Problem
With this toy problem, I want to understand why a list is not updated when a function calls another function. The same list is updated if I run the code outside of the second function.
Reproducible Example:
The following function updates a previously empty list SControlModelStates():
library(pracma)
func_1 <- function(VTimeStamp,
iSample,
SParameters){
# is this the first time step?
if (pracma::isempty(SControlModelStates)){
stopifnot(iSample == 1)
# set default values for any unspecified model parameters
if (!('my_param' %in% names(SParameters))){
SParameters[['my_param']] <- 5
}
SControlModelStates[['my_val']] <- rep(0, length(VTimeStamp))
} else {
# this is not the first time step
SControlModelStates[['my_val']][iSample] <- SControlModelStates[['my_val']][iSample-1] + SParameters[['my_param']]
}
res <- list('SParameters' = SParameters,
'SControlModelStates' = SControlModelStates)
res
}
Running the function:
When I run the above function, it successfully updates SControlModelStates():
SControlModelStates <- list()
Parameters <- list()
VTimeStamp <- 1:5
for (i in 1:length(VTimeStamp)){
foo <- func_1 (VTimeStamp,
i,
Parameters)
Parameters <- foo[["SParameters"]]
SControlModelStates <- foo[["SControlModelStates"]]
}
> SControlModelStates
$my_val
[1] 0 5 10 15 20
Calling the above function in another function produces an error:
After restarting R I did the following:
## Function definition
func_2 <- function(VTimeStamp){
SControlModelStates <- list()
Parameters <- list()
VTimeStamp <- VTimeStamp
for (i in 1:length(VTimeStamp)){
foo <- func_1 (VTimeStamp,
i,
Parameters)
Parameters <- foo[["SParameters"]]
SControlModelStates <- foo[["SControlModelStates"]]
}
return(SControlModelStates[["my_val"]])
}
## Running the function:
> func_2(1:5)
Error in pracma::isempty(SControlModelStates) :
object 'SControlModelStates' not found
This indicates that SControlModelStates() is not updated when iSample == i == 1. It remains empty when i==2. How can I fix func_2 to resolve this issue?
Your problem is one of variable scope.
The for loop run as is works because you define SControlModelStates as a global variable and func_1 can access it.
When you run func_2 then func_1 doesn't have access to the calling environment (func_1 and global) but only the environment it was defined in (global) but the global environment doesn't have a variable SControlModelStates in a clean R runtime.
If you want to reference an existing SControlModelStates you either need to pass it as a parameter to func_1 (preferred) or use a global variable.
Related
I've just read about delayedAssign(), but the way you have to do it is by passing the name of the delayed variable as the first parameter. Is there a way to do it via direct assignment?
e.g.:
x <- delayed_variable("Hello World")
rather than
delayedAssign("x","Hello World")
I want to create a variable that will throw an error if accessed (use-case is obviously more complex), so for example:
f <- function(x){
y <- delayed_variable(stop("don't use y"))
x
}
f(10)
> 10
f <- function(x){
y <- delayed_variable(stop("don't use y"))
y
}
f(10)
> Error in f(10) : don't use y
No, you can't do it that way. Your example would be fine with the current setup, though:
f <- function(x){
delayedAssign("y", stop("don't use y"))
y
}
f(10)
which gives exactly the error you want. The reason for this limitation is that delayed_variable(stop("don't use y")) would create a value which would trigger the error when evaluated, and assigning it to y would evaluate it.
Another version of the same thing would be
f <- function(x, y = stop("don't use y")) {
...
}
Internally it's very similar to the delayedAssign version.
I reached a solution using makeActiveBinding() which works provided it is being called from within a function (so it doesn't work if called directly and will throw an error if it is). The main purpose of my use-case is a smaller part of this, but I generalised the code a bit for others to use.
Importantly for my use-case, this function can allow other functions to use delayed assignment within functions and can also pass R CMD Check with no Notes.
Here is the function and it gives the desired outputs from my question.
delayed_variable <- function(call){
#Get the current call
prev.call <- sys.call()
attribs <- attributes(prev.call)
# If srcref isn't there, then we're not coming from a function
if(is.null(attribs) || !"srcref" %in% names(attribs)){
stop("delayed_variable() can only be used as an assignment within a function.")
