R: removing rows based on row value in a column of a data frame

Question

I have a dataframe a with row names. The row names are unique string names, something like this:

       order..new..i...2.ncol.new..i.....
J.TYMO                                620
J.TTMO                               2851
J.NTT                                1972
J.ABOT                                565
J.NNDO                               1025
J.SFTB                               2509

when I execute a one-liner code as.data.frame(a[a[1] <= 2000]) to remove rows less than or equal to a value (as in 2000) my result does not include the row names.

I would EXPECT my code to do something like this:

J.TYMO                                620
J.NTT                                1972
J.ABOT                                565
J.NNDO                               1025

Instead it does this:

1                                620
2                                1972
3                                565
4                                1025

Any ideas? Thanks.

Answer 1

EDIT I reproduced your error, you need to add the drop = FALSE option in your subsetting to get a data.frame as result and not a vector :

df_a <- structure(list(order..new..i...2.ncol.new..i..... = c(620L, 2851L, 1972L, 565L, 1025L, 2509L)), row.names = c("J.TYMO", "J.TTMO", "J.NTT", "J.ABOT", "J.NNDO", "J.SFTB"), class = "data.frame")
str(df_a)
#> 'data.frame':    6 obs. of  1 variable:
#>  $ order..new..i...2.ncol.new..i.....: int  620 2851 1972 565 1025 2509
names(df_a) <- "V1"

df_a[df_a[[1]] <= 1000 , , drop = FALSE]
#>         V1
#> J.TYMO 620
#> J.ABOT 565

OLD ANSWER

The best with-row-names-dataset I though of was the mtcars dataset. Building from that I found that adding a comma in your call solves the problem :

dfr <- head(mtcars)
dfr
#>                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
#> Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
#> Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
#> Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
#> Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
#> Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
#> Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

as.data.frame(dfr[dfr[1]<20 , ])
#>                    mpg cyl disp  hp drat   wt  qsec vs am gear carb
#> Hornet Sportabout 18.7   8  360 175 3.15 3.44 17.02  0  0    3    2
#> Valiant           18.1   6  225 105 2.76 3.46 20.22  1  0    3    1

Thus with your peculiar a object, if it is a data.frame the answer should be :

as.data.frame(a[a[1] <= 333 , ])

Answer 2

You're extracting one variable as a vector and then converting that vector into a data frame. You should just use a[a[1] <= 333,] or subset(a, blah <= 333) to subset your dataframe. Example:

a <- `row.names<-`(data.frame(blah = 332:335, bleh = "bleh"),
                   c("row1", "row2", "row3", "row4")
                   )

#### OUTPUT ####

     blah bleh
row1  332 bleh
row2  333 bleh
row3  334 bleh
row4  335 bleh

Now subset with a[a[1] <= 333,] or subset(a, blah <= 333) :

     blah bleh
row1  332 bleh
row2  333 bleh

If you're trying to remove rows less than or equal to a value (as in 333) then you should instead use a[a[1] > 333,] or subset(a, blah > 333) :

     blah bleh
row3  334 bleh
row4  335 bleh