[R] Variables captured in closures get copied?

[Peter Dalgaard]

Titus von der Malsburg wrote:
> Hi list!  I have a data frame called fix and a list of index vectors
> called rois:
> 
>   > head(rois, 3)
>   [[1]]
>   [1] 2 1
> 
>   [[2]]
>   [1] 3
> 
>   [[3]]
>   [1]  6  7 28 26 27 24 25
> 
> The part that's causing the issue is the following line:
> 
>   lapply(rois, function(roi) fix$x[roi] <- 100)
> 
> So for every index vector I'd like to set the respective entries in the
> data frame (fix) to 100.
> 
> I expected the data frame would be changed after lapply but instead it
> remains unchanged.  I understand that when I pass an argument into a
> function it gets passed as a value and not as a reference.  But here fix
> is not an argument but captured in the closure.  Do my questions are:
> What's going on here and what is the idiomatic way of achieving my goal?
> 

It's a local variable in the function. Not in principle different from

function(roi) { fix <- fix ;  ... }

You could use superassignment (<<-), but a simpler idiom is

for (roi in rois) fix$x[roi] <- 100

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