I am converting data.frame-based code to data.table, for performance reasons. Is there a way to refer to columns using expressions in the "i"-part? Here is an example: a <- data.table(x=rep(LETTERS[1:10],each=2), y=1:20) v <- "x" For the j-part, I can access the column whose name is stored in v as a[,..v] However, for the i-part I did not find a good way to achieve the same. For a simple case, the following works xx <- "B" a[xx, on=v] However, I can't see how this is easily expanded to more complex logical expressions. Pascal
data.table: reference column in "i"-part by column name in string object
3 messages · Rui Barradas, Pascal A. Niklaus
Hello, I don't understand the question. Like this? xx <- "B" yy <- "D" a[xx, on = v] a[c(xx, yy), on = v] Note that .(xx, yy) doesn't work. It outputs something else. Could you give an example of more complicated expressions you have doubts with? Hope this helps, Rui Barradas ?s 14:10 de 21/02/2019, Pascal A. Niklaus escreveu:
I am converting data.frame-based code to data.table, for performance reasons. Is there a way to refer to columns using expressions in the "i"-part? Here is an example: a <- data.table(x=rep(LETTERS[1:10],each=2), y=1:20) v <- "x" For the j-part, I can access the column whose name is stored in v as a[,..v] However, for the i-part I did not find a good way to achieve the same. For a simple case, the following works xx <- "B" a[xx, on=v] However, I can't see how this is easily expanded to more complex logical expressions. Pascal
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On 21.02.19 20:43, Rui Barradas wrote:
Hello, I don't understand the question. Like this? xx <- "B" yy <- "D" a[xx, on = v] a[c(xx, yy), on = v] Note that .(xx, yy) doesn't work. It outputs something else. Could you give an example of more complicated expressions you have doubts with?
Let's assume I'd like to create a subset like this, in data.frame syntax: d[ (d[[v]] == a | d$b == "c") & d$c>2, ] Here I have a column referenced by its name (stored in v), and compared to a value stored in a, and the whole thing is part of a more complex logical expression. I could write it in this conventional way, but I wondered whether there is a better way that would leverage some of data.tables syntax. Pascal