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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div class="bloop_markdown"><p>I think you mean:</p>
<pre><code>dta[dtb, b:=b, by=.EACHI]
</code></pre>
<p>and not <code>.EACHI = TRUE</code>. Not sure what’s the use of <code>nomatch=0L</code> along with <code>:=</code>.</p>
<p><code>by=.EACHI</code> does exactly what it means, really. It evaluates <code>j</code> for each <code>i</code> match. Let’s first see the matches:</p>
<pre><code>dta[dtb, which=TRUE]
# [1] 1 1 3
</code></pre>
<p>So, first row of <code>dtb</code> matches with first of <code>dta</code>. The second of <code>dtb</code> matches with 1st of <code>dta</code> and so on.</p>
<p>When you add <code>by=.EACHI</code>, as shown on the top, <code>j-expression</code> is evaluated on each of these matches. So, it’ll be evaluated 3-times here. On the other hand, without it, <code>j</code> is evaluated once. In this case, it doesn’t make a difference either way. So you should avoid <code>by=.EACHI</code>, as it’ll be slower with it.</p>
<p>It’s particularly useful when you’d like to perform operations in <code>j</code>, that depends on the values in <code>j</code> on <em>that</em> group. For example, consider these data.tables <code>dt1</code> and <code>dt2</code>:</p>
<pre><code>dt1 = data.table(x=rep(1:4, each=2), y=1:8, key="x")
dt2 = data.table(x=3:5, z=10, key="x")
</code></pre>
<p>And, you’d like to get <code>sum(y)*z</code> while joining.. If not for the <code>by=.EACHI</code> feature.. you’d approach the problem like this:</p>
<pre><code>dt1[dt2][, list(agg = sum(y)*z[1]), by=x]
</code></pre>
<p>With <code>by=.EACHI</code>, this is simply:</p>
<pre><code>dt1[dt2, list(agg=sum(y)*z), by=.EACHI]
</code></pre>
<p>Here, your expression is evaluated on each <code>i</code>.</p>
<p>Another interesting use case is, say, you’d like to create a lagged vector of <code>y</code>:</p>
<pre><code>dt1[dt2, list(y=y, lagy = c(NA, head(y,-1)), z=z), by=.EACHI]
</code></pre>
<p>It’s that simple.. really. Basically, as long as the operation you’re performing in <code>j</code> affects it depending on whether j is executed for that group or as a whole, then you’re most likely looking for <code>by=.EACHI</code>. If not, <code>by=.EACHI</code> has no effect, and therefore you’re wanting to use a <code>normal join</code> there..</p>
<p>This is not a text book definition, rather my understanding of this awesome feature!</p>
<p>Hope this helps.</p>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><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_1410534341499977216" 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">Juan Manuel Truppia</span> <a href="mailto:jmtruppia@gmail.com"><jmtruppia@gmail.com></a><br>Reply: <span style="color:black">Juan Manuel Truppia</span> <a href="mailto:jmtruppia@gmail.com"><jmtruppia@gmail.com>></a><br>Date: <span style="color:black">September 11, 2014 at 10:16:41 PM</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"> [datatable-help] Update table from other table <br></span></div><br> <blockquote type="cite" class="clean_bq"><span><div><div></div><div>
<title></title>
<div dir="ltr">What is the best data.table way of doing something
similar to UPDATE FROM in SQL?
<div><br>
<div>I used to do something like</div>
<div><br></div>
<div>dta = data.table(idx = c(1, 2, 3), a = runif(3), key =
"idx")</div>
<div>dtb = data.table(idx = c(1, 3), b = runif(3), key =
"idx")</div>
<div>dta[dtb, b := b]</div>
<div><br></div>
<div>However, after the 1.9.3 and the explicit .EACHI, it fails
sometimes, but I can't determine when.</div>
<div><br></div>
<div>So, just to be sure, I do </div>
<div><br></div>
<div>dta[dtb, b := b, .EACHI = TRUE, nomatch = 0]<br></div>
</div>
<div><br></div>
<div>Is the .EACHI and the nomatch necessary?</div>
<div><br></div>
<div>In this case, I want the row with idx 1 and 3 (the matching
ones) to end with a b value from the matching b column in dtb, and
the row with idx 2 (the one that isn't in dtb) to end up with NA in
column b.</div>
<div><br></div>
<div><br></div>
</div>
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