[datatable-help] data.table BUG : data.table assignment

[Ken Williams]

One thing that would be super-awesome is a lazy copy-on-write mechanism.  By which I mean table2 would initially point to the same memory location as table1, but as soon as it's modified, the portions being modified would be copied to a new location.

It's pretty rare use case to make a copy of a data structure without intending to modify it.  The only instance I can really think of is passing arguments to a function, which IIUC is already copy-on-write in R?

Also, I do think Natus' example is a little different from your example on SO.  In his, a new column is being added, but in yours, an existing column is being modified.

Is there a doc reference showing which circumstances make `<-` do a copy-by-reference, and which do a deep copy?  For example, if I do `table2 <- table1[x>1, list(id)]` , it seems to do a deep copy:

> table1<-data.table(id=c(1,2,3),x=c(1,2,3))
> table2 <- table1[x>1, list(id)]
> table2[, id := 3:4]
> table1
   id x
1:  1 1
2:  2 2
3:  3 3

Sorry if this has already been hashed out a million times, I'm pretty new to data.table.

-Ken

From: datatable-help-bounces at lists.r-forge.r-project.org [mailto:datatable-help-bounces at lists.r-forge.r-project.org] On Behalf Of Christoph Jäckel
Sent: Thursday, October 04, 2012 7:07 AM
To: natus
Cc: datatable-help at lists.r-forge.r-project.org
Subject: Re: [datatable-help] data.table BUG : data.table assignment

This is actually intended behaviour and I had the problem once as well. Here is my question and the solution to it:

http://stackoverflow.com/questions/8030452/pass-by-reference-the-operator-in-the-data-table-package

In a nutshell: Use copy() if you don't want table2 to have y as well.

I hope this helps,

Christoph
On Thu, Oct 4, 2012 at 1:56 PM, natus <niparisco at gmail.com<mailto:niparisco at gmail.com>> wrote:
Hello,

see this example :

require(data.table)

table1<-data.table(id=c(1,2,3),x=c(1,2,3))
table2<-table1
table1[,y:=sum(x)]
table1
table2

The problem ? Both of table1 and table2 have the variable 'y' BUT only
table1 should.

Thx



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