Retrieve the first, second and last line that matches the criterion
I would like to know how to extract the last row following the criterion. I have seen a solution to get the first one using the "duplicate" function in the following link How do I select the first row in an R data frame that meets certain criteria? ...
However, is it possible to get the second or last row that meets the criterion?
I would like to create a loop for each Class
(I only put two here) and select the first, second and last row that match the criterion Weight >= 10
. And if there is no row that meets the criterion to get NA.
Finally, I want to store three values (first, second and last row) in a list containing the values for each class.
Class Weight
1 A 20
2 A 15
3 B 10
4 B 23
5 A 11
6 B 12
7 B 11
8 A 25
9 A 7
10 B 3
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A datasheet can help with this. This edit David's comment to move it into the answers as his approach is the correct way to do it.
library(data.table)
DT <- as.data.table(db)
DT[Weight >= 10][, .SD[c(1, 2, .N)], by = Class]
As with the faster alternative, also from David see
indx <- DT[Weight >= 10][, .I[c(1, 2, .N)], by = Class]$V1 ; DT[indx]
Creates the desired index using .I and then DT subsets based on those strings.
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