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I have the following table
DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=rep(4:6, 3))
I want to count how many rows meet the condition (y==3 & v==5).
I can get the rows that meet the condition, so I could save them and then count the rows. However, I know it can be done more efficiently with .N , I just don't know how. My code:
require(data.table)
keycols = c("y","v")
setkeyv(DT,keycols)
DT[J(3,5)] # This gets the subset I am interested in
DT[ , `:=` (count = .N), by = J(3,5)] # This is one of the multiple unsuccessful ways I have been trying to count the rows.
Anyone has any idea on how to make the last line work?
534
If you need a quick way to count rows that contain data, select all the cells in the first column of that data (it may not be column A). Just click the column header. The status bar, in the lower-right corner of your Excel window, will tell you the row count.
DataSet.Tables.Count() - Gives the number of tables available within a dataset. DataTable.Rows.Count() - Gives the number of rows available within a datatable.
Use the SUBTOTAL function to count the number of values in an Excel table or range of cells.
How about just
DT[.(3,5), .N]
# [1] 3
## These are also equivalent
## DT[J(3,5), .N]
## DT[list(3,5), .N]
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