[R] Apply a function to each cell of a ragged matrix

[Gabor Grothendieck]

rowsum(x,f)

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Date:   Tue, 17 Feb 2004 17:38:46 -0500 
From:   XIAO LIU <xiaoliu at jhmi.edu>
To:   R Help <r-help at stat.math.ethz.ch> 
Subject:   [R] Apply a function to each cell of a ragged matrix 

 
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