Get a single vector with `[[`

[reinholdsson]

Often one is only interested in one vector and then it's not too obvious to type a['var'][[1]] (since [ always returns a table - even if it only returns a single variable). However, a complementary solution could be to make use of [[, e.g. a[['var']].

It should through an error or only select the first variable if multiple variables are asked for. Also, it should only allow for one dimension. Perhaps the easiest way would be to only return the first match y[[1]] in y <- list_to_query_repr(x$variable_match(name = i, dims = j)).

[reinholdsson]

Some example code:

setMethod(
  f = "[[",
  signature = "cdb",
  definition = function(x, i, j, na = NA) {

    # Get database, e.g. a[], a[, ._] 
    if (missing(i) || is.null(i)) {
      i <- ._
      if (missing(j)) j <- .all
      # Get variable, e.g. a[._], a[.all]
    } else {
      if (any(.all %in% i)) i <- ._
      if (missing(j)) j <- NULL
    }

    y <- list_to_query_repr(x$variable_match(name = i, dims = j))

    if (length(y) == 1) {
      v <- x$get_variable(name = y[[1]]$name, dims = y[[1]]$dims, na = na)
    } else if (length(y) > 1) {
      err(27)
    } else v <- NULL

    return(v)
  }
)