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POR - data.table by group by - order of keys and missing keys
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If I have a data.table <pre><code>> DT1 <- data.table(A=rep(c('A', 'B'), 3), B=rep(c(1,2,3), 2), val=rnorm(6), key='A,B') > DT1 A B val 1: A 1 -1.6283314 2: B 2 0.5337604 3: A 3 0.9991301 4: B 1 1.1421400 5: A 2 0.1230095 6: B 3 0.4988504 </code></pre> and I want to subset by more than one key, like so: <pre><code>> DT1[J('A', 1)] A B val 1: A 1 -0.004898047 </code></pre> However, the join is dependent on the order of the keys, so the value for key A must always come first. This will not work, even if you specify names (either as <code>J()</code> or as a <code>list()</code>): <pre><code>> DT1[J(1, 'A')] Error in `[.data.table`(DT1, J(1, "A")) : x.'A' is a character column being joined to i.'V1' which is type 'double'. Character columns must join to factor or character columns. > DT1[J(B=1, A='A')] Error in `[.data.table`(DT1, J(B = 1, A = "A")) : x.'A' is a character column being joined to i.'B' which is type 'double'. Character columns must join to factor or character columns. </code></pre> Is there a syntax where you can do this kind of grouping by <code>i</code> without knowing the order of the keys? Added: Another use case would be if I wanted to subset by B only and not by A -- is there a way to skip keys in the subsetting? The current answers that create wrapper functions for J don't seem to allow this. EDIT: Some have mentioned doing it the data.frame way. I know that you can use a vector of logical values to subset, but this is slow as it does a scan of the entire table: <pre><code>> DT1 <- data.table(A=rep(c(1,2,3), 100000), B=rep(c('A', 'B'), 150000), val=rnorm(300000), key='A,B') > system.time(DT1[DT1$A==1, DT1$B=="A"]) user system elapsed 0.080 0.000 0.054 > system.time(DT1[J(1, 'A')]) user system elapsed 0.004 0.000 0.004 </code></pre> Some references to related discussions: <a href="https://stackoverflow.com/questions/8509595/how-do-i-select-rows-by-two-criteria-in-data-table-in-r">(1)</a>
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R - data.table by group by - order of keys and missing keys
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