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<div class="moz-cite-prefix">On 11/01/14 14:31, Arunkumar Srinivasan
wrote:<br>
</div>
<blockquote
cite="mid:etPan.52d155d9.71f32454.175@Arunkumars-MacBook-Pro.local"
type="cite">
<p>Thanks for reporting. That's expected behaviour. Use an
explicit <code>copy</code>.</p>
<p>In short, when you do: <code>DT1 <- DT2</code>, there's <strong><code>no
copy</code></strong> being made. They still reference/point
to the same location (try doing <code>tracemem(DT1)</code> and
<code>tracemem(DT2)</code>).</p>
</blockquote>
Just to be clear that's no different to base. DF1 <- DF2 makes
no copy in base either. In fact x <- y never makes a copy in R
regardless of what x and y are.<br>
<br>
The phrase "copy-on-write" is terribly named because it might imply
DF1 <- DF2 copies. I think the term should be
"copy-on-subassign" because that's really what R does. Only at the
point of changing a sub-element of an object, does <- copy (if
another symbol is pointing to that same object). It is switching
from <- to set and := that does things by reference. Not
switching from data.frame to data.table. Subassigning to a
data.table using <- will still copy the entire data.table, just
like base. Only set* and := can modify by reference. In
fact, set* can be used on data.frame too, and other objects; e.g.
setattr is often useful on non-data.table's and therefore copy() is
useful on non-data.table's too. Hope that clarifies.<br>
<blockquote
cite="mid:etPan.52d155d9.71f32454.175@Arunkumars-MacBook-Pro.local"
type="cite">
<p>So when you change the names of one <code>DT</code> by
reference, the other one will get changed as well - they're both
pointing to the same location.</p>
<p>To overcome this, when you want to duplicate a <code>DT</code>,
explicitly use <code>copy</code>. That is, <code>DT1 <-
copy(DT2)</code>. Now if you <code>setnames(DT1, c("x", "y"))</code>,
then <code>DT2</code> names won't get changed.</p>
<p>I think there's a FR somewhere on documenting this… Thanks
again for reporting (with nice example).</p>
</blockquote>
<br>
<br>
<blockquote
cite="mid:etPan.52d155d9.71f32454.175@Arunkumars-MacBook-Pro.local"
type="cite">
<p>
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<div id="bloop_sign_1389450647203549952" class="bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">Arun</div>
</div>
<div style="color:gray">
<hr style="style:linear">From: <span style="color:black">Holger
Kirsten</span> <a moz-do-not-send="true"
href="mailto:hkirsten@imise.uni-leipzig.de">Holger Kirsten</a><br>
Reply: <span style="color:black">Holger Kirsten</span> <a
moz-do-not-send="true"
href="mailto:hkirsten@imise.uni-leipzig.de">hkirsten@imise.uni-leipzig.de</a><br>
Date: <span style="color:black">January 11, 2014 at 3:19:02 PM</span><br>
To: <span style="color:black"><a class="moz-txt-link-abbreviated" href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a></span>
<a moz-do-not-send="true"
href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a><br>
Subject: <span style="color:black"> [datatable-help] setnames
changes names of other data.table <br>
</span></div>
<blockquote type="cite" class="clean_bq"><span>
<div>
<div>
<title></title>
In a debugging session, I found that setnames changed the
names of
an identical data.table although having a different
name><br>
<br>
<font size="2">> ############### using setnames()<br>
> require(data.table)<br>
> mytab = data.table(a = letters[1:4], b = 1:4 )<br>
> str(mytab)<br>
Classes ‘data.table’ and 'data.frame': 4 obs.
of 2 variables:<br>
$ a: chr "a" "b" "c" "d"<br>
$ b: int 1 2 3 4<br>
- attr(*, ".internal.selfref")=<externalptr><br>
> mytab<br>
a b<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
><br>
> othertab = mytab<br>
> othertab<br>
a b<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
> setnames(othertab, c("a", "b"), c("aa","bb"))<br>
> othertab<br>
aa bb<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
> mytab ## names have unexpectedly changed too<br>
aa bb<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
><br>
> ############### using names()<br>
> mytab = data.table(a = letters[1:4], b = 1:4 )<br>
> str(mytab)<br>
Classes ‘data.table’ and 'data.frame': 4 obs.
of 2 variables:<br>
$ a: chr "a" "b" "c" "d"<br>
$ b: int 1 2 3 4<br>
- attr(*, ".internal.selfref")=<externalptr><br>
> mytab<br>
a b<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
><br>
> othertab = mytab<br>
> othertab<br>
a b<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
> names(othertab) = c("aa","bb")<br>
Warning message:<br>
In `names<-.data.table`(`*tmp*`, value = c("aa",
"bb")) :<br>
The names(x)<-value syntax copies the whole table.
This
is due to <- in R itself. Please change to
setnames(x,old,new)
which does not copy and is faster. See help('setnames').
You can
safely ignore this warning if it is inconvenient to
change right
now. Setting options(warn=2) turns this warning into an
error, so
you can then use traceback() to find and change your
names<-
calls.<br>
> othertab<br>
aa bb<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
> mytab ## names unchanged as expected<br>
a b<br>
1: a 1<br>
2: b 2<br>
3: c 3<br>
4: d 4<br>
><br>
> sessionInfo()<br>
R version 3.0.1 (2013-05-16)<br>
Platform: x86_64-w64-mingw32/x64 (64-bit)<br>
<br>
locale:<br>
[1] LC_COLLATE=German_Germany.1252
LC_CTYPE=German_Germany.1252
LC_MONETARY=German_Germany.1252
LC_NUMERIC=C
LC_TIME=German_Germany.1252 <br>
<br>
attached base packages:<br>
[1] stats graphics grDevices
utils datasets methods
base <br>
<br>
other attached packages:<br>
[1] data.table_1.8.10<br>
<br>
loaded via a namespace (and not attached):<br>
[1] tools_3.0.1</font><br>
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