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RClasses.html
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RClasses.html
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<!DOCTYPE html>
<!-- saved from url=(0014)about:internet -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<meta http-equiv="x-ua-compatible" content="IE=9" >
<title>Classes & Environments</title>
<style type="text/css">
body, td {
font-family: sans-serif;
background-color: white;
font-size: 12px;
margin: 8px;
}
tt, code, pre {
font-family: 'DejaVu Sans Mono', 'Droid Sans Mono', 'Lucida Console', Consolas, Monaco, monospace;
}
h1 {
font-size:2.2em;
}
h2 {
font-size:1.8em;
}
h3 {
font-size:1.4em;
}
h4 {
font-size:1.0em;
}
h5 {
font-size:0.9em;
}
h6 {
font-size:0.8em;
}
a:visited {
color: rgb(50%, 0%, 50%);
}
pre {
margin-top: 0;
max-width: 95%;
border: 1px solid #ccc;
white-space: pre-wrap;
}
pre code {
display: block; padding: 0.5em;
}
code.r, code.cpp {
background-color: #F8F8F8;
}
table, td, th {
border: none;
}
blockquote {
color:#666666;
margin:0;
padding-left: 1em;
border-left: 0.5em #EEE solid;
}
hr {
height: 0px;
border-bottom: none;
border-top-width: thin;
border-top-style: dotted;
border-top-color: #999999;
}
@media print {
* {
background: transparent !important;
color: black !important;
filter:none !important;
-ms-filter: none !important;
}
body {
font-size:12pt;
max-width:100%;
}
a, a:visited {
text-decoration: underline;
}
hr {
visibility: hidden;
page-break-before: always;
}
pre, blockquote {
padding-right: 1em;
page-break-inside: avoid;
}
tr, img {
page-break-inside: avoid;
}
img {
max-width: 100% !important;
}
@page :left {
margin: 15mm 20mm 15mm 10mm;
}
@page :right {
margin: 15mm 10mm 15mm 20mm;
}
p, h2, h3 {
orphans: 3; widows: 3;
}
h2, h3 {
page-break-after: avoid;
}
}
</style>
<!-- Styles for R syntax highlighter -->
<style type="text/css">
pre .operator,
pre .paren {
color: rgb(104, 118, 135)
}
pre .literal {
color: rgb(88, 72, 246)
}
pre .number {
color: rgb(0, 0, 205);
}
pre .comment {
color: rgb(76, 136, 107);
}
pre .keyword {
color: rgb(0, 0, 255);
}
pre .identifier {
color: rgb(0, 0, 0);
}
pre .string {
color: rgb(3, 106, 7);
}
</style>
<!-- R syntax highlighter -->
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</head>
<body>
<h1>Classes & Environments</h1>
<p>Taken from <a href="http://adv-r.had.co.nz/">Advanced R programming</a> by Hadley Wickham</p>
<h2>Reference Classes</h2>
<blockquote>
<p>Reference classes (or RC for short) are the newest OO system in R. They were introduced in version 2.12. They are fundamentally different to S3 and S4 because:
RC methods belong to objects, not functions
RC objects are mutable: the usual R copy-on-modify semantics do not apply
These properties make RC objects behave more like objects do in most other programming languages, e.g., Python, Ruby, Java and C#. <a href="http://adv-r.had.co.nz/OO-essentials.html#rc">ref</a></p>
</blockquote>
<pre><code class="r">Account <- setRefClass("Account") # a new class
my.account <- Account$new() # a new object
</code></pre>
<p>Classes have attributes:</p>
<pre><code class="r">Account <- setRefClass("Account", fields = list(balance = "numeric"))
my.account <- Account$new(balance = 100)
my.account$balance
</code></pre>
<pre><code>## [1] 100
</code></pre>
<pre><code class="r">my.account$balance <- 200
my.account$balance
</code></pre>
<pre><code>## [1] 200
</code></pre>
<p>Unlike other R objects, RC objects are mutable and are not copied when modified:</p>
<pre><code class="r">b <- my.account
b$balance
</code></pre>
<pre><code>## [1] 200
</code></pre>
<pre><code class="r">my.account$balance <- 0
b$balance
</code></pre>
<pre><code>## [1] 0
</code></pre>
<pre><code class="r"># for a copy:
