Default arguments

Introduzione alla scrittura di funzioni 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)
}

A template with defaults

my_fun <- function(data_arg1, data_arg2, detail_arg1 = default1) {
  # Do something
}

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, ...)

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)

Categorical defaults

Pass a character vector in the signature.
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)

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?

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")

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))

Let's practice!

Introduzione alla scrittura di funzioni in R