Renaming multiple columns in R data.table

Question

This is related to this question from Henrik Assign multiple columns using := in data.table, by group

But what if I want to create a new data.table with given column names instead of assigning new columns to an existing one?

f <- function(x){list(head(x,2),tail(x,2))}
dt <- data.table(group=sample(c('a','b'),10,replace = TRUE),val=1:10)

> dt
    group val
 1:     b   1
 2:     b   2
 3:     a   3
 4:     b   4
 5:     a   5
 6:     b   6
 7:     a   7
 8:     a   8
 9:     b   9
10:     b  10

I want to get a new data.table with predefined column names by calling the function f:

dt[,c('head','tail')=f(val),by=group]

I wish to get this:

   group head tail
1:     a    1    8
2:     a    3   10
3:     b    2    6
4:     b    5    9

But it gives me an error. What I can do is create the table then change the column names, but that seems cumbersome:

> dt2 <- dt[,f(val),by=group]

> dt2
   group V1 V2
1:     a  1  8
2:     a  3 10
3:     b  2  6
4:     b  5  9

> colnames(dt2)[-1] <- c('head','tail')

> dt2
   group head tail
1:     a    1    8
2:     a    3   10
3:     b    2    6
4:     b    5    9

Is it something I can do with one call?

Why don't you just define f with the names in it? f <- function(x){list(head = head(x,2), tail = tail(x,2))}? Or, I guess dt[, setNames(f(val), c("head", "tail")), group] should work. — A5C1D2H2I1M1N2O1R2T1
– A5C1D2H2I1M1N2O1R2T1, Commented Aug 9, 2017 at 17:21
@A5C1D2H2I1M1N2O1R2T1 Downside to the second way is that data.table complains (albeit only quietly, when verbose = TRUE). I guess the real issue is that when using data.table programmatically (at least in ways as general as the OP wants), it's best to construct expressions or full [.data.table calls to evaluate... — Frank
– Frank, Commented Aug 9, 2017 at 17:33
@A5C1D2H2I1M1N2O1R2T1 As Frank said, the names in the data table will be determined programmatically so I cannot hard-code it in the function. But your second way does work. Thanks. — Bill
– Bill, Commented Aug 9, 2017 at 19:32
@Frank by 'it's best to construct expressions', do you mean to have an inline function instead of calling an existing one? — Bill
– Bill, Commented Aug 9, 2017 at 19:33
I mean something like github.com/franknarf1/r-tutorial/issues/11 It gets pretty messy, so I rarely build tables + functions where I'll need to determine input or output column names dynamically. Here's another example: stackoverflow.com/questions/37007282/… and more generally google.com/… — Frank
– Frank, Commented Aug 9, 2017 at 19:42

Nathan Werth · Accepted Answer · 2017-08-09 18:41:59Z

0

From running your code as-is, this is the error I get:

dt[,c('head','tail')=f(val),by=group]
# Error: unexpected '=' in "dt2[,c('head','tail')="

The problem is using = instead of := for assignment.

On to your problem of wanting a new data.table:

dt2 <- dt[, setNames(f(val), c('head', 'tail')), by = group]

answered Aug 9, 2017 at 18:41

Nathan Werth

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1 Comment

Bill Over a year ago

Thanks, this works. Although @A5C1D2H2I1M1N2O1R2T1 gave this solution earlier in the comment. Still accepting your answer.

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