Maintain attributes of data frame columns after merge
If you don't mind using dplyr, this one seems to work.
Your data:
attr(mtcars$mpg, "units") <- "miles.per.gallon"
new.df <- data.frame(gear=3:5, my.opinion=c("not enough", "just right", "too many"))
> attr(mtcars$mpg, "units")
[1] "miles.per.gallon"
Function inner_join from dplyr.
inner.df<-dplyr::inner_join(new.df, mtcars,"gear")
The resulting data frame is as follows:
> inner.df
gear my.opinion mpg cyl disp hp drat wt qsec vs am carb
1 3 not enough 21.4 6 258.0 110 3.08 3.215 19.44 1 0 1
2 3 not enough 18.7 8 360.0 175 3.15 3.440 17.02 0 0 2
3 3 not enough 18.1 6 225.0 105 2.76 3.460 20.22 1 0 1
4 3 not enough 14.3 8 360.0 245 3.21 3.570 15.84 0 0 4
5 3 not enough 16.4 8 275.8 180 3.07 4.070 17.40 0 0 3
6 3 not enough 17.3 8 275.8 180 3.07 3.730 17.60 0 0 3
7 3 not enough 15.2 8 275.8 180 3.07 3.780 18.00 0 0 3
8 3 not enough 10.4 8 472.0 205 2.93 5.250 17.98 0 0 4
9 3 not enough 10.4 8 460.0 215 3.00 5.424 17.82 0 0 4
10 3 not enough 14.7 8 440.0 230 3.23 5.345 17.42 0 0 4
11 3 not enough 21.5 4 120.1 97 3.70 2.465 20.01 1 0 1
12 3 not enough 15.5 8 318.0 150 2.76 3.520 16.87 0 0 2
13 3 not enough 15.2 8 304.0 150 3.15 3.435 17.30 0 0 2
14 3 not enough 13.3 8 350.0 245 3.73 3.840 15.41 0 0 4
15 3 not enough 19.2 8 400.0 175 3.08 3.845 17.05 0 0 2
16 4 just right 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4
17 4 just right 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4
18 4 just right 22.8 4 108.0 93 3.85 2.320 18.61 1 1 1
19 4 just right 24.4 4 146.7 62 3.69 3.190 20.00 1 0 2
20 4 just right 22.8 4 140.8 95 3.92 3.150 22.90 1 0 2
21 4 just right 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4
22 4 just right 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4
23 4 just right 32.4 4 78.7 66 4.08 2.200 19.47 1 1 1
24 4 just right 30.4 4 75.7 52 4.93 1.615 18.52 1 1 2
25 4 just right 33.9 4 71.1 65 4.22 1.835 19.90 1 1 1
26 4 just right 27.3 4 79.0 66 4.08 1.935 18.90 1 1 1
27 4 just right 21.4 4 121.0 109 4.11 2.780 18.60 1 1 2
28 5 too many 26.0 4 120.3 91 4.43 2.140 16.70 0 1 2
29 5 too many 30.4 4 95.1 113 3.77 1.513 16.90 1 1 2
30 5 too many 15.8 8 351.0 264 4.22 3.170 14.50 0 1 4
31 5 too many 19.7 6 145.0 175 3.62 2.770 15.50 0 1 6
32 5 too many 15.0 8 301.0 335 3.54 3.570 14.60 0 1 8
Where the attribute is mantained:
> attr(inner.df$mpg, "units")
[1] "miles.per.gallon"
There's also data.table
library(data.table)
dt1 = as.data.table(mtcars)
dt2 = as.data.table(new.df)
inner.dt <- dt1[dt2, on = "gear"]
attr(inner.dt$mpg, "units")
...
> attr(inner.dt$mpg, "units")
[1] "miles.per.gallon"
but...
library(microbenchmark)
microbenchmark(dplyr::inner_join(new.df, mtcars,"gear"),
dt1[dt2, on = "gear"])
...
> microbenchmark(dplyr::inner_join(new.df, mtcars,"gear"),
+ dt1[dt2, on = "gear"])
Unit: microseconds
expr min lq mean median uq max neval
dplyr 544.877 568.5840 625.6442 606.319 658.870 1005.197 100
data.table 860.186 892.1915 961.2788 938.618 979.711 1510.166 100