Writing your own functions

You have already used many functions. Now it is time to write your own. A function can accept one or more arguments, listed in parentheses. Its body, enclosed in curly braces, contains the operations to perform. For example, the mean of a numeric vector without missing values is its sum divided by its length. We can write sum(x) / length(x) as a function:

function(x){sum(x)/length(x)}

To access this function later, you can also save it as an object in your environment:

my.mean <- function(x){sum(x)/length(x)}

For an initial test, create a vector X (uppercase), call my.mean(), and compare the result with mean():

X <- c(50:100)

my.mean(X)
mean(X)

You can also define default argument values:

my.mean <- function(x, na.rm = TRUE){
  if(na.rm == TRUE){
    sum(x, na.rm = TRUE)/length(x[!is.na(x)])
  } else {
    if(any(is.na(x) == TRUE)){
      cat("Error: you need to change your NA settings!")
    }
    
    sum(x)/length(x)
  }
}

X[sample(1:50, 4)] <- NA

my.mean(X)
my.mean(X, na.rm = FALSE)

In this example, na.rm has the default value TRUE, specified in the argument list. If you call my.mean() without supplying na.rm, the function excludes missing values before calculating the mean. With na.rm = FALSE, the function takes the else branch. Here is another example:

my_function <- function(x, y = 10) {
  result <- x + y
  return(result)
}

If the caller omits y, the function uses its default value of 10 and adds it to x.

my_function(5)  # This will result in 15 (5 + 10)

You can override the default by supplying a value for y when calling the function:

my_function(5, 20)  # This will result in 25 (5 + 20)

Test what you’ve learned so far!

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