expression {base}

R Documentation

Unevaluated Expressions

Description

Creates or tests for objects of mode "expression".

Usage

expression(...)

is.expression(x)
as.expression(x, ...)

Arguments

...	expression: R objects, typically calls, symbols or constants. as.expression: arguments to be passed to methods.
x	an arbitrary R object.

Details

‘Expression’ here is not being used in its colloquial sense, that of mathematical expressions. Those are calls (see call) in R, and an R expression vector is a list of calls, symbols etc, typically as returned by parse.

As an object of mode "expression" is a list, it can be subsetted by both [ and by [[, the latter extracting individual calls etc.

Value

expression returns a vector of type "expression" containing its arguments (unevaluated).

is.expression returns TRUE if expr is an expression object and FALSE otherwise.

as.expression attempts to coerce its argument into an expression object. It is generic, and only the default method is described here. NULL, calls, symbols (see as.symbol) and pairlists are returned as the element of a length-one expression vector. Vectors (including lists) are placed element-by-element into an expression vector. Other types are not currently supported.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

call, eval, function. Further, text and legend for plotting mathematical expressions.

Examples

length(ex1 <- expression(1+ 0:9))# 1
ex1
eval(ex1)# 1:10

length(ex3 <- expression(u,v, 1+ 0:9))# 3
mode(ex3 [3]) # expression
mode(ex3[[3]])# call
rm(ex3)

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