How to aggregate in R with a custom function that uses two columns -


is possible aggregate custom function uses 2 columns return 1 column?

say have dataframe:

x <- c(2,4,3,1,5,7) y <- c(3,2,6,3,4,6) group <- c("a","a","a","a","b","b")  data <- data.frame(group, x, y) data #   group x y # 1     2 3 # 2     4 2 # 3     3 6 # 4     1 3 # 5     b 5 4 # 6     b 7 6

and have function want use on 2 columns (x , y):

pathlength <- function(xy) {   out <- as.matrix(dist(xy))   sum(out[row(out) - col(out) == 1]) }

i tried following aggregate:

out <- aggregate(cbind(x, y) ~ group, data, fun = pathlength)   out <- aggregate(cbind(x, y) ~ group, data, function(x) pathlength(x))

however, calls pathlength on x , y separately instead of together, giving me: 

#  group x y #1     5 8 #2     b 2 2

what want call pathlength on x , y , aggregate way. here want aggregate do:

reala <- matrix(c(2,4,3,1,3,2,6,3), nrow=4, ncol=2) pathlength(reala) # [1] 9.964725  realb <- matrix(c(5,7,4,6), nrow=2, ncol=2) pathlength(realb) # [1] 2.828427  group <- c("a", "b")  pathlength <- c(9.964725,2.828427) real_out <- data.frame(group, pathlength) real_out #   group pathlength # 1       9.964725 # 2     b   2.828427

does have suggestions? or there other function can't find on google let me this? i'd rather not work around using loop, i'm assuming slow big dataset.

as you've found out, base aggregate() function works on 1 column @ time. instead use by() function

by(data[,c("x","y")], data$group, pathlength) data$group: [1] 9.964725 -----------------------------------------------------------------------  data$group: b [1] 2.828427

or split()/lapply()

lapply(split(data[,c("x","y")], data$group), pathlength) $a [1] 9.964725  $b [1] 2.828427

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