Facilitate expression substitution
Utility function based on substitute(), to recover an unquoted input.
recreate(x, snames = NULL)
x |
A substituted input. |
snames |
A character string containing set names. |
This function is especially useful when users have to provide lots of quoted inputs, such as the name of the columns from a data frame to be considered for a particular function.
This is actually one of the main uses of the base function
substitute(), but here it can be employed to also
detect SOP (sum of products) expressions, explained for instance in function
translate().
Such SOP expressions are usually used in contexts of sufficieny and necessity,
which are indicated with the usual signs -> and <-. These are
both allowed by the R parser, indicating standard assignment. Due to the R's
internal parsing system, a sufficient expression using -> is automatically
flipped to a necessity statement <- with reversed LHS to RHS, but this
function is able to determine what is the expression and what is the output.
The other necessity code <= is also recognized, but the equivalent
sufficiency code => is not allowed in unquoted expressions.
A quoted, equivalent expression or a substituted object.
Adrian Dusa
recreate(substitute(A + ~B*C)) foo <- function(x, ...) recreate(substitute(list(...))) foo(arg1 = 3, arg2 = A + ~B*C) df <- data.frame(A = 1, B = 2, C = 3, Y = 4) # substitute from the global environment # the result is the builtin C() function res <- recreate(substitute(C)) is.function(res) # TRUE # search first within the column name space from df recreate(substitute(C), colnames(df)) # "C" # necessity well recognized recreate(substitute(A <- B)) # but sufficiency is flipped recreate(substitute(A -> B)) # more complex SOP expressions are still recovered recreate(substitute(A + ~B*C -> Y))
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