directlabels - utility.function - Positioning Method - check.for.columns

Stop if a data.frame does not have some columns.

check.for.columns <- function
### Stop if a data.frame does not have some columns.
(d,
### data.frame to check.
 must.have
### column names to check.
 ){
  stopifnot(is.character(must.have))
  for(N in must.have){
    if(! N %in% names(d)){
      stop("data must have a column named ",N)
    }
  }
}