CODEWITHSUNDEEP
rexpressionevaldata.table
Alex
Alex

Reputation: 19793

expression syntax for data.table := in R

I am having some trouble getting an eval within data.table in R to work with an expression. Here is some code:

dtb = data.table(a=1:100, b=100:1, id=1:10)
dtb[,`:=`(c=a+b, d=a/b),by=id] #this works fine
expr = expression({`:=`(c=a+b, d=a/b)}) #try to couch everything in an expression
dtb[,eval(expr),by=id] #this does not work
Error in `:=`(c = a + b, d = a/b) : 
   unused argument(s) (c = a + b, d = a/b)

expr = expression(`:=`(c=a+b, d=a/b)) #this works fine
dtb[,eval(expr),by=id]

Why does including {} break this?

Upvotes: 4

Views: 655

Answers (2)

Arun
Arun

Reputation: 118779

Issue #376 to trap the { around := has now been implemented in v1.8.11. From NEWS:

o FR #2496 is now implemented to trap and remove the { around := in j to obtain desired result. Now, DT[,{`:=`(...)}] and DT[, {`:=`(...)}, by=(...)] both work as intended but with a warning. Thanks to Alex for reporting on SO: expression syntax for data.table := in R

Upvotes: 5

user1935457
user1935457

Reputation: 721

See the definition of :=:

function (LHS, RHS) 
stop(":= is defined for use in j only, and (currently) only once; i.e., DT[i,col:=1L] and DT[,newcol:=sum(colB),by=colA] are ok, but not DT[i,col]:=1L, not DT[i]$col:=1L and not DT[,{newcol1:=1L;newcol2:=2L}]. Please see help(\":=\"). Check is.data.table(DT) is TRUE.")

The assignment of a column doesn't happen within a call of :=--the function itself doesn't do anything besides produce an error. The assignment happens when [.data.table detectsj is an expression of the form `:=`(...) and then sets everything up for a call to the C code. When you enclose expr in brackets, you're making the first part of the expression { instead of :=, which passes by the above detection and eventually results in an evaluation of := with arguments c and d.

I guess that leads to the question, why do you need to enclose it in { }?

Upvotes: 7

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