My question is: I have a data frame with some factor variables. I now want to assign a new vector to this data frame, which creates an index for each subset of those factor variables.
data <-data.frame(fac1=factor(rep(1:2,5)), fac2=sample(letters[1:3],10,rep=T))
Gives me something like:
fac1 fac2
1 1 a
2 2 c
3 1 b
4 2 a
5 1 c
6 2 b
7 1 a
8 2 a
9 1 b
10 2 c
And what I want is a combination counter which counts the occurrence of each factor combination. Like this
fac1 fac2 counter
1 1 a 1
2 2 c 1
3 1 b 1
4 2 a 1
5 1 c 1
6 2 b 1
7 1 a 2
8 2 a 2
9 1 b 2
10 1 a 3
So far I thought about using tapply to get the counter over all factor-combinations, which works fine
counter <-tapply(data$fac1, list(data$fac1,data$fac2), function(x) 1:length(x))
But I do not know how I can assign the counter list (e.g. unlisted) to the combinations in the data-frame without using inefficient looping :)
This is a job for the ave()
function:
# Use set.seed for reproducible examples
# when random number generation is involved
set.seed(1)
myDF <- data.frame(fac1 = factor(rep(1:2, 7)),
fac2 = sample(letters[1:3], 14, replace = TRUE),
stringsAsFactors=FALSE)
myDF$counter <- ave(myDF$fac2, myDF$fac1, myDF$fac2, FUN = seq_along)
myDF
# fac1 fac2 counter
# 1 1 a 1
# 2 2 b 1
# 3 1 b 1
# 4 2 c 1
# 5 1 a 2
# 6 2 c 2
# 7 1 c 1
# 8 2 b 2
# 9 1 b 2
# 10 2 a 1
# 11 1 a 3
# 12 2 a 2
# 13 1 c 2
# 14 2 b 3
Note the use of stringsAsFactors=FALSE
in the data.frame()
step. If you didn't have that, you can still get the output with: myDF$counter <- ave(as.character(myDF$fac2), myDF$fac1, myDF$fac2, FUN = seq_along)
.
A data.table solution
library(data.table)
DT <- data.table(data)
DT[, counter := seq_len(.N), by = list(fac1, fac2)]
This is a base R way that avoids (explicit) looping.
data$counter <- with(data, {
inter <- as.character(interaction(fac1, fac2))
names(inter) <- seq_along(inter)
inter.ordered <- inter[order(inter)]
counter <- with(rle(inter.ordered), unlist(sapply(lengths, sequence)))
counter[match(names(inter), names(inter.ordered))]
})
Here a variant with a little looping (I have renamed your variable to "x" since "data" is being used otherwise):
x <-data.frame(fac1=rep(1:2,5), fac2=sample(letters[1:3],10,rep=T))
x$fac3 <- paste( x$fac1, x$fac2, sep="" )
x$ctr <- 1
y <- table( x$fac3 )
for( i in 1 : length( rownames( y ) ) )
x$ctr[x$fac3 == rownames(y)[i]] <- 1:length( x$ctr[x$fac3 == rownames(y)[i]] )
x <- x[-3]
No idea whether this is efficient over a large data.frame but it works!
来源:https://stackoverflow.com/questions/13071669/how-to-assign-a-counter-to-a-specific-subset-of-a-data-frame-which-is-defined-by