column in) the other_study object:

other_study\$age_group
##  [1] "Younger" "Younger" "Younger" "Younger" "Younger" "Younger" "Younger" "Younger" "Younger" "Older"

But the data frames themselves are not data masked. They do exist as standalone objects in our global environment:

other_study
## # A tibble: 10 × 2
##       id age_group
##    <int> <chr>
##  1     1 Younger
##  2     2 Younger
##  3     3 Younger
##  4     4 Younger
##  5     5 Younger
##  6     6 Younger
##  7     7 Younger
##  8     8 Younger
##  9     9 Younger
## 10    10 Older

Therefore, there is no need to wrap them with double curly braces. Having said that, it doesn’t appear as though doing so will hurt anything:

cat_stats <- function(data, var) {
{{data}} %>%
count({{ var }}) %>%
mutate(percent = n / sum(n) * 100)
}
cat_stats(other_study, age_group)
## # A tibble: 2 × 3
##   age_group     n percent
##   <chr>     <int>   <dbl>
## 1 Older         1      10
## 2 Younger       9      90

That pretty much wraps up this chapter on the basics of writing function to reduce unnecessary repetition in your R code. If you’re feeling good about writing your own functions, great! If you want to dig even deeper, take a look at the functions chapter of the Advanced R book.

If you’re still feeling a little apprehensive or confused, don’t feel bad. It takes most people (myself included) awhile to get comfortable with writing functions. Just remember, functions can be complicated, but they don’t have to be. Even very simple functions can sometimes be useful. So, start simple and get more complex as your skills and confidence grow.

If you find that you’ve written a function that is really useful, consider saving it for use again in the future. Early on, I would just save my functions as R scripts in a folder on my computer. I could then copy and paste those functions from the scripts into my R programs as needed. Then, I learned about the source() function, which allowed me to use my saved functions without having to manually copy and paste them. Eventually, I learned how to make my own packages that contained groups of related functions and save them to my Github account. From there, I could use my functions on any computer, and even share them with others. Finally, you can even publish your packages on CRAN if you want to them with the broadest possible audience.

### References

9.
Grolemund G, Wickham H. R for Data Science.; 2017.