\name{QSkillNames}
\alias{QSkillNames}
\alias{QSkillNames<-}
\title{Extracts names of the proficiency variables from a Q-Matrix.}
\description{
  Extracts (or sets) the names of the columns of the data frame
  associated with the proficiency variables of Q-Matrix.
}
\usage{
QSkillNames(Qmat)
QSkillNames(Qmat) <- value
}
\arguments{
  \item{Qmat}{A data frame object containing a Q-Matrix (see details).}
  \item{value}{New names for proficiency variable columns.}
}
\details{
  The Q-Matrix is basically a \code{data.frame} object with special
  columns (see \code{\link{read.Q}}).  In particular, the column called
  \code{Difficulty} marks a transition from meta-data to the Q-Matrix
  proper.  This function returns (sets) the names of the columns after
  the \code{Difficulty} column.

  If there is no \code{Difficulty} column, then an error will be
  generated. 
}
\value{
  For \sQuote{QSkillNames}, a character vector of length equal to the
  number of proficiency variables supported by the Q-Matrix.

  For \sQuote{QSkillNames<-}, the updated data frame.
}
\author{Russell Almond}
\seealso{\code{\link{read.Q}} }
\examples{
  readingQ <- read.Q(paste(library(help="SSX")$path,
                           "testFiles","Q-Reading.csv",
                           sep=.Platform$file.sep))

  QSkillNames(readingQ)
  \dontshow{
    stopifnot(all.equal(QSkillNames(readingQ),
                       c("WordMeaning", "SpecificInformation",
                         "ConnectInformation", "SynthesizeOrganize")))
  }
  QSkillNames(readingQ) <- c("ReadingWordMeaning", "ReadingSpecificInfo",
                          "ReadingConnectInfo", "ReadingSynOrganize")
  \dontshow{
    stopifnot(all.equal(QSkillNames(readingQ),
                        c("ReadingWordMeaning", "ReadingSpecificInfo",
                          "ReadingConnectInfo", "ReadingSynOrganize")))
  }
}
\keyword{attribute}% at least one, from doc/KEYWORDS