r - convert data.frame to data.table column missing -


had case this. tried convert "mtcars" class data.frame data.table.

"mtcars" data:

> mtcars                      mpg cyl  disp  hp drat    wt  qsec vs gear carb mazda rx4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4 mazda rx4 wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4 datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1

original class "data.frame".

> str(mtcars) 'data.frame':   32 obs. of  11 variables:  $ mpg : num  21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...  $ cyl : num  6 6 4 6 8 6 8 4 4 6 ...  $ disp: num  160 160 108 258 360 ...  $ hp  : num  110 110 93 110 175 105 245 62 95 123 ...  $ drat: num  3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...  $ wt  : num  2.62 2.88 2.32 3.21 3.44 ...  $ qsec: num  16.5 17 18.6 19.4 17 ...  $ vs  : num  0 0 1 1 0 1 0 1 1 1 ...  $  : num  1 1 1 0 0 0 0 0 0 0 ...  $ gear: num  4 4 4 3 3 3 3 4 4 4 ...  $ carb: num  4 4 1 1 2 1 4 2 2 4 ...

convert data.table. found column car brands gone. why? how retain column of brands?

> mtcars2 <- data.table(mtcars) > mtcars2      mpg cyl  disp  hp drat    wt  qsec vs gear carb  1: 21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4  2: 21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4  3: 22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1

would have data.table final data format below -- having new column name "brands" first column of brands. how code add column "brands" original "mtcars" dataset?

brands              mpg cyl  disp  hp drat    wt  qsec vs gear carb mazda rx4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4 mazda rx4 wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4 datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1

it's rownames missing, not 1 of columns.

if want brands column, manual approach is:

data.table(brands = rownames(mtcars), mtcars)

alternately:

data.table(mtcars, keep.rownames = true)

however, not make resulting data.table have old rownames, makes column them, called "rn". in documentation, ?data.table.

alternately, modify table in place, df = mtcars:

setdt(df, keep.rownames = "brands")

minor point: cannot setdt(mtcars, ...), since mtcars built-in table.


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