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POCreate Sequence Number for a block of records in an R Data Frame
 

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    <p>I have a fairly large dataset (by my standards) and I want to create a sequence number for blocks of records. I can use the plyr package, but the execution time is very slow. The code below replicates a comparable size dataframe.</p> <pre><code>## simulate an example of the size of a normal data frame N &lt;- 30000 id &lt;- sample(1:17000, N, replace=T) term &lt;- as.character(sample(c(9:12), N, replace=T)) date &lt;- sample(seq(as.Date("2012-08-01"), Sys.Date(), by="day"), N, replace=T) char &lt;- data.frame(matrix(sample(LETTERS, N*50, replace=T), N, 50)) val &lt;- data.frame(matrix(rnorm(N*50), N, 50)) df &lt;- data.frame(id, term, date, char, val, stringsAsFactors=F) dim(df) </code></pre> <p>In reality, this is a little smaller than what I work with, as the values are typically larger...but this is close enough. </p> <p>Here is the execution time on my machine:</p> <pre><code>&gt; system.time(test.plyr &lt;- ddply(df, + .(id, term), + summarise, + seqnum = 1:length(id), + .progress="text")) |===============================================================================================| 100% user system elapsed 63.52 0.03 63.85 </code></pre> <p>Is there a "better" way to do this? Unfortunately, I am on a Windows machine.</p> <p>Thanks in advance.</p> <p>EDIT: Data.table is extremely fast, but I can't get my sequence numbers to calc correctly. Here is what my ddply version created. The majority only have one record in the group, but some have 2 rows, 3 rows, etc.</p> <pre><code>&gt; with(test.plyr, table(seqnum)) seqnum 1 2 3 4 5 24272 4950 681 88 9 </code></pre> <p>And using data.table as shown below, the same approach yields:</p> <pre><code>&gt; with(test.dt, table(V1)) V1 1 24272 </code></pre>
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