Option to print the number of columns

[KyleHaynes]

Could data.table:::print.data.table look to gain an option to print the number of columns?

Happy to put a PR in if so.

If yes, things to consider

• Parameter name (e.g. ncols to match nrows?)
• Option name (e.g. getOption("datatable.print.ncols")
• Label (e.g. ncol)
• Position (header vs footer?)
• Don't print if 0 rows (print.data.table already communicates this)
• On or off by default?

e.g.

> options("datatable.print.ncols" = TRUE)
> iris
Key: <Petal.Width>
ncol: 5
     Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
            <num>       <num>        <num>       <num>    <char>
  1:          4.9         3.1          1.5         0.1    setosa
  2:          4.8         3.0          1.4         0.1    setosa
  3:          4.3         3.0          1.1         0.1    setosa
  4:          5.2         4.1          1.5         0.1    setosa
  5:          4.9         3.6          1.4         0.1    setosa
 ---
146:          6.3         3.4          5.6         2.4 virginica
147:          6.7         3.1          5.6         2.4 virginica
148:          6.3         3.3          6.0         2.5 virginica
149:          7.2         3.6          6.1         2.5 virginica
150:          6.7         3.3          5.7         2.5 virginica

[KyleHaynes]

Closing, this should go in #1523

[MichaelChirico]

Re-opened -- #1523 is more of an umbrella issue. I think this will keep more attention as a standalone issue.

I think it's a good idea -- it can be hard to tell the exact number of columns on tables that require wrapping, especially multiple times.

Perhaps the row with Key: should be turned into a row for various metadata, the second element being # columns.

OTOH, Index:(/Indices:) already gets its own row, so Number of columns: fits in better on its own line.

Parameter name (e.g. ncols to match nrows?)

No, nrows is quite different. This parameter is more related to print.keys and show.indices (hmm, why do they have such similar-but-different names 😆). Maybe show.ncols instead, although I wonder whether this should just be on-by-default.

Most important will be the interaction with trunc.cols, which already records the number of columns:

DT=data.table(a=1,b=2,c=3)
setkey(DT)
setindex(DT,a,b)
setindex(DT,a,c)
setindex(DT,b,c)
DT
# Key: <a, b, c>
# Indices: <a__b>, <a__c>, <b__c>
#        a     b     c
#    <num> <num> <num>
# 1:     1     2     3
options(width=10)
print(DT, trunc.cols=TRUE)
# Key: <a, b, c>
# Indices: <a__b>, <a__c>, <b__c>, <b__a>, <c__a>, <c__b>
#        a
#    <num>
# 1:     1
# 2 variable(s) not shown: [b <num>, c <num>]

@TysonStanley WDYT about something like:

• if trunc.cols=FALSE, record Number of columns: <n>
• if trunc.cols=TRUE, record Number of columns: <n>, of which <k> are not shown: [...]

That would mean moving the note from the footer to the header.

[rikivillalba]

Imho for coherence, the output should be similar to:

print(data.table(a=integer(0)))
# Empty data.table (0 rows and 1 cols): a
print(data.table())
# Null data.table (0 rows and 0 cols)

[KyleHaynes]

Imho for coherence, the output should be similar to:

print(data.table(a=integer(0)))
# Empty data.table (0 rows and 1 cols): a
print(data.table())
# Null data.table (0 rows and 0 cols)

Agree! I'm not proposing any changes to those current messages.

[KyleHaynes]

ITMT:

# install.packages("devtools")
devtools::install_github("KyleHaynes/data.table", build_vignettes=FALSE, ref="print_col_nums")
require(data.table)
dt = data.table(iris)
dt
# Number of columns: 5
#      Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
#             <num>       <num>        <num>       <num>    <fctr>
#   1:          5.1         3.5          1.4         0.2    setosa
#   2:          4.9         3.0          1.4         0.2    setosa
#   3:          4.7         3.2          1.3         0.2    setosa
#   4:          4.6         3.1          1.5         0.2    setosa
#   5:          5.0         3.6          1.4         0.2    setosa
#  ---
# 146:          6.7         3.0          5.2         2.3 virginica
# 147:          6.3         2.5          5.0         1.9 virginica
# 148:          6.5         3.0          5.2         2.0 virginica
# 149:          6.2         3.4          5.4         2.3 virginica
# 150:          5.9         3.0          5.1         1.8 virginica

setkey(dt)
setindex(dt,Species,Sepal.Width)
dt
# Key: <Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species>
# Index: <Species__Sepal.Width>
# Number of columns: 5
#      Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
#             <num>       <num>        <num>       <num>    <fctr>
#   1:          4.3         3.0          1.1         0.1    setosa
#   2:          4.4         2.9          1.4         0.2    setosa
#   3:          4.4         3.0          1.3         0.2    setosa
#   4:          4.4         3.2          1.3         0.2    setosa
#   5:          4.5         2.3          1.3         0.3    setosa
#  ---
# 146:          7.7         2.6          6.9         2.3 virginica
# 147:          7.7         2.8          6.7         2.0 virginica
# 148:          7.7         3.0          6.1         2.3 virginica
# 149:          7.7         3.8          6.7         2.2 virginica
# 150:          7.9         3.8          6.4         2.0 virginica

data.table()
# Null data.table (0 rows and 0 cols)
data.table(as.character())
# Empty data.table (0 rows and 1 cols): V1
options(width=10)
print(dt, trunc.cols=TRUE)
# Key: <Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species>
# Index: <Species__Sepal.Width>
# Number of columns: 5, of which 5 are not shown: [Sepal.Length <num>, Sepal.Width <num>, Petal.Length <num>, Petal.Width <num>, Species <fctr>]

options(width=200)
options("datatable.show.ncols" = FALSE)
# Key: <Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species>
# Index: <Species__Sepal.Width>
#      Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
#             <num>       <num>        <num>       <num>    <fctr>
#   1:          4.3         3.0          1.1         0.1    setosa
#   2:          4.4         2.9          1.4         0.2    setosa
#   3:          4.4         3.0          1.3         0.2    setosa
#   4:          4.4         3.2          1.3         0.2    setosa
#   5:          4.5         2.3          1.3         0.3    setosa
#  ---
# 146:          7.7         2.6          6.9         2.3 virginica
# 147:          7.7         2.8          6.7         2.0 virginica
# 148:          7.7         3.0          6.1         2.3 virginica
# 149:          7.7         3.8          6.7         2.2 virginica
# 150:          7.9         3.8          6.4         2.0 virginica