Row/column counter in 'apply' functions

Question

What if one wants to apply a functon i.e. to each row of a matrix, but also wants to use as an argument for this function the number of that row. As an example, suppose you wanted to get the n-th root of the numbers in each row of a matrix, where n is the row number. Is there another way (using apply only) than column-binding the row numbers to the initial matrix, like this?

test <- data.frame(x=c(26,21,20),y=c(34,29,28))

t(apply(cbind(as.numeric(rownames(test)),test),1,function(x) x[2:3]^(1/x[1])))

P.S. Actually if test was really a matrix : test <- matrix(c(26,21,20,34,29,28),nrow=3) , rownames(test) doesn't help :( Thank you.

Answer 1

What I usually do is to run sapply on the row numbers 1:nrow(test) instead of test, and use test[i,] inside the function:

t(sapply(1:nrow(test), function(i) test[i,]^(1/i)))

I am not sure this is really efficient, though.

Answer 2

Actually, in the case of a matrix, you don't even need "apply". Just:

test^(1/row(test))

does what you want, I think. I think the "row" function is the thing you are looking for.

Answer 3

cbind()ing the row numbers seems a pretty straightforward approach. For a matrix (or a data frame) the following should work:

apply( cbind(1:(dim(test)[1]), test), 1, function(x) plot(x[-1], main=x[1]) )

or whatever you want to plot.