Rescore Items from Scoring Key - r

Question

I have a set of data on which respondents were given a series of questions, each with five response options (e.g., 1:5). Given those five options, I have a scoring key for each question, where some responses are worth full points (e.g., 2), others half points (1), and others no points (0). So, the data frame is n (people) x k (questions), and the scoring key is a k (questions) x m (responses) matrix.
What I am trying to do is to programmatically create a new dataset of the rescored items. Trivial dataset:
x <- sample(c(1:5), 50, replace = TRUE)
y <- sample(c(1:5), 50, replace = TRUE)
z <- sample(c(1:5), 50, replace = TRUE)
dat <- data.frame(cbind(x,y,z)) # 3 items, 50 observations (5 options per item)
head(dat)
x y z
1 3 1 2
2 2 1 3
3 5 3 4
4 1 4 5
5 1 3 4
6 4 5 4
# Each option is scored 0, 1, or 2:
key <- matrix(sample(c(0,0,1,1,2), size = 15, replace = TRUE), ncol=5)
key
[,1] [,2] [,3] [,4] [,5]
[1,] 0 0 0 1 2
[2,] 2 1 1 1 2
[3,] 2 2 1 1 2

Answer 1

Some other options, firstly using Map:
data.frame(Map( function(x,y) key[y,x], dat, seq_along(dat) ))
# x y z
#1 0 2 2
#2 0 2 1
#3 2 1 1
#4 0 1 2
#5 0 1 1
#6 1 2 1
Secondly using matrix indexing on key:
newdat <- dat
newdat[] <- key[cbind( as.vector(col(dat)), unlist(dat) )]
newdat
# x y z
#1 0 2 2
#2 0 2 1
#3 2 1 1
#4 0 1 2
#5 0 1 1
#6 1 2 1
Things would be even simpler if you specified key as a list:
key <- list(x=c(0,0,0,1,2),y=c(2,1,1,1,2),z=c(2,2,1,1,2))
data.frame(Map("[",key,dat))
# x y z
#1 0 2 2
#2 0 2 1
#3 2 1 1
#4 0 1 2
#5 0 1 1
#6 1 2 1

Answer 2

For posterity, I was discussing this issue with a friend, who suggested another approach. The benefits of this is that it still uses mapvalues() to do the rescoring, but does not require a for loop, instead uses "from" in sapply to do the indexing.
library(plyr)
scored <- sapply(1:ncol(raw), function(x, dat, key){
mapvalues(dat[,x], from = 1:ncol(key), to = key[x,])
}, dat = dat, key = key)

Answer 3

My current working approach is to use 1) mapvalues, which lives within package:plyr to do the heavy lifting: it takes a vector of data to modify, and two additional parameters "from", which is the original data (here 1:5), and "to", or what we want to convert the data to; and, 2) A for loop with index notation, in which we cycle through the available questions, extract the vector pertaining to each using the current loop value, and use it to select the proper row from our scoring key.
library(plyr)
newdat <- matrix(data=NA, nrow=nrow(dat), ncol=ncol(dat))
for (i in 1:3) {
newdat[,i] <- mapvalues(dat[,i], from = c(1,2,3,4,5),
to = c(key[i,1], key[i,2], key[i,3], key[i,4], key[i,5]))
}
head(newdat)
[,1] [,2] [,3]
[1,] 0 2 2
[2,] 0 2 1
[3,] 2 1 1
[4,] 0 1 2
[5,] 0 1 1
[6,] 1 2 1
I am pretty happy with this solution, but if anyone has any better approaches, I would love to see them!