in R, eval is not evaluating in the correct environment

Question

z <- 1
a <- function() {
  print(parent.frame())
  eval(quote({print(z)}))
}
b <- function() {
  z <- 2
  print(environment())
  a()
}
b()
# output:
<environment: 0x55f0020b1af0>
<environment: 0x55f0020b1af0>
[1] 1

According to documentation:

Usage

eval(expr, envir = parent.frame(),
           enclos = if(is.list(envir) || is.pairlist(envir))
                       parent.frame() else baseenv())

Since the envir argument defaults to parent.frame(), which is the environment of function b, it is the z inside of b that is supposed to be printed. But rather it is the z in the global environment that is printed.

Note that default parameters are evaluated in the running function environment but parameters you pass in are evaluated in the calling environment. So you get a very different `parent.frame()` if you pass it in as a parameter vs if you use the default value. Try `evalq(print(z), envir=parent.frame())` instead of the `eval()` you currently have. — MrFlick, Oct 24 '20 at 02:16

G. Grothendieck · Answer 1 · 2020-10-24T02:20:49.830

The parent frame of eval is the caller of eval, not the caller of a.

# same as eval except for addition of print statement
my.eval <- function (expr, envir = parent.frame(), enclos = if (is.list(envir) || 
    is.pairlist(envir)) parent.frame() else baseenv())  {
  print(envir)
  .Internal(eval(expr, envir, enclos))
}

z <- 1
a <- function() {
  print(environment())
  my.eval(quote({print(z)}))
}

b <- function() {
  z <- 2
  a()
}
b()
## <environment: 0x000000000b208d20>
## <environment: 0x000000000b208d20>

in R, eval is not evaluating in the correct environment

1 Answers1