c <- my.account$copy()
my.account$balance <- 100
c$balance
</code></pre>
<pre><code>## [1] 0
</code></pre>
<p>And we can add methods also:</p>
<pre><code class="r">Account <- setRefClass("Account", fields = list(balance = "numeric"), methods = list(withdraw = function(x) {
balance <<- balance - x # use <<- to modify attributes
}, deposit = function(x) {
balance <<- balance + x
}))
a <- Account$new(balance = 100)
a$deposit(100)
a$balance
</code></pre>
<pre><code>## [1] 200
</code></pre>
<p>For subclassing use contains:</p>
<pre><code class="r">NoOverdraft <- setRefClass("NoOverdraft", contains = "Account", methods = list(withdraw = function(x) {
# overwrites superclass 'withdraw'
if (balance < x) stop("Not enough money")
balance <<- balance - x
}))
accountJohn <- NoOverdraft$new(balance = 100)
accountJohn$deposit(50)
accountJohn$balance
</code></pre>
<pre><code>## [1] 150
</code></pre>
<pre><code class="r">accountJohn$withdraw(200)
</code></pre>
<pre><code>## Error: Not enough money
</code></pre>
<h2>Environments</h2>
<blockquote>
<p>The job of an environment is to associate, or bind, a set of names to a set of values. Environments are the data structures that power scoping. An environment is very similar to a list, with three important exceptions:</p>
<p>1.Environments have reference semantics. So R's usual copy on modify rules do not apply. Whenever you modify an environment, you modify every copy.
In the following code chunk, we create a new environment, create a “copy” and then modify the original environment. Notice that the copy also changes. If you change e to a list (or any other R datastructure), f will become a list. e and f are identical. </p>
</blockquote>
<pre><code class="r">e <- new.env()
f <- e
e$a <- 10
f$a
</code></pre>
<pre><code>## [1] 10
</code></pre>
<blockquote>
<p>As well as powering scoping, environments can also be useful data structures because they have reference semantics and can work like a hashtable.</p>
<ol>
<li>Environments have parents. If an object is not found in an environment, then R will look at its parent (and so on). There is only one exception: the empty environment does not have a parent.</li>
</ol>
</blockquote>
<pre><code class="r">e <- new.env()
# the default parent provided by new.env() is environment from which it is
# called
parent.env(e)
</code></pre>
<pre><code>## <environment: R_GlobalEnv>
</code></pre>
<blockquote>
<ol>
<li>Every object in an environment must have a name. And, those names must be unique.</li>
</ol>
<p>Technically, an environment is made up of a frame, a collection of named objects (like a list), and a reference to a parent environment. <a href="http://adv-r.had.co.nz/Environments.html">ref</a></p>
</blockquote>
<pre><code class="r">e <- new.env()
identical(e, globalenv())
</code></pre>
<pre><code>## [1] FALSE
</code></pre>
<pre><code class="r">e$a <- 1
ls(e) # check contents of e (does not show names beginning with .)
</code></pre>
<pre><code>## [1] "a"
</code></pre>
<pre><code class="r">e$a
</code></pre>
<pre><code>## [1] 1
</code></pre>
<pre><code class="r">e$.b <- 0
ls(e)
</code></pre>
<pre><code>## [1] "a"
</code></pre>
<pre><code class="r">ls(e, all = TRUE)
</code></pre>
<pre><code>## [1] ".b" "a"
</code></pre>
<pre><code class="r"># we can coerce to a list:
as.list(e)
</code></pre>
<pre><code>## $a
## [1] 1
</code></pre>
<pre><code class="r">str(as.list(e, all = TRUE))
</code></pre>
<pre><code>## List of 2
## $ a : num 1
## $ .b: num 0
</code></pre>
<p>We can use $ or [[ to extract an element which looks only within the environment. Function <code>get</code> will also looks into its parents:</p>
<pre><code class="r">e$a
</code></pre>
<pre><code>## [1] 1
</code></pre>
<pre><code class="r">e[["a"]]
</code></pre>
