Quickly remove zero variance variables from a data.frame

Question

I have a large data.frame that was generated by a process outside my control, which may or may not contain variables with zero variance (i.e. all the observations are the sa

Answer 1

I think having zero variance is equivalent to being constant and one can get around without doing any arithmetic operations at all. I would expect that range() outperforms var(), but I have not verified this:

removeConstantColumns <- function(a_dataframe, verbose=FALSE) {
  notConstant <- function(x) {
    if (is.factor(x)) x <- as.integer(x)
    return (0 != diff(range(x, na.rm=TRUE)))
  }
  bkeep <- sapply(a_dataframe, notConstant)
  if (verbose) {
    cat('removeConstantColumns: '
      , ifelse(all(bkeep)
        , 'nothing'
        , paste(names(a_dataframe)[!bkeep], collapse=',')
      , ' removed',  '\n')
  }
  return (a_dataframe[, bkeep])
}