Add a variable to a data frame giving a mean of a set of quiz grades, dropping n grades

dropquizzes.R

## Note: we're assuming each grade should be weighted equally here.
dropmean <- function(x, drop=2) {
    count <- length(x)-drop
    round(sum(sort(x, decreasing=TRUE)[seq(1, count)], na.rm=T)/count, 2)
}

## Same formula for multiple sections; note list.files uses a regex pattern, not a glob pattern
infiles <- list.files(pattern='^gradebook-quizzes-[0-9]+\\.csv$')

for(filename in infiles) {
    dat <- read.csv(filename)
    newdat <- dat[,1:3]

    quizzes <- dat[,grep('^Quiz...Chapter.([0-9]|1[012])$', colnames(dat))]

    ## Quiz grades are out of 10, so I multiplied here by 10 to make it a percentage.
    newdat$DroppedAverage <- dat$DroppedAverage <- apply(quizzes, 1, dropmean)*10
    str(dat)

    write.csv(newdat, paste("fixed", filename, sep='-'), row.names=FALSE)
}