Print list-column dims

[TysonStanley]

Closes #3671

Prints dims with list-columns. For example:

DT = data.table(
  x = 1:2,
  y = list(data.table(x = 1,
                      y = 1),
           data.table(x = 2,
                      y = 2))
)
print(DT, class=TRUE)
#>        x                 y
#>    <int>            <list>
#> 1:     1 <data.table[1x2]>
#> 2:     2 <data.table[1x2]>

DT = data.table(
  x = 1:2,
  y = list(list(x = 1,
                y = c("yes", "no")),
           list(x = 2,
                y = 2))
)
print(DT, class=TRUE)
#>        x         y
#>    <int>    <list>
#> 1:     1 <list[2]>
#> 2:     2 <list[2]>

This provides some additional information without printing too much and costs relatively little since it only applies to what is being printed, not the entire data table.

[codecov]

Codecov Report

Merging #4154 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4154      +/-   ##
==========================================
+ Coverage   99.53%   99.54%   +<.01%     
==========================================
  Files          72       72              
  Lines       13911    13915       +4     
==========================================
+ Hits        13847    13851       +4     
  Misses         64       64

Impacted Files	Coverage Δ
R/print.data.table.R	100% <100%> (ø)	⬆️

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[jangorecki]

Thanks for PR! looks good but we need more tests. Please add tests for: S4 class object, named vector, matrix, arrays (1D, 3D and 4D). 1D array - a vector having a dimension of length 1: array(1:4, c(2,2))[1,, drop=FALSE].

[TysonStanley]

If I understand correctly, the current way for data.table to print named vectors and arrays is to present the first 6 elements and then ... so we wouldn't be printing the dimensions for those. As it is, the print method uses is.atomic() to make the decision about the printing of the class name (and with this PR the dimensions). Would we want to change that and have the class and dimensions printed for named vectors and arrays?

As for the s4 classes, I've added functionality to handle those and have tests for it.

[TysonStanley]

To clarify:

s4class <- setClass("ex_class", slots = list(x = "integer", y = "character", z = "numeric"))
array1 = array(1:4, 2)
array2 = array(1:8, c(2,2))
array3 = array(1:16, c(2,2,2))

DT = data.table::data.table(
  int = 1L:2L,
  list = list(list(x = 1,
                y = c("yes", "no")),
           list(x = 2,
                y = 2)),
  s4 = list(s4class(x = 1L, y = "thing1", z = 3.4),
            s4class(x = 2L, y = "thing2", z = 2.5)),
  array1 = list(array1[1, drop = FALSE],
                array1[2, drop = FALSE]),
  array2 = list(array2[,1, drop = FALSE],
                array2[,2, drop = FALSE]),
  array3 = list(array3[,,1, drop = FALSE],
                array3[,,2, drop = FALSE]),
  named_vec = list(c("var1" = 1, "var2" = 2),
                   c("var1" = 3, "var2" = 4))
  )
print(DT, class=TRUE)
#>      int      list            s4 array1 array2  array3 named_vec
#>    <int>    <list>        <list> <list> <list>  <list>    <list>
#> 1:     1 <list[2]> <ex_class[3]>      1    1,2 1,2,3,4       1,2
#> 2:     2 <list[2]> <ex_class[3]>      2    3,4 5,6,7,8       3,4

So the arrays and named vectors don't print the class as of right now.

[MichaelChirico]

I'm happy if codecov is happy re:tests :)

Looks good overall; I just wonder whether we should patch up #3414 first and add this on to that

[TysonStanley]

That makes sense to me. Seems like #3414 is definitely broader in scope.

Question: Does #3414 mostly have to do with printing list columns with custom classes and arrays/named vectors/matrices? That’s what it seemed to me from the discussion about using toString(). Or does it also include individuals being able to customize printing of, say, data tables in a list column?

[MichaelChirico]

Technically that's enabled, yep, through the format_list_item generic. So users could define format_list_item.data.table.

[TysonStanley]

I like the flexibility. As for this PR, I’ll wait to hear back from you once things are more clear on #3414

[mattdowle]

Btw, why is your id @TysonStanley but your name Tyson Barrett? It doesn't matter really other than it catches me out w.r.t. to checking your name is in DESCRIPTION, and when thanking you in news.

[TysonStanley]

Great question. My middle name is Stanley and I thought I was being creative but, in hindsight, would have gone with something like "@tysonbarrett" instead... Thanks for working around it.

[jangorecki]

I really like the idea that Michael said about using format_list_item to achieve what we need in this PR. Do you think you could follow-up later on with required changes when we have generic method in place?

[mattdowle]

@jangorecki anyone can follow up. Tyson has achieved what he wanted, which closes an issue. We can't ask for more than that.