[R] holding objects in dataframes

[Uwe Ligges]

I haven't seen an answer so far, hence I try:

As far as I can see, everything is correct here, since you have an 
element polygons in you data.frame that is a valid vector of mode list.

Nevertheless, since printing /plotting etc. on the dataframe won't give 
desired results now, it would be more natural to introduce a new class 
for your kind of object and write some nice methods for it.

Best,
Uwe Ligges





On 14.01.2011 21:59, Alexander Shenkin wrote:
> Hello list,
>
> I need to keep track of objects that are related to particular
> observations.  In this case, I need to keep track of polygons that are
> associated with observations.  What I would ideally have is one column
> of a dataframe hold a "polygonal" object (from the spatstat package).
> My question: I seem to have managed to do it, but as I haven't read that
> dataframes are supposed to be able to hold list elements or objects this
> way, I wanted to ask if I'm violating things here in a way such that my
> code may break in future R releases.  (that is, am i relying on
> non-guaranteed behavior).  My other option is to created a named list or
> something along those lines, and just match() the dataframe and named
> list when i need access to the related objects.  Please see below for
> the code.  Thoughts and suggestions are greatly appreciated!
>
> By tinkering around, I've found that dataframe columns do seem to be
> able to hold list elements:
>
> a=data.frame(i=c(1,2),j=c(3,4))
> z=list(x=c(1,2,3))
> a$k=NA
> a[1,]$k=z
>> str(a)
>
>      'data.frame':   2 obs. of  3 variables:
>       $ i: num  1 2
>       $ j: num  3 4
>       $ k:List of 2
>        ..$ : num  1 2 3
>        ..$ : logi NA
>
>
> Extending that behavior, I can attach "polygonal" objects to dataframes
> it seems:
>
> library(spatstat)
> a=data.frame( a=c(1,2), b=c(3,4) )
> a$polygons=NA
> 	
>> str(a)
>
>      'data.frame':   2 obs. of  3 variables:
>       $ a       : num  1 2
>       $ b       : num  3 4
>       $ polygons:List of 2
>        ..$ :List of 5
>        .. ..$ type  : chr "polygonal"
>        .. ..$ xrange: num  -50 50
>        .. ..$ yrange: num  -50 50
>        .. ..$ bdry  :List of 1
>        .. .. ..$ :List of 4
>        .. .. .. ..$ x   : num  50 49.9 49.8 49.5 49 ...
>        .. .. .. ..$ y   : num  0 2.45 4.9 7.34 9.75 ...
>        .. .. .. ..$ area: num 7851
>        .. .. .. ..$ hole: logi FALSE
>        .. ..$ units :List of 3
>        .. .. ..$ singular  : chr "unit"
>        .. .. ..$ plural    : chr "units"
>        .. .. ..$ multiplier: num 1
>        .. .. ..- attr(*, "class")= chr "units"
>        .. ..- attr(*, "class")= chr "owin"
>        ..$ : logi NA
>
>> area.owin(a[1,]$polygons[[1]])
> [1] 7850.828
>
> thanks,
> allie
>
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