<pre><code>## [1] 1
</code></pre>
<pre><code class="r">c <- 1 # global env
e$c
</code></pre>
<pre><code>## NULL
</code></pre>
<pre><code class="r">get("c", e)
</code></pre>
<pre><code>## [1] 1
</code></pre>
<p>To remove an element use <code>rm</code>:</p>
<pre><code class="r">e <- new.env()
e$a <- 1
e$a <- NULL
ls(e)
</code></pre>
<pre><code>## [1] "a"
</code></pre>
<pre><code class="r">rm("a", envir = e)
ls(e)
</code></pre>
<pre><code>## character(0)
</code></pre>
<p>Generally, when you create your own environment, you want to manually set the parent environment to the empty environment. This ensures you don't accidentally inherit objects from somewhere else:</p>
<pre><code class="r">x <- 1
e1 <- new.env()
get("x", e1)
</code></pre>
<pre><code>## [1] 1
</code></pre>
<pre><code class="r">e2 <- new.env(parent = emptyenv())
get("x", e2)
</code></pre>
<pre><code>## Error: object 'x' not found
</code></pre>
<p>You can determine if a binding exists in a environment with the exists() function. Like get(), the default is to follow regular scoping rules and look in parent environments. If you don't want this behavior, use inherits = FALSE:</p>
<pre><code class="r">exists("x", e1)
</code></pre>
<pre><code>## [1] TRUE
</code></pre>
<pre><code class="r">exists("x", e1, inherits = FALSE)
</code></pre>
<pre><code>## [1] FALSE
</code></pre>
<p>There are a few special environments that you can access directly:</p>
<ul>
<li>globalenv(): the user's workspace</li>
<li>baseenv(): the environment of the base package</li>
<li>emptyenv(): the ultimate ancestor of all environments, the only environment without a parent.</li>
</ul>
<pre><code class="r">search() # lists all environments between and including the global and base environments
</code></pre>
<pre><code>## [1] ".GlobalEnv" "package:knitr" "package:stats"
## [4] "package:graphics" "package:grDevices" "package:utils"
## [7] "package:datasets" "package:methods" "Autoloads"
## [10] "package:base"
</code></pre>
<h2>Function environments</h2>
<p>The environment where the function is created:</p>
<pre><code class="r">y <- 1
f <- function(x) x + y
environment(f)
</code></pre>
<pre><code>## <environment: R_GlobalEnv>
</code></pre>
<p>To make an equivalent function that is safer (it throws an error if the input isn't a function), more consistent (it can take a function name as an argument not just a function), and more informative (it has a better name), we'll create funenv():</p>
<pre><code class="r">funenv <- function(f) {
f <- match.fun(f) # extract the desired function object
environment(f)
}
funenv(plot)
</code></pre>
<pre><code>## <environment: namespace:graphics>
</code></pre>
<pre><code class="r">environment(plot)
</code></pre>
<pre><code>## <environment: namespace:graphics>
</code></pre>
<pre><code class="r">
funenv("plot")
</code></pre>
<pre><code>## <environment: namespace:graphics>
</code></pre>
<pre><code class="r">environment("plot")
</code></pre>
<pre><code>## NULL
</code></pre>
<p>The environment created when a function is run: each time a function is called, a new environment is created to host execution.</p>
<pre><code class="r">f <- function(x) {
list(e = environment(), p = parent.env(environment()))
}
str(f())
</code></pre>
<pre><code>## List of 2
## $ e:<environment: 0x046c4c0c>
## $ p:<environment: R_GlobalEnv>
</code></pre>
<pre><code class="r">str(f())
</code></pre>
<pre><code>## List of 2
## $ e:<environment: 0x046d3458>
## $ p:<environment: R_GlobalEnv>
</code></pre>
<pre><code class="r">funenv("f")
</code></pre>
<pre><code>## <environment: R_GlobalEnv>
</code></pre>
<p>The environment where the function is called:</p>
<pre><code class="r">f <- function() {
x <- 10
function() {
x
}
}
g <- f()
x <- 20
g()
</code></pre>
