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<h2>Just a quick sum up.</h2> <code><-</code> with <code>data.table</code> is just like base; i.e., no copy is taken until a subassign is done afterwards with <code><-</code> (such as changing the column names or changing an element such as <code>DT[i,j]<-v</code>). Then it takes a copy of the whole object just like base. That's known as copy-on-write. Would be better known as copy-on-subassign, I think! It DOES NOT copy when you use the special <code>:=</code> operator, or the <code>set*</code> functions provided by <code>data.table</code>. If you have large data you probably want to use them instead. <code>:=</code> and <code>set*</code> will NOT COPY the <code>data.table</code>, EVEN WITHIN FUNCTIONS. Given this example data : <pre><code>DT <- data.table(a=c(1,2), b=c(11,12)) </code></pre> The following just "binds" another name <code>DT2</code> to the same data object bound currently bound to the name <code>DT</code> : <pre><code>DT2 <- DT </code></pre> This never copies, and never copies in base either. It just marks the data object so that R knows that two different names (<code>DT2</code> and <code>DT</code>) point to the same object. And so R will need to copy the object if either are subassigned to afterwards. That's perfect for <code>data.table</code>, too. The <code>:=</code> isn't for doing that. So the following is a deliberate error as <code>:=</code> isn't for just binding object names : <pre><code>DT2 := DT # not what := is for, not defined, gives a nice error </code></pre> <code>:=</code> is for subassigning by reference. But you don't use it like you would in base : <pre><code>DT[3,"foo"] := newvalue # not like this </code></pre> you use it like this : <pre><code>DT[3,foo:=newvalue] # like this </code></pre> That changed <code>DT</code> by reference. Say you add a new column <code>new</code> by reference to the data object, there is no need to do this : <pre><code>DT <- DT[,new:=1L] </code></pre> because the RHS already changed <code>DT</code> by reference. The extra <code>DT <-</code> is to misunderstand what <code>:=</code> does. You can write it there, but it's superfluous. <code>DT</code> is changed by reference, by <code>:=</code>, EVEN WITHIN FUNCTIONS : <pre><code>f <- function(X){ X[,new2:=2L] return("something else") } f(DT) # will change DT DT2 <- DT f(DT) # will change both DT and DT2 (they're the same data object) </code></pre> <code>data.table</code> is for large datasets, remember. If you have a 20GB <code>data.table</code> in memory then you need a way to do this. It's a very deliberate design decision of <code>data.table</code>. Copies can be made, of course. You just need to tell data.table that you're sure you want to copy your 20GB dataset, by using the <code>copy()</code> function : <pre><code>DT3 <- copy(DT) # rather than DT3 <- DT DT3[,new3:=3L] # now, this just changes DT3 because it's a copy, not DT too. </code></pre> To avoid copies, don't use base type assignation or update : <pre><code>DT$new4 <- 1L # will make a copy so use := attr(DT,"sorted") <- "a" # will make a copy use setattr() </code></pre> If you want to be sure that you are updating by reference use <code>.Internal(inspect(x))</code> and look at the memory address values of the constituents (see Matthew Dowle's answer). Writing <code>:=</code> in <code>j</code> like that allows you subassign by reference by group. You can add a new column by reference by group. So that's why <code>:=</code> is done that way inside <code>[...]</code> : <pre><code>DT[, newcol:=mean(x), by=group] </code></pre>
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