}
# Extract the call including the assignment operator
this_call <- parse(text=as.character(attribs$srcref))[[1]]
# Check if this is an assignment `<-` or `=`
if(!(identical(this_call[[1]],quote(`<-`)) ||
identical(this_call[[1]],quote(`=`)))){
stop("delayed_variable() can only be used as an assignment within a function.")
}
# Get the variable being assigned to as a symbol and a string
var_sym <- this_call[[2]]
var_str <- deparse(var_sym)
#Get the parent frame that we will be assigining into
p_frame <- parent.frame()
var_env <- new.env(parent = p_frame)
#Create a random string to be an identifier
var_rand <- paste0(sample(c(letters,LETTERS),50,replace=TRUE),collapse="")
#Put the variables into the environment
var_env[["p_frame"]] <- p_frame
var_env[["var_str"]] <- var_str
var_env[["var_rand"]] <- var_rand
# Create the function that will be bound to the variable.
# Since this is an Active Binding (AB), we have three situations
# i) It is run without input, and thus the AB is
# being called on it's own (missing(input)),
# and thus it should evaluate and return the output of `call`
# ii) It is being run as the lhs of an assignment
# as part of the initial assignment phase, in which case
# we do nothing (i.e. input is the output of this function)
# iii) It is being run as the lhs of a regular assignment,
# in which case, we want to overwrite the AB
fun <- function(input){
if(missing(input)){
# No assignment: variable is being called on its own
# So, we activate the delayed assignment call:
res <- eval(call,p_frame)
rm(list=var_str,envir=p_frame)
assign(var_str,res,p_frame)
res
} else if(!inherits(input,"assign_delay") &&
input != var_rand){
# Attempting to assign to the variable
# and it is not the initial definition
# So we overwrite the active binding
res <- eval(substitute(input),p_frame)
rm(list=var_str,envir=p_frame)
assign(var_str,res,p_frame)
invisible(res)
}
# Else: We are assigning and the assignee is the output
# of this function, in which case, we do nothing!
}
#Fix the call in the above eval to be the exact call
# rather than a variable (useful for debugging)
# This is in the line res <- eval(call,p_frame)
body(fun)[[c(2,3,2,3,2)]] <- substitute(call)
#Put the function inside the environment with all
# all of the variables above
environment(fun) <- var_env
# Check if the variable already exists in the calling
# environment and if so, remove it
if(exists(var_str,envir=p_frame)){
rm(list=var_str,envir=p_frame)
}
# Create the AB
makeActiveBinding(var_sym,fun,p_frame)
# Return a specific object to check for
structure(var_rand,call="assign_delay")
}
I am doing some heavy computations which I would like to speed up by performing it in a parallel loop. Moreover, I want the result of each calculation to be assigned to the global environment based on the name of the data currently processed:
fun <- function(arg) {
assign(arg, arg, envir = .GlobalEnv)
}
For loop
In a simple for loop, that would be the following and this works just fine:
for_fun <- function() {
data <- letters[1:10]
for(i in 1:length(data)) {
dat <- quote(data[i])
call <- call("fun", dat)
eval(call)
}
}
# Works as expected
for_fun()
In this function, I first get some data, loop over it, quote it (although not necessary) to be used in a function call. In reality, this function name is also dynamic which is why I am doing it this way.
Foreach
Now, I want to speed this up. My first thought was to use the foreach package (with a doParallel backend):
foreach_fun <- function() {
# Set up parallel backend
cl <- parallel::makeCluster(parallel::detectCores())
doParallel::registerDoParallel(cl)
data <- letters[1:10]
foreach(i = 1:length(data)) %dopar% {
dat <- quote(data[i])
call <- call("fun", dat)
eval(call)
}
# Stop the parallel backend
parallel::stopCluster(cl)
doParallel::stopImplicitCluster()
}
# Error in { : task 1 failed - "could not find function "fun""
foreach_fun()
Replacing the whole quote-call-eval procedure with simply fun(data[i]) resolves the error but still nothing gets assigned.
