unstack_column_quiz_mrchypark.R

grade <- 
    as.data.frame(cbind(
        ID = c(LETTERS[1:5]),
        GPA = list('A+','F', c('C-','B','A+'), c('A','A+'), 'D')
    ))

unstack <- function(grade){
    do.call(rbind, apply(grade, 1, function(x)
        data.frame(ID = x$ID, GPA = x$GPA)))
}