extract_values: Extract and Modify Values in an Expression
In ryan-heslin/matador: Linear Algebra Visualization

Description Usage Arguments Value Examples

This function extracts symbols from an expression and evaluates them in a manner the user specifies. First, each symbol is evaluated in the supplied envir. If it is defined in that environment and is not bound to a function, extract_values then applies pred to the value. If pred returns TRUE, the value is then transformed by fun.

1	extract_values(exp, env, pred = function(val) TRUE, fun = identity, ...)

`exp`	A quoted expression.
`env`	An environment in which to evaluate values in the expression. Defaults to the caller environment.
`pred`	A predicate function that returns a length-one logical when applied to evaluated values within the expression. Defaults to `TRUE`.
`fun`	An optional function to apply to each evaluated value. `extract_values` will return whatever this function returns for each value successfully evaluated in `env`. Defaults to `identity` - which leaves the value unchanged.
`...`	Additional arguments to `fun`.

A named vector or list of the successfully evaluated values. If none were successfully evaluated, an empty list.

 envir <- new.env()
 envir$A <- square(1:9)
 B <- square(4, 5, 6, 7)
 k <- 2
 extract_values(quote(A + B), env = envir, pred = function(x) nrow(x) >2, fun = mean)
 extract_values(quote(k * A), pred = is.matrix, fun = function(x) x %^% 3)