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POWhat are the restrictions for the column classes in data.table?
 

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  1. POWhat are the restrictions for the column classes in data.table?
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    <p><strong>UPDATE</strong> This problem is not relevant anymore for <code>data.table</code> versions 1.8.0 and higher. From the NEWS file:</p> <blockquote> <p>character columns are now allowed in keys and are preferred to factor. data.table() and setkey() no longer coerce character to factor. Factors are still supported. Implements FR#1493, FR#1224 and (partially) FR#951.</p> </blockquote> <p><strong>Original question</strong></p> <p>I try to join two data.tables. However, the success of the join is dependent on the classes of the columns I use to match the data.tables. More precisely, it seems that the columns should not have the class "character". I don't quite understand the reason, but I'm sure I'm missing something obvious here. So help is really appreciated. </p> <p>Here is an example:</p> <pre><code>#Objective: Select all rows from DT for which Region=="US", Year &gt;= 5 &amp; Year&lt;=8, Cat="A" library(data.table) #Set-up data.table DT DT &lt;- data.table(Year=1:20, value=rnorm(20), Region=c(rep("US", 10), rep("EU", 10)), Cat=c(rep("A", 7), rep("B", 7), rep("C", 6))) setkey(DT, Region, Cat, Year) #Set-up data.table int_DT to join with DT years &lt;- 5:8 df &lt;- data.frame(Region=c("US", "EU"), Categ=c("A", "B")) int_DT &lt;- J(cbind(df[1, ], years)) #Join them: Works like a charm! DT[int_DT] #Let's assume that for any reason the columns in df are of class "character" df$Region &lt;- as.character(df$Region) df$Categ &lt;- as.character(df$Categ) #Rebuild int_DT int_DT &lt;- J(cbind(df[1, ], years)) DT[int_DT] #Error in `[.data.table`(DT, int_DT) : # unsorted column Region of i is not internally type integer. #OK, maybe the problem is that the column classes in DT are factors, so change those: DT[, Cat:=as.character(Cat)] DT[, Region:=as.character(Region)] DT[int_DT] #Error in `[.data.table`(DT, int_DT) : # When i is a data.table, x must be sorted to avoid a vector scan of x per row of i </code></pre> <p>Still doesn't work. Why? What is the restriction? What do I miss? Additionally information: I'm using data.table 1.6.6 and R version 2.13.2 (2011-09-30) on Platform: x86_64-pc-linux-gnu (64-bit).</p>
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    1. USChristoph_J
    1. USChristoph_J
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    1. COWhy did you have the thought of changing DT's key columns to character? data.table keys work with integers (or factors) only. Assigning to DT's key dropped the key (correctly).
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      1. USMatt Dowle
    2. COTo answer your question, @MatthewDowle, in my real problem, I create the inner data.table i on the run and based on a data.frame with character classes. So it was the other way round (I start with character columns and get the error). However, thanks to your answer I know the way forward: just convert the columns of the data.frame before I create the data.table i.
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      1. USChristoph_J
    3. COThanks to your question the error message has been improved in v1.7.1. See item in [latest NEWS](https://r-forge.r-project.org/scm/viewvc.php/pkg/NEWS?view=markup&root=datatable). @Andrie, too.
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      1. USMatt Dowle
 

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