# Writing functions – Part one

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(This post originally appeared on my R blog)

## Writing functions

This post outlines the writing of a basic function. Writing functions in R (R Core Team 2017) is fairly simple, and the usefulness of function writing cannot be conveyed in a single post. I have included “Part one” in the title, and I will add follow-up posts in time.

The basic code to write a function looks like this:

function_name <- function(){}

The code for the task you want your function to perform goes inside the curly brackets `{}`

, and the object you wish the function to work on goes inside the parenthesis`()`

.

### The problem

I have often found myself using a number of different functions together for multiple variables. For each variable, I need re-type each function. For example, when looking at a variable, I would often run the functions `mean()`

, `sd()`

, `min()`

, `max()`

, and `length()`

together. Each time I wanted to inspect a new variable, I had to type all five functions for the variable in question. For example, looking at the `Temp`

variable, from the `airquality`

dataset in the `datasets`

package, would require typing the following: `mean(airquality$Temp)`

, `sd(airquality$Temp)`

, `min(airquality$Temp)`

, `max(airquality$Temp)`

, `length(airquality$Temp)`

. This can get very tedious and repetitive.

### The solution

In response to repeatedly typing these functions together, I created the `descriptives()`

function which combines these frequently used functions into a single function.

#### The `descriptives()`

function

The `descriptives()`

function combines the functions `mean()`

, `sd()`

, `min()`

, `max()`

, and `length()`

to return a table displaying the mean, standard deviation, minimum, maximum, and length of a vector.^{1} The code for creating this function is below, each line of code within the function is explained in the comment above (denoted with the `#`

symbol). The code below can be copied and pasted into your R session to create the `descriptives()`

function.

descriptives <- function(x){ # create an object "mean" which contains the mean of x mean <- mean(x, na.rm = TRUE) # create an object "sd" which contains the sd of x sd <- sd(x, na.rm = TRUE) # create an object "min" which contains the min of x min <- min(x, na.rm = TRUE) # create an object "max" which contains the max of x max <- max(x, na.rm = TRUE) # create an object "len" which contains the length of x len <- length(x) # combine the objects created into a table data.frame(mean, sd, min, max, len) }

When you pass a vector `x`

through the function `descriptives()`

, it creates 5 objects which are then combined into a table. Running the function returns the table:

descriptives(airquality$Temp) ## mean sd min max len ## 1 77.88235 9.46527 56 97 153

### Things to bear in mind when writing functions

- Try to give your function a name that is short and easy to remember.
- If you are writing a longer more complex function, it may be useful to test it line-by-line, before seeing if it “works”; this will help to identify any errors before they cause your function to fail.
- If the function returns an error, testing the code line by line will help you find the source of the error.
- The final line of code in a function will be the “output” of the function.
- Objects created within the function are not saved in the global environment: in the
`descriptives()`

function, all that is returned is a table containing the variables specified. The individual objects that were created disappear when the function has finished running. - The disappearing of objects created within a function described above can be very useful for keeping a tidy working environment.

### Conclusion

I find myself writing functions regularly, for various tasks. Often a function may be specific to a particular task, or even to a particular dataset. One example of such a function builds on the previous post, in which I described how to create a dataframe from multiple files. In practice, I rarely create data frames exactly as described. I usually nest the “read.csv” function within a larger function that also sorts the data, creating a more manageable dataframe, better suited to my purposes; e.g., removing variables that are of no interest or computing/recoding variables. I can then run this function to build my dataframe at the start of a session.

### References

R Core Team. 2017. *R: A Language and Environment for Statistical Computing*. Vienna, Austria: R Foundation for Statistical Computing. https://www.R-project.org/.

Most of what

`descriptives()`

does can also be achieved by the`summary()`

function, however`sd()`

and`length()`

are missing.↩

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