<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><div id="bloop_customfont" style="margin: 0px;">My 2 cents here.</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">There are several reasons why I don’t think, IMHO, allowing multiple columns with the same name is a good idea:</div><div id="bloop_customfont" style="margin: 0px;"> </div><div id="bloop_customfont" style="margin: 0px;">- It will force the code to use column numbers to access all the data in a predictable fashion (since depending on your code you might now know which of the two columns with the same name will be the first), so we’ll lose all the delicious syntactic sugar painstakingly added to data.table.</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">- For people learning data.table and having data.frame or even the concept of a relational table as a reference, this is a definite WTF and will cause confusion and complicate troubleshooting. I speak from experience on this matter. :)</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">Even though there might be some situations where this might be a plus, I imagine they are few and far between and could be worked around. I could be wrong, it’s been know to happen :) - but I have never seen and can’t even imagine a situation where multiple columns with the same name would be essential. So in the balance I consider keeping this behavior as a bad trade-off for most users.</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">Having said that, this is a design decision and it's up to the data.table demigods to decide. :)</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">BTW, is there any part of the data.table documentation that covers this? If you choose to maintain this property, I strongly suggest it be documented somewhere that most beginners would read.</div><div id="bloop_customfont" style="margin: 0px;"><br></div><div id="bloop_customfont" style="margin: 0px;">In my personal example, I ran into this problem after a rather long troubleshooting of a very esoteric problem that was happening in my code. I was renaming a column to a name that already existed, and this broke things in a completely different part of my code. If ‘setnames()’ had at least warned me that a duplicate column name was created, I would have been able to detect the source cause much faster.</div><div id="bloop_customfont" style="margin: 0px;"><br></div></div> <div id="bloop_sign_1383394197815133184"><span style="font-family:helvetica,arial;font-size:13px"></span><div style="font-family: Helvetica; line-height: normal; ">-- </div><div style="font-family: Helvetica; line-height: normal; "><span style="font-family: arial; font-size: small; ">Alexandre Sieira</span><br style="font-family: arial; font-size: small; "><span style="font-family: arial; font-size: small; ">CISA, CISSP, ISO 27001 Lead Auditor</span><br style="font-family: arial; font-size: small; "><br style="font-family: arial; font-size: small; "><span style="font-family: arial; font-size: small; ">"The truth is rarely pure and never simple."</span><br style="font-family: arial; font-size: small; "><span style="font-family: arial; font-size: small; ">Oscar Wilde, The Importance of Being Earnest, 1895, Act I</span></div></div> <br><p style="color:#A0A0A8;">On 1 de novembro de 2013 at 21:10:45, Arunkumar Srinivasan (<a href="mailto://aragorn168b@gmail.com">aragorn168b@gmail.com</a>) wrote:</p> <blockquote type="cite" class="clean_bq"><span><div><div>
<title></title>
<div>Hm, I've not encountered that use myself, can't comment there.
Probably then it should be allowed everywhere except where deciding
which column could be an issue? Ex:
subsetting/aggregating/grouping/by-without-by etc.. should result
in error (if one has the time, one could do this by checking if the
duplicate column is in use actually or not and then issue an
error/warning). </div>
<div><br></div>
<div>At the moment, I'm not convinced that it's worth that much
trouble to help data presentation.</div>
<div>
<div><br></div>
<div>Arun</div>
<div><br></div>
</div>
<p style="color: #A0A0A8;">On Saturday, November 2, 2013 at 12:05
AM, Eduard Antonyan wrote:</p>
<blockquote type="cite" style="border-left-style:solid;border-width:1px;margin-left:0px;padding-left:10px;">
<div>
<div>
<div dir="ltr"><span>Because it's very useful for e.g. data
presentation purposes.</span></div>
<div><span><br>
