R: Substituting and Quoting Expressions (original) (raw)
| substitute {base} | R Documentation |
|---|
Description
substitute returns the parse tree for the (unevaluated) expression expr, substituting any variables bound inenv.
quote simply returns its argument. The argument is not evaluated and can be any R expression.
enquote is a simple one-line utility which transforms a call of the form Foo(....) into the call quote(Foo(....)). This is typically used to protect a [call](../../base/help/call.html) from early evaluation.
Usage
substitute(expr, env)
quote(expr)
enquote(cl)
Arguments
| expr | any syntactically valid R expression. |
|---|---|
| cl | a call, i.e., an R object ofclass (and mode) "call". |
| env | an environment or a list object. Defaults to the current evaluation environment. |
Details
The typical use of substitute is to create informative labels for data sets and plots. The myplot example below shows a simple use of this facility. It uses the functions [deparse](../../base/help/deparse.html) and substituteto create labels for a plot which are character string versions of the actual arguments to the function myplot.
Substitution takes place by examining each component of the parse tree as follows: If it is not a bound symbol in env, it is unchanged. If it is a promise object, i.e., a formal argument to a function or explicitly created using [delayedAssign](../../base/help/delayedAssign.html)(), the expression slot of the promise replaces the symbol. If it is an ordinary variable, its value is substituted, unless env is[.GlobalEnv](../../base/help/.GlobalEnv.html) in which case the symbol is left unchanged.
Both quote and substitute are ‘special’primitive functions which do not evaluate their arguments.
Value
The [mode](../../base/help/mode.html) of the result is generally "call" but may in principle be any type. In particular, single-variable expressions have mode "name" and constants have the appropriate base mode.
Note
substitute works on a purely lexical basis. There is no guarantee that the resulting expression makes any sense.
Substituting and quoting often cause confusion when the argument isexpression(...). The result is a call to the[expression](../../base/help/expression.html) constructor function and needs to be evaluated with [eval](../../base/help/eval.html) to give the actual expression object.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)The New S Language. Wadsworth & Brooks/Cole.
See Also
[missing](../../base/help/missing.html) for argument ‘missingness’,[bquote](../../base/help/bquote.html) for partial substitution,[sQuote](../../base/help/sQuote.html) and [dQuote](../../base/help/dQuote.html) for adding quotation marks to strings.[Quotes](../../base/help/Quotes.html) about forward, back, and double quotes ‘'’, ‘`’, and ‘"’.
[all.names](../../base/help/all.names.html) to retrieve the symbol names from an expression or call.
Examples
require(graphics)
(s.e <- substitute(expression(a + b), list(a = 1))) #> expression(1 + b)
(s.s <- substitute( a + b, list(a = 1))) #> 1 + b
c(mode(s.e), typeof(s.e)) # "call", "language"
c(mode(s.s), typeof(s.s)) # (the same)
# but:
(e.s.e <- eval(s.e)) #> expression(1 + b)
c(mode(e.s.e), typeof(e.s.e)) # "expression", "expression"
substitute(x <- x + 1, list(x = 1)) # nonsense
myplot <- function(x, y)
plot(x, y, xlab = deparse1(substitute(x)),
ylab = deparse1(substitute(y)))
## Simple examples about lazy evaluation, etc:
f1 <- function(x, y = x) { x <- x + 1; y }
s1 <- function(x, y = substitute(x)) { x <- x + 1; y }
s2 <- function(x, y) { if(missing(y)) y <- substitute(x); x <- x + 1; y }
a <- 10
f1(a) # 11
s1(a) # 11
s2(a) # a
typeof(s2(a)) # "symbol"
[Package _base_ version 4.6.0 Index]