# Lesson 4. Build a function in R - Efficient scientific programming

## Learning Objectives

After completing this tutorial, you will be able to:

• Create a new function in R
• Understand the concept of a function argument.

## What you need

You will need a computer with internet access to complete this lesson.

In this lesson, we’ll learn how to write a function so that we can repeat several operations with a single command.

### Defining a Function

Let’s start by defining a function fahr_to_kelvin that converts temperatures from Fahrenheit to Kelvin:

fahr_to_kelvin <- function(fahr) {
kelvin <- ((fahr - 32) * (5 / 9)) + 273.15
kelvin
}


We define fahr_to_kelvin by assigning it to the output of function. The list of argument names are contained within parentheses - in this case temp is the only argument that this function requires.

Next, the body of the function–the statements that are executed when it runs–is contained within curly braces ({}). The statements in the body are indented by two spaces, which makes the code easier to read but does not affect how the code operates.

When we call the function, the values we pass to it are assigned to those variables so that we can use them inside the function.

Inside the function, we use a return statement to send a result back to whoever asked for it.

## Automatic Returns

In R, it is not necessary to include the return statement. R automatically returns whichever variable is on the last line of the body of the function.

Let’s try running our function. Calling our own function is no different from calling any other function:

# freezing point of water
fahr_to_kelvin(32)
## [1] 273.1
# boiling point of water
fahr_to_kelvin(212)
## [1] 373.1


We’ve successfully called the function that we defined, and we have access to the value that we returned.

### Composing Functions

Now that we’ve seen how to turn Fahrenheit into Kelvin, it’s easy to turn Kelvin into Celsius:

kelvin_to_celsius <- function(kelvin) {
celsius <- kelvin - 273.15
celsius
}

# absolute zero in Celsius
kelvin_to_celsius(0)
## [1] -273.1


What about converting Fahrenheit to Celsius? We could write out the formula, but we don’t need to. Instead, we can compose the two functions we have already created:

fahr_to_celsius <- function(fahr) {
kelvin <- fahr_to_kelvin(fahr)
celsius <- kelvin_to_celsius(kelvin)
celsius
}

# freezing point of water in Celsius
fahr_to_celsius(32.0)
## [1] 0


This is our first taste of how larger programs are built: we define basic operations, then combine them in ever-larger chunks to get the effect we want. Real-life functions will usually be larger than the ones shown here–typically half a dozen to a few dozen lines–but they shouldn’t ever be much longer than that, or the next person who reads it won’t be able to understand what’s going on. Even more important than avoiding long functions is ensuring that the logic of your function is expressed by the code that comprises your function. For instance, it is nearly always better to use meaningful variable names such as fahr instead of simply temp, which could be taken to mean temporary, or temperature (in what units?). You might be surprised at the human mental expense required to comprehend all of the objects and operations in long functions.

## Chaining Functions

This example showed the output of fahr_to_kelvin assigned to temp_k, which is then passed to kelvin_to_celsius to get the final result. It is also possible to perform this calculation in one line of code, by “chaining” functions together, like so:

# freezing point of water in Celsius
kelvin_to_celsius(fahr_to_kelvin(32.0))
## [1] 0


## Create a Function

We can use the c function to concatenate elements. We have done this when we classified rasters. For example

x <- c("A", "B", "C") creates a vector x with three elements. Furthermore, we can extend that vector again using c.

For example,

y <- c(x, "D") creates a vector y with four elements.

## Optional challenge

Write a function called fence that takes two vectors as arguments, called original and wrapper, and returns a new vector that has the wrapper vector at the beginning and end of the original. An example of how the function should run is below.

best_practice <- c("Write", "programs", "for", "people", "not", "computers")
asterisk <- "***"  # R interprets a variable with a single value as a vector
# with one element.
fence(best_practice, asterisk)
## [1] "***"       "Write"     "programs"  "for"       "people"    "not"
## [7] "computers" "***"


## Optional challenge 2

If the variable v refers to a vector, then v[1] is the vector’s first element and v[length(v)] is its last (the function length returns the number of elements in a vector). Write a function called outside that returns a vector made up of just the first and last elements of its input:

 dry_principle <- c("Don't", "repeat", "yourself", "or", "others")
outside(dry_principle)
## [1] "Don't"  "others"
`