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R version 4.5.0 (2025-04-11 ucrt) RGui console in Windows 11.

I happened to come across the following problem. Consider the global assignment statement:

xxx <<- c(1,2)
xxx[1:2]
ls()

This gives

[1] 1 2
[1] "xxx"

However, this example:

xxx <<- c(1,2)
xxx[1:2]
ls()
xxx[1:2] <<- c(11, 22)

Results in

Error: object 'xxx' not found

Q: Why is object xxx not found? Where to find the relevant documentation?

Note: Above statement are executed immediately after startup in the R console for Windows 11. This issue may occur when copying instructions from a function to the global environment.

Environment

  globalenv()
# environment: R_GlobalEnv>
  environment()
# environment: R_GlobalEnv>
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7
  • 1
    I don't know if you'll find this documented because it doesn't really make sense to use <<- in the global environment. Commented yesterday
  • 1
    The statement is copied from a function for testing. Commented yesterday
  • 1
    I suggest you edit the question to make it clear which environment xxx was created in and in which the assignment is taking place. The purpose of this operator is to assign in the parent environment, which certainly isn't happening in the question as it's currently written. Commented yesterday
  • 1
    Note this in the ?"<<-" help page: The operators ‘<<-’ and ‘->>’ are normally only used in functions. Commented yesterday
  • The notion of a “global vector” is misleading. The <<- function is used to assign a value to a vector in the next “higher” environment, but it appears you are executing it in the base environment for which there is nothing “higher”. Commented yesterday

2 Answers 2

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9

I think this is a result of bad design in R (which might be excused because it was inherited from S, I'm not sure). Here's what is going on:

When you run

xxx <<- c(1,2)

R will look for xxx in the parent environments of the current environment (which is shown by printing the result of environment(), i.e. <environment: R_GlobalEnv>). This search fails, because there's no xxx higher up in the environment tree. At this point, a good design would signal an error and you wouldn't try that again.

However, R doesn't signal an error, it defaults to making the assignment in globalenv(). This leads to tons of confusion, e.g. to people thinking <<- means "global assignment", when really it means "assignment to an ancestor environment or maybe global". It would be better if it was just "assignment to an ancestor environment".

Your second assignment is more complicated, because it assigns to a subset of xxx. If you had used the regular assignment operator

xxx[1:2] <- c(11, 22)

R would go through several steps. As the Language manual explains, these are equivalent to

`*tmp*` <- xxx
xxx <- "[<-"(`*tmp*`, 1:2, value=c(11, 22))
rm(`*tmp*`)

The manual also explains what happens when you do both, as in your

xxx[1:2] <<- c(11, 22)

though I think there are typos. Correcting those, this is equivalent to

`*tmp*` <- get("xxx", envir=parent.env(), inherits=TRUE)
`*tmp*`[1:2] <- c(11, 22)
xxx <<- `*tmp*`
rm(`*tmp*`)

and here the first line fails, because xxx is not in the parent environment. This is a good thing. We should have had a failure in

xxx <<- c(1,2)

too, but we don't because of the bad design.

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It's because it assign to an existing variable in parent environment, but it does not create the variable when used through a sub assignment.

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Harman Khaira is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.

4 Comments

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How to create a variable through a sub assignment?
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You cannot create a variable through a sub assignment, you cannot subset what does not exist yet.
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I have clarified my question. The slicing is after the previous statements, not as a single statement.
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