expression               package:base               R Documentation

Unevaluated Expressions

Description:

     Creates or tests for objects of mode '"expression"'.

Usage:

     expression(...)

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

Arguments:

     ...: valid R calls, symbols or constants.

       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 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.

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.

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)

