R-Helpers:
There are a matrix x and a factor f. nrow(x) == length(f), e.g.:
x <- matrix(1:6, nrow = 3)
f <- factor(c("daytime", "daytime", "night"))
I want the sum of all elements of rows of "x" for each corresponding level in factor
"f",
In this case, I want output like:
"daytime" [1] x[1,1]+x[2,1]+x[1,2]+x[2,2]
"night" [2] x[3,1]+x[3,2]
But, tapply(x,f,sum) or by(x,f,sum) do not work. What other functions can I use?
Thank you very much
Xiao
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