问题
I am trying to write a function that will sum the column(s) in the data frame according to the values in the first two columns.For example I have a matrix M,
Crs gr P_7 P_8
38 1 3 16
38 1 12 45
38 1 9 28
40 2 3 9
40 2 14 29
40 1 4 3
40 2 8 2
I want to sum the columns according to column1(crs) first and then column2(gr). Result will be,
Crs gr P_7 P_8
38 1 24 89
40 2 25 40
40 1 4 3
Currently I am using,
M <- M[, list(sum(P_7),sum(P_8)), by=list(Crs,gr)]
But the problem with this, is that I have to define the names of columns which wont be fixed. So, I was wondering how can I do this without defining the names of the columns. Thanks in advance!
回答1:
You're looking for this:
M[, lapply(.SD, sum), by = list(Crs, gr)]
回答2:
The package plyr has some magic for situations just like this. Use a combination of ddply and numcolwise, like this:
library(plyr)
ddply(dat, .(Crs, gr), numcolwise(sum))
results in:
Crs gr P_7 P_8
1 38 1 24 89
2 40 1 4 3
3 40 2 25 40
来源：https://stackoverflow.com/questions/17407506/sum-multiple-columns