<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><p>Looking at <code>?data.table</code>, there’s another way which doesn’t require sorting on “Group, Date”:</p>
<pre><code>DT[DT[, .I[which.max(idx)], by=Group]$V1]
# Group Value Date idx
# 1: 1 yyy July 7
# 2: 2 qqqq August 8
</code></pre>
<p>HTH</p>
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.error { color:#AA0000; }</style></p><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <div id="bloop_sign_1405726158853142016" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">Arun</div></div> <div style="color:black"><br>From: <span style="color:black">Arunkumar Srinivasan</span> <a href="mailto:aragorn168b@gmail.com">aragorn168b@gmail.com</a><br>Reply: <span style="color:black">Arunkumar Srinivasan</span> <a href="mailto:aragorn168b@gmail.com">aragorn168b@gmail.com</a><br>Date: <span style="color:black">July 19, 2014 at 1:20:40 AM</span><br>To: <span style="color:black">bgoldstein</span> <a href="mailto:ben.goldstein@gmail.com">ben.goldstein@gmail.com</a>, <span style="color:black">datatable-help@lists.r-forge.r-project.org</span> <a 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"> Re: [datatable-help] Subsetting By Row Function <br></span></div><br> <blockquote type="cite" class="clean_bq"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div></div><div>
<title></title>
<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>
<blockquote type="cite" class="clean_bq" style="color: rgb(0, 0, 0); font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);">
I was wondering - if you don't mind - you could briefly explain
some of the <br>
syntax. I have seen .I and .SD but am not familiar quite with what
they <br>
mean. I'm assuming .N is last? Is there a syntax for the first
(.n?) or the <br>
5th (.5?) </blockquote>
</div>
<p>All special variables are explained in `?data.table`. It'd be
much easier for you in the future if you go through it and try it
out yourself with some dummy examples. They can be very powerful
tools!</p>
<p>Briefly: .N contains the number of observations for each group -
integer vector of length 1. If you want to refer to the first
value, then you can just use 1 = .I[1], .I[5] for 5th value.. and
if 5 > .N, .I[5] will return NA (like base R does when we access
beyond a vector's allocated length).</p>
<p>.I contains the row number of the original data.table for each
group. Ex: `DT <- data.table(x=c(1,2,1,1,2,1,2), y=10:16); DT[,
print(.I), by=x]` gives the position of all the 1's in `DT`
corresponding to x=1 first followed by all 2's in DT corresponding
to x=2.</p>
<p>HTH</p>
</div>
<div id="bloop_sign_1405725201931048960" class="bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">Arun</div>
</div>
<div style="color:black"><br>
From: <span style="color:black">bgoldstein</span> <a href="mailto:ben.goldstein@gmail.com">ben.goldstein@gmail.com</a><br>
Reply: <span style="color:black">bgoldstein</span> <a href="mailto:ben.goldstein@gmail.com">ben.goldstein@gmail.com</a><br>
Date: <span style="color:black">July 19, 2014 at 1:12:17
AM</span><br>
To: <span style="color:black">datatable-help@lists.r-forge.r-project.org</span>
<a 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">Re: [datatable-help]
Subsetting By Row Function<br></span></div>
<br>
<blockquote type="cite" class="clean_bq">
<div>
<div><span>Arun,<br>
<br>
This worked perfectly - Thank you. The dates are actually full
Chron dates<br>
so it was easy to sort first by date. I ended up using your Method
2 for<br>
speed.<br>
<br>
I was wondering - if you don't mind - you could briefly explain
some of the<br>
syntax. I have seen .I and .SD but am not familiar quite with what
they<br>
mean. I'm assuming .N is last? Is there a syntax for the first
(.n?) or the<br>
5th (.5?)<br>
<br>
Is '.I' saying find the index that meets this criterion? And .SD
find the<br>
group that meet the criterion?<br>
<br>
Thank you,<br>
<br>
Ben<br>
<br>
<br>
<br>
--<br>
View this message in context:
http://r.789695.n4.nabble.com/Subsetting-By-Row-Function-tp4694221p4694227.html<br>
Sent from the datatable-help mailing list archive at
Nabble.com.<br>
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</div>
</blockquote>
</div></div></span></blockquote><p></p></body></html>