Selecting columns by name
DT[, c("col1", "col2")] returns a data.table with two columns, just like a data.frame.
Alternatively, you can also select columns by passing a list with each element referring to the column name as if it were a variable, i.e., DT[, .(col1, col2)]. Note the absence of quotes, i.e., "" around column names here.
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Data Manipulation with data.table in R
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# Select bike_id and trip_id using a character vector
df_way <- batrips[___]
df_way