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POHow to do a basic left outer join with data.table in R?
 

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  1. POHow to do a basic left outer join with data.table in R?
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    <p>I have a data.table of a and b that I've partitioned into <code>below</code> with b &lt; .5 and <code>above</code> with b > .5:</p> <pre><code>DT = data.table(a=as.integer(c(1,1,2,2,3,3)), b=c(0,0,0,1,1,1)) above = DT[DT$b &gt; .5] below = DT[DT$b &lt; .5, list(a=a)] </code></pre> <p>I'd like to do a left outer join between <code>above</code> and <code>below</code>: for each <code>a</code> in <code>above</code>, count the number of rows in <code>below</code>. This is equivalent to the following in SQL:</p> <pre><code>with dt as (select 1 as a, 0 as b union select 1, 0 union select 2, 0 union select 2, 1 union select 3, 1 union select 3, 1), above as (select a, b from dt where b &gt; .5), below as (select a, b from dt where b &lt; .5) select above.a, count(below.a) from above left outer join below on (above.a = below.a) group by above.a; a | count ---+------- 3 | 0 2 | 1 (2 rows) </code></pre> <p>How do I accomplish the same thing with data.tables? This is what I tried so far:</p> <pre><code>&gt; key(below) = 'a' &gt; below[above, list(count=length(b))] a count [1,] 2 1 [2,] 3 1 [3,] 3 1 &gt; below[above, list(count=length(b)), by=a] Error in eval(expr, envir, enclos) : object 'b' not found &gt; below[, list(count=length(a)), by=a][above] a count b [1,] 2 1 1 [2,] 3 NA 1 [3,] 3 NA 1 </code></pre> <p>I should also be more specific in that I already tried <code>merge</code> but that blows through the memory on my system (and the dataset takes only about 20% of my memory).</p>
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      1. POHow to do a basic left outer join with data.table in R?
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    1. COCan you just write in human terms what you want to achieve?
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    2. COpossible duplicate of [How to join data frames in R (inner, outer, left, right)?](http://stackoverflow.com/questions/1299871/how-to-join-data-frames-in-r-inner-outer-left-right)
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    3. COYou could have tried looking for the answer first. a data.table is an extension of data.frame, and has a merge function.
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