<br></span>
<div><span>On Fri, Nov 1, 2013 at 6:02 PM, Arunkumar Srinivasan
<span dir="ltr"><<a href="mailto:aragorn168b@gmail.com" target="_blank">aragorn168b@gmail.com</a>></span> wrote:<br></span>
<blockquote type="cite">
<div>
<div>Yes, it chooses the first. But we won't be able to perform any
operation as intended. So why allow duplicate names (ex: in
`setnames` as Alexandre asks)?</div>
<div>
<div><br></div>
<div>Arun</div>
<div><br></div>
</div>
<div>
<div>
<p style="color:#a0a0a8">On Friday, November 1, 2013 at 11:57 PM,
Eduard Antonyan wrote:</p>
<blockquote type="cite">
<div>
<div>
<div>
<div dir="ltr"><span>I think currently it chooses the first "x",
but it's definitely a good idea to add a warning
there.</span></div>
<div><span><br>
<br></span>
<div><span>On Fri, Nov 1, 2013 at 5:51 PM, Arunkumar Srinivasan
<span dir="ltr"><<a href="mailto:aragorn168b@gmail.com" target="_blank">aragorn168b@gmail.com</a>></span> wrote:<br></span>
<blockquote type="cite">
<div>
<div>Ricardo added a bug report here on this topic: <a href="https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5008&group_id=240&atid=975" target="_blank">https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5008&group_id=240&atid=975</a></div>
<div>
<div>But I don't think having duplicate names is an
easy-to-implement concept. For ex:</div>
<div><br></div>
<div>dt <- data.table(x=1:3, x=4:6, y=c(1,1,2))</div>
<div>dt[, print(.SD), by=y]</div>
<div>
<div> x</div>
<div>1: 1</div>
<div>2: 2</div>
<div> x</div>
<div>1: 3</div>
</div>
<div><br></div>
<div>.SD loses the second "x". Also, some other questions become
difficult to handle. Ex: </div>
<div><br></div>
<div>
<div>dt <- data.table(x=c(1,1,2,2), y=c(1,2,3,4),
x=c(2,2,1,1))</div>
<div>dt[, list(x=x/x[1], y=y), by=x]</div>
</div>
<div><br></div>
<div>Which "x" should be choose for which operation?</div>
<div><br></div>
<div>Arun</div>
<div><br></div>
</div>
<div>
<div>
<p style="color:#a0a0a8">On Friday, November 1, 2013 at 10:59 PM,
Eduard Antonyan wrote:</p>
<blockquote type="cite">
<div>
<div>
<div>
<div dir="ltr"><span>Having duplicate names is allowed and not that
unusual in data.table framework, so there is no need to signal
anything here.</span>
<div><span><br></span></div>
<div><span>A different question is whether there should be a
warning here:</span></div>
<div><span><br></span></div>
<div><span> dt = data.table(a = 1, a = 2)</span></div>
<div><span> dt[, a]</span></div>
<div><span><br></span></div>
<div><span>and I think that'd be a pretty good FR to
have.</span></div>
</div>
<div><span><br>
<br></span>
<div><span>On Fri, Nov 1, 2013 at 4:49 PM, Alexandre Sieira
<span dir="ltr"><<a href="mailto:alexandre.sieira@gmail.com" target="_blank">alexandre.sieira@gmail.com</a>></span>
wrote:<br></span>
<blockquote type="cite">
<div>
<div style="word-wrap:break-word">
<div style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto">
I found this behavior during a debugging session: </div>
<div style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto">
<br></div>
<div style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto">
<div style="margin:0px">> d = data.table(a=1, b=2, c=3)</div>
<div style="margin:0px">> setnames(d, "a", "b")</div>
<div style="margin:0px">> d</div>
<div style="margin:0px"> b b c</div>
<div style="margin:0px">1: 1 2 3</div>
<div style="margin:0px"><br></div>
<div style="margin:0px">Shouldn’t setnames() check if the new
column names already exist before renaming, and signal an error or
at least a warning if they do?</div>
</div>
<span><font color="#888888"><br></font></span>
<div>
<div style="font-family:Helvetica;line-height:normal">
<span>-- </span></div>
<div style="font-family:Helvetica;line-height:normal">
<span><span style="font-family:arial;font-size:small">Alexandre
Sieira</span><br style="font-family:arial;font-size:small">
<span style="font-family:arial;font-size:small">CISA, CISSP, ISO
27001 Lead Auditor</span><br style="font-family:arial;font-size:small">
<br style="font-family:arial;font-size:small">
<span style="font-family:arial;font-size:small">"The truth is
rarely pure and never simple."</span><br style="font-family:arial;font-size:small">
<span style="font-family:arial;font-size:small">Oscar Wilde, The
Importance of Being Earnest, 1895, Act I</span></span></div>
</div>
</div>
<br>
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