Writing functions

So far you have used functions that come with R. You can also write your own. A function bundles up some code, so you can reuse it without retyping it and give it a name that says what it does.

celsius_to_kelvin <- function(temp) {
  temp + 273.15
}

Compare this with Python:

def celsius_to_kelvin(temp):
    return temp + 273.15

There are three differences:

In R, a function is an object like any other, so you create one with function() and save it with <-.
Curly braces { }, not indentation, mark the body of the function. (Indent anyway: it makes code easier to read.)
R returns the value of the last line automatically, so you rarely need return().

The chunk below loads the weather data, which a later exercise uses.

Write a function

Write the celsius_to_kelvin() function above, then use it to convert 15 °C to kelvin.

celsius_to_kelvin <- function(temp) {
  temp + 273.15
}
celsius_to_kelvin(15)

Because arithmetic in R works element by element, your function works on a whole vector too: celsius_to_kelvin(c(0, 15, 100)) returns three values.

Your turn

Write a function called mm_to_inches() that converts a length in millimetres to inches (there are 25.4 mm in an inch). Test it on 25.4, which should return 1.

mm_to_inches <- function(mm) {
  mm / 25.4
}
mm_to_inches(25.4)

More than one argument

A function can take several arguments, and any of them can have a default value. This function calculates the range of a vector: the difference between its largest and smallest values.

value_range <- function(x, na.rm = FALSE) {
  max(x, na.rm = na.rm) - min(x, na.rm = na.rm)
}

Use value_range() to find the range of tmax_c.

value_range <- function(x, na.rm = FALSE) {
  max(x, na.rm = na.rm) - min(x, na.rm = na.rm)
}
value_range(weather$tmax_c)

Ready for Week 1?

You can now load a CSV file, inspect it, pull out columns, summarise them, plot them and write functions. That is everything the Week 1 exercises assume.

Try the self-test at the end of the preparation guide. If you can predict what each line does, you are ready.