Future
To ensure it wasn't a problem with the foreach package, I also tried the future package (although I am not familiar with it).
future_fun <- function() {
# Plan a parallel future
cl <- parallel::makeCluster(parallel::detectCores())
future::plan(cluster, workers = cl)
data <- letters[1:10]
# Create an explicit future
future(expr = {
for(i in 1:length(data)) {
dat <- quote(data[i])
call <- call("fun", dat)
eval(call)
}
})
# Stop the parallel future
parallel::stopCluster(cl)
future::plan(sequential)
}
# No errors but nothing assigned
# probably the future was never evaluated
future_fun()
Forcing the future to be evaluated (f <- future(...); value(f)) triggers the same error as by using foreach: Error in { : task 1 failed - "could not find function "fun""
Summary
In short, my questions are:
How do you assign variables to the global environment in a parallel loop?
Why does the function lookup fail?
I'm trying to figure out how to allow a function to directly alter or create variables in its parent environment, whether the parent environment is the global environment or another function.
For example if I have a function
my_fun <- function(){
a <- 1
}
I would like a call to my_fun() to produce the same results as doing a <- 1.
I know that one way to do this is by using parent.frame as per below but I would prefer a method that doesn't involve rewriting every variable assignment.
my_fun <- function(){
env = parent.frame()
env$a <- 1
}
Try with:
g <- function(env = parent.frame()) with(env, { b <- 1 })
g()
b
## [1] 1
Note that normally it is preferable to pass the variables as return values rather than directly create them in the parent frame. If you have many variables to return you can always return them in a list, e.g. h <- function() list(a = 1, b = 2); result <- h() Now result$a and result$b have the values of a and b.
Also see Function returning more than one value.
Suppose we have this functions in a R package.
prova <- function() {
print(attr(prova, 'myattr'))
print(myattr(prova))
invisible(TRUE)
}
'myattr<-' <- function(x, value) {
attr(x, 'myattr') <- value
x
}
myattr <- function(x) attr(x, 'myattr')
So, I install the package and then I test it. This is the result:
prova()
# NULL
# NULL
myattr(prova) <- 'ciao' # setting 'ciao' for 'myattr' attribute
prova()
# NULL
# NULL # Why NULL here ?
myattr(prova)
# [1] "ciao"
attr(prova, 'myattr')
# [1] "ciao"
The question is: how to get the attribute of the function from within itself?
Inside the function itself I cannot get its attribute, as demonstrated by the example.
I suppose that the solution will be of the serie "computing on the language" (match.call()[[1L]], substitute, environments and friends). Am I wrong?
I think that the important point here is that this function is in a package (so, it has its environment and namespace) and I need its attribute inside itself, in the package, not outside.
you can use get with the envir argument.
prova <- function() {
print(attr(get("prova", envir=envir.prova), 'myattr'))
print(myattr(prova))
invisible(TRUE)
}
eg:
envir.prova <- environment()
prova()
# NULL
# NULL
myattr(prova) <- 'ciao'
prova()
# [1] "ciao"
# [1] "ciao"
Where envir.prova is a variable whose value you set to the environment in which prova is defined.
Alternatively you can use get(.. envir=parent.frame()), but that is less reliable as then you have to track the calls too, and ensure against another object with the same name between the target environment and the calling environment.
Update regarding question in the comments:
regarding using parent.frame() versus using an explicit environment name: parent.frame, as the name suggests, goes "up one level." Often, that is exactly where you want to go, so that works fine. And yet, even when your goal is get an object in an environment further up, R searches up the call stack until it finds the object with the matching name. So very often, parent.frame() is just fine.
HOWEVER if there are multiple calls between where you are invoking parent.frame() and where the object is located AND in one of the intermediary environments there exists another object with the same name, then R will stop at that intermediary environment and return its object, which is not the object you were looking for.
Therefore, parent.frame() has an argument n (which defaults to 1), so that you can tell R to begin it's search at n levels back.
This is the "keeping track" that I refer to, where the developer has to be mindful of the number of calls in between. The straightforward way to go about this is to have an n argument in every function that is calling the function in question, and have that value default to 1. Then for the envir argument, you use: get/assign/eval/etc (.. , envir=parent.frame(n=n) )
Then if you call Func2 from Func1, (both Func1 and Func2 have an n argument), and Func2 is calling prova, you use:
Func1 <- function(x, y, ..., n=1) {
... some stuff ...
