apply a function over groups of columns

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This may be more generalizable to your situation in that you pass a list of indices. If speed is an issue (large data frame) I’d opt for lapply with do.call rather than sapply:

x <- list(1:3, 4:6)
do.call(cbind, lapply(x, function(i) rowMeans(dat[, i])))

Works if you just have col names too:

x <- list(c('a','b','c'), c('d', 'e', 'f'))
do.call(cbind, lapply(x, function(i) rowMeans(dat[, i])))

EDIT

Just happened to think maybe you want to automate this to do every three columns. I know there’s a better way but here it is on a 100 column data set:

dat <- data.frame(matrix(rnorm(16*100), ncol=100))

n <- 1:ncol(dat)
ind <- matrix(c(n, rep(NA, 3 - ncol(dat)%%3)), byrow=TRUE, ncol=3)
ind <- data.frame(t(na.omit(ind)))
do.call(cbind, lapply(ind, function(i) rowMeans(dat[, i])))

EDIT 2
Still not happy with the indexing. I think there’s a better/faster way to pass the indexes. here’s a second though not satisfying method:

n <- 1:ncol(dat)
ind <- data.frame(matrix(c(n, rep(NA, 3 - ncol(dat)%%3)), byrow=F, nrow=3))
nonna <- sapply(ind, function(x) all(!is.na(x)))
ind <- ind[, nonna]

do.call(cbind, lapply(ind, function(i)rowMeans(dat[, i])))

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