Using mapply() in R over rows, vs. columns - r

Question

I deal with a great deal of survey data and the like in my work, and I often have to make various scoring programs that process data on a row-by-row level. For instance, I am dealing with a table right now that contains 12 columns with subscale scores from a psychometric instrument. These will be converted to normalized scores using tables provided by the instrument's creator. Seems straightforward so far.
However, there are four tables - the instrument is scored differently depending on gender and age range. So, for instance, a 14-year old female and an 10 year-old male get different normalization tables. All of the normalization data is stored in a R data frame.
What I would like to do is write a function which can be applied over rows, which returns a vector looked up from the normalization data. So, something vaguely like this:
converter <- function(rawscores,gender,age) {
if(gender=="Male") {
if(8 <= age & age <= 11) {convertvec <- c(1:12)}
if(12 <= age & age <= 14) {convertvec <- c(13:24)}
}
else if(gender=="Female") {
if(8 <= age & age <= 11) {convertvec <- c(25:36)}
if(12 <= age & age <= 14) {convertvec <- c(37:48)}
}
converted_scores <- rep(0,12)
for(z in 1:12) {
converted_scores[z] <- conversion_table[(unlist(rawscores)+1)[z],
convertvec[z]]
}
rm(z)
return(converted_scores)
}
EDITED: I updated this with the code I actually got to work yesterday. This version returns a simple vector with the scores. Here's how I then implemented it.
mydata[,21:32] <- 0
for(x in 1:dim(mydata)[1]) {
tscc_scores[x,21:32] <- converter(mydata[x,7:18],
mydata[x,"gender"],
mydata[x,"age"])
}
This works, but like I said, I'm given to understand that it is bad practice?
Side note: the reason rawscores+1 is there is that the data frame has a score of zero in the first index.
Fundamentally, the function doesn't seem very complicated, and I know I could just implement it using a loop where I would do for(x in 1:number_of_records), but my understanding is that doing so is poor practice. I had hoped to simply use apply() to do this, like as follows:
apply(X=mydata[,1:12],MARGIN=1,
FUN=converter,gender=mydata[,"gender"],age=mydata[,"age"])
Unfortunately, R doesn't seem to approve of this approach, as it does not iterate through the vectors passed to subsequent arguments, but rather tries to take them as the argument as a whole. The solution would appear to be mapply(), but I can't figure out if there's a way to use mapply() over rows, instead of columns.
So, I guess my questions are threefold. One, is there a way to use mapply() over rows? Two, is there a way to make apply() iterate over arguments? And three, is there a better option out there? I've seen and heard a lot about the plyr package, but I didn't want to jump to that before I fully investigated the options present in Base R.

Answer 1

You could rewrite 'converter' so that it takes vectors of gender, age, and a row index which you then use to do lookups and assignments to converted_scores using a conversion array and a data array that is jsut the numeric score columns. There is an additional problem with using apply since it will convert all its x arguments to "character" class because of the gender class being "character". It wasn't clear whether your code normdf[ rawscores+1, convertvec] was supposed to be an array extraction or a function call.
Untested in absence of working example (with normdf, mydata):
converted_scores <- matrix(NA, nrow=NROW(rawscores), ncol=12)
converter <- function(idx,gender,age) {
gidx <- match(gender, c("Male", "Female") )
aidx <- findInterval(age, c(8,12,15) )
ag.idx <- gidx + 2*aidx -1
# the aidx factor needs to be the same number of valid age categories
cvt <- cvt.arr[ ag.idx, ]
converted_scores[idx] <- normdf[rawscores+1,convertvec]
return(converted_scores)
}
cvt.arr <- matrix(1:48, nrow=4, byrow=TRUE)[1,3,2,4] # the genders alternate
cvt.scores <- mapply(converter, 1:NROW(mydata), mydata$gender, mydata$age)

Answer 2

I'd advise against applying this stuff by row, but would rather apply this by column. The reason is that there are only 12 columns, but there might be many rows.
The following piece of code works for me. There might be better ways, but it might be interesting for you nevertheless.
offset <- with(mydata, 24*(gender == "Female") + 12*(age >= 12))
idxs <- expand.grid(row = 1:nrow(mydata), col = 1:12)
idxs$off <- idxs$col + offset
idxs$val <- as.numeric(mydata[as.matrix(idxs[c("row", "col")])]) + 1
idxs$norm <- normdf[as.matrix(idxs[c("val", "off")])]
converted <- mydata
converted[,1:12] <- as.matrix(idxs$norm, ncol=12)
The tricky part here is this idxs data frame which combines all the rest. It has the folowing columns:
row and column: Position in the original data
off: column in normdf, based on gender and age
val: row in normdf, based on original value + 1
norm: corresponding normalized value
I'll post this here with this first thought, and see whether I can come up with a better answer, either based on jorans comment, or using a three- or four-dimensional array for normdf. Not sure yet.