[R] Interpreting the output of str on a data frame created using aggregate function

[Duncan Murdoch]

I'll answer your question inline.

On 2025-01-24 2:03 p.m., Sorkin, John wrote:
> I ran the following code:
> marginalcats <- aggregate(meanbyCensusIDAndDay3$cats,
>                            list(meanbyCensusIDAndDay3$CensusID),table)
> followed by
> str(marginalcats)
> 
> I received the following output:
> 'data.frame':	844 obs. of  2 variables:
>   $ Group.1: num  6e+09 6e+09 6e+09 6e+09 6e+09 ...
>   $ x      : int [1:844, 1:7] 14 14 14 14 14 14 14 14 14 14 ...
>    ..- attr(*, "dimnames")=List of 2
>    .. ..$ : NULL
>    .. ..$ : chr [1:7] "Good" "Moderate" "Unhealthy For Some" "Unhealthy" ...
> 
> I am trying to understand the output. I believe it says that marginalcats
> (1) is a data frame
> (2) the df has two elements (I) Group.1 and (II) x

Correct so far.

> (3) Group.1 is a ?list? of number

No, it's a numeric vector.  The length doesn't print because it's in a 
dataframe, so the length is 844, the number of dataframe observations.


> (4) x which is a 844x7 matrix having value "Good", "Moderate", etc.

Correct.

> 
> A few questions:
> (A) Is the interpretation given above correct?
> (B) Does the .. ..$ : NULL mean that the matrix has no row names?

Yes.

> (C) What does "attr(*, "dimnames")=List of 2" mean?

That says x has an attribute called "dimnames", which is a list with two 
elements.  They are the row names (which is NULL, you don't have any) 
and the column names.

> (D) Does it mean that the dimensions of the matrix are stored as two separate lists?

No.  The dim is an attribute which is shown implicitly as "[1:844, 
1:7]", i.e. c(844, 7).

Duncan Murdoch

> (E) If so, how do I access the lists?
> When I enter
> dimnames(marginalcatsx$x)
> I receive:
> 
> [[1]]
> NULL
> 
> [[2]]
> [1] "Good"               "Moderate"           "Unhealthy For Some" "Unhealthy"          "Very Unhealthy"     "Hazardous1"
> [7] "Hazardous2"
> 
> Thank you,
> John
> 
> John David Sorkin M.D., Ph.D.
> Professor of Medicine, University of Maryland School of Medicine;
> Associate Director for Biostatistics and Informatics, Baltimore VA Medical Center Geriatrics Research, Education, and Clinical Center;
> PI Biostatistics and Informatics Core, University of Maryland School of Medicine Claude D. Pepper Older Americans Independence Center;
> Senior Statistician University of Maryland Center for Vascular Research;
> 
> Division of Gerontology and Paliative Care,
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> Cell phone 443-418-5382
> 
> 
> 
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