Default arguments

Introduction to Writing Functions in R

Richie Cotton

Data Evangelist at DataCamp

toss_coin() troubles

toss_coin <- function(n_flips, p_head) {
  coin_sides <- c("head", "tail")
  weights <- c(p_head, 1 - p_head)
  sample(coin_sides, n_flips, replace = TRUE, prob = weights)
}

Set the default in the signature

toss_coin <- function(n_flips, p_head = 0.5) {
  coin_sides <- c("head", "tail")
  weights <- c(p_head, 1 - p_head)
  sample(coin_sides, n_flips, replace = TRUE, prob = weights)
}
Introduction to Writing Functions in R

A template with defaults

my_fun <- function(data_arg1, data_arg2, detail_arg1 = default1) {
  # Do something
}
Introduction to Writing Functions in R

Other types of default

args(median)

function (x, na.rm = FALSE, ...)

library(jsonlite)
args(fromJSON)

function (txt, simplifyVector = TRUE, simplifyDataFrame = simplifyVector, 
    simplifyMatrix = simplifyVector, flatten = FALSE, ...)
Introduction to Writing Functions in R

NULL defaults

By convention, this means

The function will do some special handling of this argument. Please read the docs.

args(set.seed)

function (seed, kind = NULL, normal.kind = NULL)
Introduction to Writing Functions in R

Categorical defaults

  1. Pass a character vector in the signature.
  2. Call match.arg() in the body.
args(prop.test)

function (x, n, p = NULL, alternative = c("two.sided", "less", "greater"), 
  conf.level = 0.95, correct = TRUE)

Inside the body

alternative <- match.arg(alternative)
Introduction to Writing Functions in R

Cutting a vector by quantile

cut_by_quantile <- function(x, n, na.rm, labels, interval_type) {
  probs <- seq(0, 1, length.out = n + 1)
  quantiles <- quantile(x, probs, na.rm = na.rm, names = FALSE)
  right <- switch(interval_type, "(lo, hi]" = TRUE, "[lo, hi)" = FALSE)
  cut(x, quantiles, labels = labels, right = right, include.lowest = TRUE)
}
  • x: A numeric vector to cut
  • n: The number of categories to cut x into
  • na.rm: Should missing value be removed?
  • labels: Character labels for the categories
  • interval_type: Should ranges be open on the left or right?
Introduction to Writing Functions in R

Cat heart weights

A strip plot of cat's heart weights. The range is from 3 to 20 grams.

quantile(cats$Hwt)

    0%    25%    50%    75%   100% 
 6.300  8.950 10.100 12.125 20.500
1 data(cats, package = "MASS")
Introduction to Writing Functions in R

Cutting by quantile

The strip plot from the previous slide has been given color, with red points on the left, then green, then turquoise, then purple on the right.

cut(x, quantile(x))
Introduction to Writing Functions in R

Let's practice!

Introduction to Writing Functions in R

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