Is there a way for an R function to tell if it's being called from a `for` or `while` loop?

Question

The title of the question pretty much says it: is there a way for an R function to find out if it is being called directly or from inside a for or while loop? sys.frame(0) or parent.frame(1) returns .GlobalEnv whether the function was called directly or from inside one of those loops. So, is there some other way?

Thanks.

Answer 1

This is not a definitive answer, but I think your solution will be to look at the sys.status, specifically the sys.parents. The second example is for when the function is called within another function, and within a loop. Not sure how you differentiate this without knowing explicitly.

test <- function() sys.status()

for(i in 1:2){
  print(test())
 }


## $sys.calls
## $sys.calls[[1]]
## print(test())
## 
## $sys.calls[[2]]
## test()
## 
## $sys.calls[[3]]
## sys.status()
## 
##
## $sys.parents
## [1] 0 0 2
##
## $sys.frames
## $sys.frames[[1]]
## <environment: 0x0479a1c8>
##
## $sys.frames[[2]]
## <environment: 0x0479a2fc>
##
## $sys.frames[[3]]
## <environment: 0x0479a334>
##
##
## $sys.calls
## $sys.calls[[1]]
## print(test())
##
## $sys.calls[[2]]
## test()
## 
## $sys.calls[[3]]
## sys.status()
##
##
## $sys.parents
## [1] 0 0 2
## 
## $sys.frames
## $sys.frames[[1]]
## <environment: 0x047993cc>
## 
## $sys.frames[[2]]
## <environment: 0x04799570>

## $sys.frames[[3]]
## <environment: 0x047995a8>

and

test()

## $sys.calls
## $sys.calls[[1]]
## test()
## 
## $sys.calls[[2]]
## sys.status()
## 
## 
## $sys.parents
## [1] 0 1
## 
## $sys.frames
## $sys.frames[[1]]
## <environment: 0x04775500>
##   
##   $sys.frames[[2]]
## <environment: 0x04775538>

and

test_sq <- function() test()

for(i in 1:2){
  print(test_sq())
 }

## $sys.calls
## $sys.calls[[1]]
## print(test_sq())
## 
## $sys.calls[[2]]
## test_sq()
## 
## $sys.calls[[3]]
## test()
## 
## $sys.calls[[4]]
## sys.status()
## 
## 
## $sys.parents
## [1] 0 0 2 3
## 
## $sys.frames
## $sys.frames[[1]]
## <environment: 0x04766c60>
##   
##   $sys.frames[[2]]
## <environment: 0x04766dcc>
##   
##   $sys.frames[[3]]
## <environment: 0x04766e04>
##   
##   $sys.frames[[4]]
## <environment: 0x04766e3c>
##   
##   
##   $sys.calls
## $sys.calls[[1]]
## print(test_sq())
## 
## $sys.calls[[2]]
## test_sq()
## 
## $sys.calls[[3]]
## test()
## 
## $sys.calls[[4]]
## sys.status()
## 
## 
## $sys.parents
## [1] 0 0 2 3
## 
## $sys.frames
## $sys.frames[[1]]
## <environment: 0x04765ac8>
##   
##   $sys.frames[[2]]
## <environment: 0x04765c34>
##   
##   $sys.frames[[3]]
## <environment: 0x04765c6c>
##   
##   $sys.frames[[4]]
## <environment: 0x04765d30>

and

test_sq()


## $sys.calls
## $sys.calls[[1]]
## test_sq()
## 
## $sys.calls[[2]]
## test()
## 
## $sys.calls[[3]]
## sys.status()
## 
## 
## $sys.parents
## [1] 0 1 2
## 
## $sys.frames
## $sys.frames[[1]]
## <environment: 0x0475ce40>
##   
##   $sys.frames[[2]]
## <environment: 0x0475cee8>
##   
##   $sys.frames[[3]]
## <environment: 0x0475cf20>

Answer 2

Unfortunately for doesn't appear in sys.calls normally. While its a bit of a hack, you can actually override for, causing it to be included:

`for` = function(iter, vec, expr) eval.parent(replace(sys.call(), 1, list(.Primitive('for'))))
in.for = function() '`for`' %in% lapply(sys.calls(), `[[`, 1)
my.fun = function() { print('before'); print(in.for()); print('after') }

my.fun()
# [1] "before"
# [1] FALSE
# [1] "after"
for (x in 1) my.fun()
# [1] "before"
# [1] TRUE
# [1] "after"