How to execute logical operators by indexing in data.table in R?

Question

I am trying to figure out how can I execute logical operators when I use indexing in the data.table package in R ?

The following is an example. I am making a datatable with the name dt . and then make var2 as the key in my datatable :

 > dt = data.table(var1 = rep(LETTERS[1:5],2), var2 = seq(1,20, 2), var3 = ceiling(rnorm(10, 3, 2))) > dt var1 var2 var3 1: A 1 5 2: B 3 3 3: C 5 0 4: D 7 6 5: E 9 3 6: A 11 4 7: B 13 2 8: C 15 1 9: D 17 3 10: E 19 7 > setkey(dt, var2) 

So now I want to identify all the values ​​in my already defined key (var2) that are less than 10 ( <10) . Performing the following attempts will give me errors .

 > dt[ < 10] Error: unexpected '<' in "dt[ <" > dt[ .< 10] Error in eval(expr, envir, enclos) : object '.' not found > dt[ .(< 10)] 

my expectation:

  var1 var2 var3 1: A 11 4 2: B 13 2 3: C 15 1 4: D 17 3 5: E 19 7 

By the way, I know that just by making dt[var2 <10] , I will get the result. BUT, please think that I want to get the concept of indexing in data.table and understand and know how to do this without calling key(var2) in each of my commands!

Any help with explanation is greatly appreciated.

Answer 1

From ?setkey , key(dt) get the key columns in the character vector. Assuming there is one key column in your table, you can get what you want:

 dt[dt[[key(dt)]] < 10] 

Thanks to David Arenburg, you can also use get() :

 dt[get(key(dt)) < 10] 

It is a little shorter and probably the way to go.

Another way I can do this is much worse:

 dt[eval(parse(text = paste(key(dt), "< 10")))] 

Answer 2

from the documentation https://www.rdocumentation.org/packages/data.table/versions/1.10.4/topics/setkey

Here is the key to the solution, if possible

 > library(data.table) data.table 1.10.4 The fastest way to learn (by data.table authors): https://www.datacamp.com/courses/data-analysis-the-data-table-way Documentation: ?data.table, example(data.table) and browseVignettes("data.table") Release notes, videos and slides: http://r-datatable.com > data(mtcars) > head(mtcars) 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 > mtcars=data.table(mtcars) > setkey(mtcars,mpg) > key(mtcars) [1] "mpg" > mtcars[mpg<15,,] mpg cyl disp hp drat wt qsec vs am gear carb 1: 10.4 8 472 205 2.93 5.250 17.98 0 0 3 4 2: 10.4 8 460 215 3.00 5.424 17.82 0 0 3 4 3: 13.3 8 350 245 3.73 3.840 15.41 0 0 3 4 4: 14.3 8 360 245 3.21 3.570 15.84 0 0 3 4 5: 14.7 8 440 230 3.23 5.345 17.42 0 0 3 4 > mtcars["mpg"<15,,] Empty data.table (0 rows) of 11 cols: mpg,cyl,disp,hp,drat,wt... 

The problem is that the key (DT) gives "var2" and the subset in datatable requires var2 (without quotes) - we get this with get

Now use Remove quotes from character vector in R

This is the easiest way.

 #get(key(mtcars)) > mtcars[get(key(mtcars))<15] mpg cyl disp hp drat wt qsec vs am gear carb 1: 10.4 8 472 205 2.93 5.250 17.98 0 0 3 4 2: 10.4 8 460 215 3.00 5.424 17.82 0 0 3 4 3: 13.3 8 350 245 3.73 3.840 15.41 0 0 3 4 4: 14.3 8 360 245 3.21 3.570 15.84 0 0 3 4 5: 14.7 8 440 230 3.23 5.345 17.42 0 0 3 4 

For your datatable will be

 DT[get(key(DT))<10] 

which is the same as @DavidArenburg's simple and elegant answer