Func2( <some, parameters, etc,> n=n+1)
}
Func2 <- function(a, b, c, ..., n=1) {
.... some stuff....
eval(quote(prova()), envir=parent.frame(n=n) )
}
As you can see, it is not complicated but it is * tedious* and sometimes what seems like a bug creeps in, which is simply forgetting to carry the n over.
Therefore, I prefer to use a fixed variable with the environment name.
The solution that I found is:
myattr <- function(x) attr(x, 'myattr')
'myattr<-' <- function(x, value) {
# check that x is a function (e.g. the prova function)
# checks on value (e.g. also value is a function with a given precise signature)
attr(x, 'myattr') <- value
x
}
prova <- function(..., env = parent.frame()) {
# get the current function object (in its environment)
this <- eval(match.call()[[1L]], env)
# print(eval(as.call(c(myattr, this)), env)) # alternative
print(myattr(this))
# print(attr(this, 'myattr')
invisible(TRUE)
}
I want to thank #RicardoSaporta for the help and the clarification about keeping tracks of the calls.
This solution doesn't work when e.g. myattr(prova) <- function() TRUE is nested in func1 while prova is called in func2 (that it's called by func1). Unless you do not properly update its parameter env ...
For completeness, following the suggestion of #RicardoSaporta, I slightly modified the prova function:
prova <- function(..., pos = 1L) {
# get the current function object (in its environment)
this <- eval(match.call()[[1L]], parent.frame(n = pos)
print(myattr(this))
# ...
}
This way, it works also when nested, if the the correct pos parameter is passed in.
With this modification it is easier to go to fish out the environment in which you set the attribute on the function prova.
myfun1 <- function() {
myattr(prova) <- function() print(FALSE)
myfun2(n = 2)
}
myfun2 <- function(n) {
prova(pos = n)
}
myfun1()
# function() print(FALSE)
# <environment: 0x22e8208>
I have a question about function environments in the R language.
I know that everytime a function is called in R, a new environment E
is created in which the function body is executed. The parent link of
E points to the environment in which the function was created.
My question: Is it possible to specify the environment E somehow, i.e., can one
provide a certain environment in which function execution should happen?
A function has an environment that can be changed from outside the function, but not inside the function itself. The environment is a property of the function and can be retrieved/set with environment(). A function has at most one environment, but you can make copies of that function with different environments.
Let's set up some environments with values for x.
x <- 0
a <- new.env(); a$x <- 5
b <- new.env(); b$x <- 10
and a function foo that uses x from the environment
foo <- function(a) {
a + x
}
foo(1)
# [1] 1
Now we can write a helper function that we can use to call a function with any environment.
with_env <- function(f, e=parent.frame()) {
stopifnot(is.function(f))
environment(f) <- e
f
}
This actually returns a new function with a different environment assigned (or it uses the calling environment if unspecified) and we can call that function by just passing parameters. Observe
with_env(foo, a)(1)
# [1] 6
with_env(foo, b)(1)
# [1] 11
foo(1)
# [1] 1
Here's another approach to the problem, taken directly from http://adv-r.had.co.nz/Functional-programming.html
Consider the code
new_counter <- function() {
i <- 0
function() {
i <<- i + 1
i
}
}
(Updated to improve accuracy)
The outer function creates an environment, which is saved as a variable. Calling this variable (a function) effectively calls the inner function, which updates the environment associated with the outer function. (I don't want to directly copy Wickham's entire section on this, but I strongly recommend that anyone interested read the section entitled "Mutable state". I suspect you could get fancier than this. For example, here's a modification with a reset option:
new_counter <- function() {
i <- 0
function(reset = FALSE) {
if(reset) i <<- 0
i <<- i + 1
i
}
}
counter_one <- new_counter()
counter_one()
counter_one()
counter_two <- new_counter()
counter_two()
counter_two()
counter_one(reset = TRUE)
I am not sure I completely track the goal of the question. But one can set the environment that a function executes in, modify the objects in that environment and then reference them from the global environment. Here is an illustrative example, but again I do not know if this answers the questioners question:
e <- new.env()
e$a <- TRUE
testFun <- function(){
print(a)
}
testFun()
Results in: Error in print(a) : object 'a' not found
testFun2 <- function(){
e$a <- !(a)
print(a)
}
environment(testFun2) <- e
testFun2()
Returns: FALSE
e$a
Returns: FALSE