<pre><code>## [1] 10
</code></pre>
<p>What value x is associated with in the environment where g() is called? x is 10 in the environment where g() is defined, but it is 20 in the environment where g() is called.</p>
<p>We can access this environment using the confusingly named parent.frame(). This function returns the environment where the function is called. We can also use this function to look up the value of names in that environment:</p>
<pre><code class="r">f2 <- function() {
x <- 10
function() {
def <- get("x", environment())
cll <- get("x", parent.frame())
list(defined = def, called = cll)
}
}
g2 <- f2()
x <- 20
str(g2())
</code></pre>
<pre><code>## List of 2
## $ defined: num 10
## $ called : num 20
</code></pre>
<p>We can get a list of all calling environments using sys.frames():</p>
<pre><code class="r">x <- 0
y <- 10
f <- function(x) {
x <- 1
g(x)
}
g <- function(x) {
x <- 2
h(x)
}
h <- function(x) {
x <- 3
i(x)
}
i <- function(x) {
x <- 4
sys.frames()
}
es <- f()
es
</code></pre>
<pre><code>## [[1]]
## <environment: 0x0338d758>
##
## [[2]]
## <environment: 0x0390a534>
##
## [[3]]
## <environment: 0x03c2e410>
##
## [[4]]
## <environment: 0x03c2d7fc>
##
## [[5]]
## <environment: 0x03c2d9f4>
##
## [[6]]
## <environment: 0x03c2db44>
##
## [[7]]
## <environment: 0x03be6a60>
##
## [[8]]
## <environment: 0x042f9470>
##
## [[9]]
## <environment: 0x042f9b28>
##
## [[10]]
## <environment: 0x04068464>
##
## [[11]]
## <environment: 0x03f1cb34>
##
## [[12]]
## <environment: 0x03f17450>
##
## [[13]]
## <environment: 0x03f1754c>
##
## [[14]]
## <environment: 0x03f143f0>
##
## [[15]]
## <environment: 0x03f14594>
##
## [[16]]
## <environment: 0x03f13be8>
##
## [[17]]
## <environment: 0x03f0da1c>
##
## [[18]]
## <environment: 0x03f0e03c>
##
## [[19]]
## <environment: 0x03f0e18c>
##
## [[20]]
## <environment: R_GlobalEnv>
##
## [[21]]
## <environment: 0x03f055a0>
##
## [[22]]
## <environment: 0x03f0562c>
##
## [[23]]
## <environment: 0x03effe4c>
##
## [[24]]
## <environment: 0x03efff10>
</code></pre>
<pre><code class="r">sapply(es, function(e) get("x", e, inherits = TRUE))
</code></pre>
<pre><code>## [[1]]
## [1] 0
##
## [[2]]
## [1] 0
##
## [[3]]
## [1] 0
##
## [[4]]
## $params
## $params$label
## [1] "unnamed-chunk-19"
##
##
## $params.src
## [1] ""
##
## attr(,"class")
## [1] "block"
##
## [[5]]
## $params
## $params$label
## [1] "unnamed-chunk-19"
##
##
## $params.src
## [1] ""
##
## attr(,"class")
## [1] "block"
##
## [[6]]
## [1] 0
##
## [[7]]
## [1] 0
##
## [[8]]
## [1] 0
##
## [[9]]
## [1] 0
##
## [[10]]
## [1] 0
##
## [[11]]
## [1] 0
##
## [[12]]
## [1] 0
##
## [[13]]
## [1] 0
##
## [[14]]
## [1] 0
##
## [[15]]
## [1] 0
##
## [[16]]
## [1] 0
##
## [[17]]
## [1] 0
##
## [[18]]
## [[18]][[1]]
## <environment: 0x0338d758>
##
## [[18]][[2]]
## <environment: 0x0390a534>
##
## [[18]][[3]]
## <environment: 0x03c2e410>
##
## [[18]][[4]]
## <environment: 0x03c2d7fc>
##
## [[18]][[5]]
## <environment: 0x03c2d9f4>
##
## [[18]][[6]]
## <environment: 0x03c2db44>
##
## [[18]][[7]]
## <environment: 0x03be6a60>
##
## [[18]][[8]]
## <environment: 0x042f9470>
##
## [[18]][[9]]
## <environment: 0x042f9b28>
##
## [[18]][[10]]
## <environment: 0x04068464>
##
## [[18]][[11]]
## <environment: 0x03f1cb34>
##
## [[18]][[12]]
## <environment: 0x03f17450>
##
## [[18]][[13]]
## <environment: 0x03f1754c>
##
## [[18]][[14]]
## <environment: 0x03f143f0>
##
## [[18]][[15]]
## <environment: 0x03f14594>
##
## [[18]][[16]]
## <environment: 0x03f13be8>
##
## [[18]][[17]]
## <environment: 0x03f0da1c>
##
## [[18]][[18]]
## <environment: 0x03f0e03c>
##
## [[18]][[19]]
## <environment: 0x03f0e18c>
##
## [[18]][[20]]
## <environment: R_GlobalEnv>
##
## [[18]][[21]]
## <environment: 0x03f055a0>
##
## [[18]][[22]]
## <environment: 0x03f0562c>
##
## [[18]][[23]]
## <environment: 0x03effe4c>
##
## [[18]][[24]]
## <environment: 0x03efff10>
##
##
## [[19]]
## [1] 0
##
## [[20]]
## [1] 0
##
## [[21]]
## [1] 1
##
## [[22]]
## [1] 2
##
## [[23]]
## [1] 3
##
## [[24]]
## [1] 4
</code></pre>
