Substituting and Quoting Expressions

Description

substitute returns the parse tree for the (unevaluated) expression expr, substituting any variables bound in env.

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 from early evaluation.

Usage

substitute(expr, env)
quote(expr)
enquote(cl)

Arguments

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 and substitute to 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(), the expression slot of the promise replaces the symbol. If it is an ordinary variable, its value is substituted, unless env is .GlobalEnv 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 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 is expression(...). The result is a call to the expression constructor function and needs to be evaluated with eval 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 for argument ‘missingness’, bquote for partial substitution, sQuote and dQuote for adding quotation marks to strings,

all.names 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 = deparse(substitute(x)),
         ylab = deparse(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"