Apply Functions - The UK's Premier R User Group · 2018. 4. 10. · Other “apply”...

Richard Pugh, Commercial Director

rpugh@mango-solutions.com

Apply Functions

Agenda

• Why Apply?

• The “apply” family of functions

• Primary Functions

• Other Functions

• The “plyr” package

• Summary

Why Apply?

• The functions we use are simple things …

• We need a framework to allow us to allow simple functions

in a more structured way

mean(x, trim = 0, na.rm = TRUE)

No “by” argument

The “Apply” family of Functions

Which Functions?

• Getting a full list is tricky

• Most challenging aspect: remembering which function to

Primary Apply Functions

The “Apply” family of Functions The apply Function

The apply Function

• Applies a simple function over dimensions of a data structure

• So input is anything that has rows, columns (matrix, data

frame, array .. NOT lists or vectors)

Structure with a dimension

Function to apply

Dimension number (1 = rows, 2 = columns)

Additional arguments to the function

Basic apply Example

Mix it with graphics …

Apply with multiple dimensions

Apply over arrays

Passing Additional Arguments

Using Custom Functions

A Quick Example

The “Apply” family of Functions The lapply Function

The lapply Function

• Applies a function over elements of a list

• Remember: a data frame is a list of named vectors!

• It always returns a list

List structure Function to apply

Additional arguments to the function

Simple lapply Example

Using split to generate a list

• Will split an object

(vector, data

frame) based on 1

or more factors

• Great input to lapply!

Using split and lapply

Using lapply with vectors

Split processing using lapply

The “Apply” family of Functions The sapply Function

The sapply Function

• Calls lapply and simplifies the output

sapply vs lapply

Using sapply and split

Bootstrap style sapply example

Using sapply with data frames

The “Apply” family of Functions The tapply Function

The tapply Function

• Applies a function to a vector BY levels of other factor(s)

• A wrapper to lapply & split

Vector to summarise

Factor(s) by which to summarise

Function to apply

Other arguments to the function

Simple examples of tapply

tapply vs sapply + split

When tapply goes wrong

The “Apply” family of Functions The by Function

The by Function

• The tapply function is restricted to vector inputs

• The by function applies functions to level of a data frame

by one or more factors

Vector to summarise

Factor(s) by which to summarise

Function to apply

Simple by example

An object of class by (example 1)

An object of class by (example 2)

The “Apply” family of Functions The aggregate Function

The aggregate Function

• The aggregate function allows us to apply functions to

one or more variables by one or more factors

• It always returns a data frame

List of variables to summarise

List of factor(s) by which to summarise

Function to apply

Simple aggregateExample

Alternative aggregate Usage

• The aggregate function allows us to apply functions to

one or more variables by one or more factors

• It always returns a data frame

Formula defining summary structure

Data frame to use

Function to apply

Simple aggregate Example

The “Apply” family of Functions Other Functions

Other “apply” Functions

Function Usage

eapply Apply a Function Over Values in an Environment

mapply Apply a Function to Multiple List or Vector Arguments

rapply Recursively Apply a Function to a List

vapply Similar to sapply with a pre-specified “template” return value

replicate Wrapper for sapply, for repeated evaluation of an expression

The eapply Function

The mapply Function

The rapply Function

The vapply Function

The replicate Function

The “plyr” package

The “plyr” package Overview

• Written and maintained by Hadley Wickham

• Widely used in the R community:

• 158 other packages depend/import/suggest plyr

• Most downloaded package from the Rstudio cran mirror last

month (19,546 downloads)

• Consists of tools for splitting data, applying a function to

each part and then combining the results

Common plyr functions

• Named according to the data structure they split

up and the data structure they return.

• Available data structures are:

• data frame, array, list, multiple inputs, repeat multiple

times, _ nothing

• For example: daply

Takes a data frame Returns an array

Some plyr alternatives to base R

• Provides an alternative to using the apply family

of functions (covered so far today):

Base function plyr function

apply aaply/alply

lapply llply

sapply laply

tapply n/a

by dlply

aggregate ddply + colwise

Simple lapply vs sapply example with plyr

plyr base

Why use plyr?

• Consistent function names make it easier to know

which apply-type function is required

• Fast and memory efficient

• Uses parallelisation through the foreach package

• Additional “helper” functions included:

• arrange

• mutate

• summarise

• join

• match_df

• colwise

• rename

• round_any

• count

Summary

• This was a quick overview of the apply family of

functions

• The key functions are apply, sapply, lapply

and aggregate

• The “plyr” package functions can be used as an

alternative to these functions

Apply Functions - The UK's Premier R User Group · 2018. 4. 10. · Other “apply”...

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