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| # a running list of really useful data.table tricks | |
| # TOP 1,2 ... last row by some group id | |
| ## source: https://stackoverflow.com/questions/16325641/is-it-possible-to-extract-the-first-2-rows-for-each-date#comment23381259_16325932 | |
| ## comment by @eddi | |
| id <- c('date', 'userid') | |
| dt[dt[, .I[1:2], by = id]$V1] # first 2 rows by id | |
| dt[dt[, .I[.N], by = id]$V1] # last row by id (.N = length of group, .I = row index) |
hmm not sure. id have to test. could be though!
Yep.. it works!
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Slightly easier syntax?
dt[, tail(.SD,1), by=.(col1, col2)]