Print zero-column data.frame properly

[heavywatal]

The attribute $row.names is preserved in zero-column base::data.frame, while not in data.table. I won't go deeply into this feature difference here, but just want to fix printing function.

attributes(iris[,FALSE])
#> $names
#> character(0)
#> 
#> $class
#> [1] "data.frame"
#> 
#> $row.names
#>   [1]   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17
#>  [18]  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34
#>  [35]  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51
#>  [52]  52  53  54  55  56  57  58  59  60  61  62  63  64  65  66  67  68
#>  [69]  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85
#>  [86]  86  87  88  89  90  91  92  93  94  95  96  97  98  99 100 101 102
#> [103] 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119
#> [120] 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136
#> [137] 137 138 139 140 141 142 143 144 145 146 147 148 149 150

base::print.data.frame(iris[FALSE,])
#> [1] Sepal.Length Sepal.Width  Petal.Length Petal.Width  Species     
#> <0 rows> (or 0-length row.names)
base::print.data.frame(iris[,FALSE])
#> data frame with 0 columns and 150 rows
base::print.data.frame(iris[FALSE,FALSE])
#> data frame with 0 columns and 0 rows

dtiris = data.table::data.table(iris)
attributes(dtiris[,FALSE]) # row.names are removed
#> $class
#> [1] "data.table" "data.frame"
#> 
#> $row.names
#> integer(0)
#> 
#> $names
#> character(0)
#> 
#> $.internal.selfref
#> <pointer: 0x7fb84001d8e0>

data.table:::print.data.table(dtiris[FALSE,])
#> Empty data.table (0 rows) of 5 cols: Sepal.Length,Sepal.Width,Petal.Length,Petal.Width,Species
data.table:::print.data.table(dtiris[,FALSE]) # OK, 0 rows
#> Null data.table (0 rows and 0 cols)
data.table:::print.data.table(dtiris[FALSE,FALSE])
#> Null data.table (0 rows and 0 cols)

data.table:::print.data.table(iris[,FALSE]) # Error
#> Error in `rownames<-`(`*tmp*`, value = paste0(format(rn, right = TRUE, : attempt to set 'rownames' on an object with no dimensions

^{Created on 2019-02-06 by the reprex package (v0.2.1)}

[jangorecki]

Before submitting PR it is best to discuss expected behaviour. There is existing issue that covers the case of non-zero rows and zero columns: #2422 please put your comment there.

[heavywatal]

Sorry I didn't check and mention that discussion. But still I don't think it matters because this PR just fixes the printing functionality for the existing base::data.frame (and tibble). The expected behavior here is to print zero-column data.frame without an error. It improves the consistency with base::print.data.frame without interfering the data.table's policy on zero-column data.

That being said, I would be happy to put my comment on that discussion.

[mattdowle]

I added a test and changed the NEWS item to reflect what's really going on here :

Calling `data.table:::print.data.table()` directly (i.e. bypassing method dispatch by
using 3 colons) and passing it a 0-column `data.frame` (not `data.table`) now
works, #3363. Thanks @heavywatal for the PR.

Please note that ::: means you get to the methods directly which are not supposed to be called directly. You're not supposed to use :::. For example, usage of ::: fails R CMD check so won't be allowed on CRAN.

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@@            Coverage Diff            @@
##             master    #3363   +/-   ##
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  Hits              ?    11513           
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Impacted Files	Coverage Δ
R/print.data.table.R	92.7% <100%> (ø)

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