R, data.table, group by column *numbers* AND sum a column

问题

Let's say I have the following data.table

> DT
#        A B C D E          N
#     1: J t X D N 0.07898388
#     2: U z U L A 0.46906049
#     3: H a Z F S 0.50826435
#    ---                     
#  9998: X b R L X 0.49879990
#  9999: Z r U J J 0.63233668
# 10000: C b M K U 0.47796539

Now I need to group by a pair of columns and calculate sum N. That's easy to do when you know column names in advance:

> DT[, sum(N), by=.(A,B)]
#      A B        V1
#   1: J t  6.556897
#   2: U z  9.060844
#   3: H a  4.293426
#  ---              
# 674: V z 11.439100
# 675: M x  1.736050
# 676: U k  3.676197

But I must do that in a function, which receives a vector of column indices to group by.

> f <- function(columns = 1:2) {
    DT[, sum(N), by=columns]
}
> f(1:2)
Error in `[.data.table`(DT, , sum(N), by = columns) : 
  The items in the 'by' or 'keyby' list are length (2). Each must be same 
  length as rows in x or number of rows returned by i (10000).

I also tried:

> f(list("A", "B"))
Error in `[.data.table`(DT, , sum(N), by = list(columns)) : 
  column or expression 1 of 'by' or 'keyby' is type list. Do not quote column
  names. Usage: DT[,sum(colC),by=list(colA,month(colB))]

How do I make this to work?

回答1:

Here's how I would approach this:

f <- function(columns) {
  Get <- if (!is.numeric(columns)) match(columns, names(DT)) else columns
  columns <- names(DT)[Get]
  DT[, sum(N), by = columns]
}

The first line (Get..) keeps "columns" as numeric if it's already numeric or it converts it from characters to numeric if they are not.

Test it out with some sample data:

set.seed(1)
DT <- data.table(
  A = sample(letters[1:3], 20, TRUE),
  B = sample(letters[1:5], 20, TRUE),
  C = sample(LETTERS[1:2], 20, TRUE),
  N = rnorm(20)
)

## Should work with either column number or name
f(1)
f("A")
f(c(1, 3))
f(c("A", "C"))

来源：https://stackoverflow.com/questions/26999291/r-data-table-group-by-column-numbers-and-sum-a-column

标签

data.table