Dear all Could someone explain me why lapply(split(x,fac),mean) returns means per levels of fac for each column of x whereas lapply(split(x,fac),sum) does not return sums per level of fac and columns of x, but adds all columns together? Hence, how can I get sum to behave as mean in this instance? Thank you very much in advance E. Castella
mean vs sum behavior
2 messages · Emmanuel Castella, Gavin Simpson
On Mon, 2008-03-31 at 18:41 +0200, Emmanuel Castella wrote:
Dear all Could someone explain me why lapply(split(x,fac),mean) returns means per levels of fac for each column of x whereas lapply(split(x,fac),sum) does not return sums per level of fac and columns of x, but adds all columns together? Hence, how can I get sum to behave as mean in this instance? Thank you very much in advance E. Castella
You didn't tell us what x is, but I suspect a data.frame. mean has a method for class "data.frame", which returns the mean of each column. Sum doesn't have any methods and hence works by summing all the numbers. If you want to replicate the mean behaviour with sum, the following would suffice:
fac <- gl(4, 50) dat <- data.frame(a = rnorm(200), b = rnorm(200), c = rnorm(200)) sp <- split(dat, fac)
then
lapply(sp, function(x) sapply(x, sum))
or even quicker and easier:
lapply(sp, colSums)
HTH G
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