Get or set a subexpression

These functions allow you to extract and/or modify a subexpression within an expression.

Usage

expr_sub(expr, idx, env = parent.frame())

expr_sub(expr, idx, env = parent.frame()) <- value

Arguments

expr

The expression to select from. Can also be a list of expressions, in which case the first element of index selects the expression from the list.

idx

A valid index: NULL or an integer vector.

env

Environment for any injections in expr (see expression).

value

Replacement; an expression.

Value

The subexpression, which can be modified.

Details

The elixir functions expr_match() and expr_locate() find matching "subexpressions" within expressions and return indices that allow accessing these subexpressions. For example, the expression 1 + 2 + 3 contains all the following subexpressions:

index	subexpression	accessed with R code
NULL	1+2+3	expr
1	+	expr[[1]]
2	1+2	expr[[2]]
3	3	expr[[3]]
c(2,1)	+	expr[[2]][[1]] or expr[[c(2, 1)]]
c(2,2)	1	expr[[2]][[2]] or expr[[c(2, 2)]]
c(2,3)	2	expr[[2]][[3]] or expr[[c(2, 3)]]

Any index returned by expr_match() or expr_locate() will either be NULL (meaning the whole expression / expression list) or an integer vector (e.g. 1 or c(2,3) in the table above).

Suppose you have an index, idx. If idx is an integer vector, you can just use expr[[idx]] to access the subexpression. But in the case where idx is NULL, R will complain that you are trying to select less than one element. The sole purpose of expr_sub() is to get around that issue and allow you to pass either NULL or an integer vector as the index you want for an expression or list of expressions.

See also

expr_match(), expr_locate() which return indices to subexpressions.

Examples

expr = quote(y == a * x + b)
expr_sub(expr, NULL)
#> y == a * x + b
expr_sub(expr, 3)
#> a * x + b
expr_sub(expr, c(3, 3))
#> b

expr_sub(expr, c(3, 3)) <- quote(q)
print(expr)
#> y == a * x + q