<pre><code class="r">sapply(es, function(e) get("y", e, inherits = TRUE))
</code></pre>
<pre><code>## [1] 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
## [24] 10
</code></pre>
<h2>Assignment</h2>
<blockquote>
<p>Assignment is the act of binding (or rebinding) a name to a value in an environment. It is the counterpart to scoping, the set of rules that determines how to find the value associated with a name.</p>
</blockquote>
<p>There are four types of binding:</p>
<ul>
<li><p>With the regular behaviour, name <- value, the name is immediately associated with the value in the current environment. assign(“name”, value) works similarly, but allows assignment in any environment.</p></li>
<li><p>The double arrow, name <<- value, assigns in a similar way to variable lookup, so that i <<- i + 1 modifies the binding of the original i, which is not necessarily in the current environment.</p></li>
<li><p>Lazy assignment, delayedAssign(“name”, expression), binds an expression that isn't evaluated until you look up the name.</p></li>
<li><p>Active assignment, makeActiveBinding(“name”, function, environment) binds the name to a function, so it is “active” and can return a different value each time the name is found.</p></li>
</ul>
<p>Regular Binding:</p>
<pre><code class="r">a <- 1
`a+b` <- 2 # a name can actually be any sequence of characters
a + `a+b`
</code></pre>
<pre><code>## [1] 3
</code></pre>
<pre><code class="r">`:)` <- "smile"
paste0(`:)`, "!")
</code></pre>
<pre><code>## [1] "smile!"
</code></pre>
<pre><code class="r">
e <- new.env()
assign("a", 1, envir = e) # same as e$a <- 1
e <- new.env()
eval(quote(a <- 2), e) # Evaluate an R expression in a specified environment
# quote simply returns its argument, which is not evaluated then
e$a
</code></pre>
<pre><code>## [1] 2
</code></pre>
<p>Constants:</p>
<pre><code class="r">x <- 10
lockBinding("x", globalenv())
x <- 15
</code></pre>
<pre><code>## Error: cannot change value of locked binding for 'x'
</code></pre>
<pre><code class="r">rm(x)
</code></pre>
<p><<-</p>
<p>The regular assignment arrow, <-, always creates a variable in the current environment. The special assignment arrow, <<-, never creates a variable in the current environment, but instead modifies an existing variable found by walking up the parent environments.</p>
<pre><code class="r">x <- 1
inc <- function() x <<- x + 1
inc()
x
</code></pre>
<pre><code>## [1] 2
</code></pre>
<pre><code class="r">inc()
x
</code></pre>
<pre><code>## [1] 3
</code></pre>
<p>Delayed bindings:</p>
<p>Another special type of assignment is a delayed binding: rather than assigning the result of an expression immediately, it creates and stores a promise to evaluate the expression when needed (much like the default lazy evaluation of arguments in R functions).</p>
<pre><code>
```r
makeActiveBinding("x1", (function() runif(1)), environment())
x1
</code></pre>
<pre><code>## [1] 0.8858
</code></pre>
<pre><code class="r">x1
</code></pre>
<pre><code>## [1] 0.8595
</code></pre>
<pre><code class="r">
f <- local({
x <- 1
function(val) {
# with 1 argument is a 'set', with 0 args is a 'get'
if (missing(val))
2^x else x <<- val
}
})
makeActiveBinding("pow2", f, environment())
pow2
</code></pre>
<pre><code>## [1] 2
</code></pre>
<pre><code class="r">pow2 <- 3
pow2
</code></pre>
<pre><code>## [1] 8
</code></pre>
<pre><code class="r">pow2 <- 4
pow2
</code></pre>
<pre><code>## [1] 16
</code></pre>
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