Add documentation regarding dimensionless units (#276)

* add note about dimensionless units to documentation

* correct bullet point and add formatting to set_attributes documentation for ease of reading

* add warning about using dimensionless units when unrecognised unit is NA. Resolve #274

* Add test for warning when unit unrecognised

* update pkgdown documentation

R/set_attributes.R

@@ -12,45 +12,56 @@
 #' headers shown here.  The attributes data frame must contain columns for required metadata.
 #' These are:
 #'
-#' For all data:
-#' - attributeName (required, free text field)
+#' \strong{For all data:}
 #' 
-#' - attributeDefinition (required, free text field)
+#' \itemize{
+#'   \item attributeName (required, free text field)
 #' 
-#' - measurementScale (required, "nominal", "ordinal", "ratio", "interval", or "dateTime",
+#'   \item attributeDefinition (required, free text field)
+#' 
+#'   \item measurementScale (required, "nominal", "ordinal", "ratio", "interval", or "dateTime",
 #'  case sensitive) but it can be inferred from col_classes.
 #'  
-#' - domain (required, "numericDomain", "textDomain", "enumeratedDomain", or "dateTimeDomain",
+#'   \item domain (required, "numericDomain", "textDomain", "enumeratedDomain", or "dateTimeDomain",
 #'  case sensitive) but it can be inferred from col_classes.
-#'
-#' For numeric (ratio or interval) data:
-#' - unit (required)
-#'
-#' For character (textDomain) data:
-#' - definition (required)
-#'
-#' For dateTime data:
-#' - formatString (required)
+#' }
+#' 
+#' \strong{For numeric (ratio or interval) data:}
+#' \itemize{
+#'   \item unit (required). Unitless values should use "dimensionless" as the unit.
+#' }
+#' 
+#' \strong{For character (textDomain) data:}
+#' \itemize{
+#'   \item definition (required)
+#' }
+#' 
+#' \strong{For dateTime data:}
+#' \itemize{
+#'   \item formatString (required)
+#' }
 #' 
 #' Other optional allowed columns in the attributes table are:
 #' source, pattern, precision, numberType, missingValueCode, missingValueCodeExplanation,
 #' attributeLabel, storageType, minimum, maximum
 #'
-#' The factors data frame, required for attributes in an enumerated domain, must use only the
+#' The \strong{factors} data frame, required for attributes in an enumerated domain, must use only the
 #'  following recognized column headers:
+#' \itemize{
+#'   \item attributeName (required)
+#'   \item code (required)
+#'   \item definition (required)
+#' }
 #' 
-#' - attributeName (required)
-#' - code (required)
-#' - definition (required)
-#'
-#' The missingValues data frame, optional, can be used in the case that multiple missing value codes
+#' The \strong{missingValues} data frame, optional, can be used in the case that multiple missing value codes
 #' need to be set for the same attribute. This table must contain the following recognized column
 #' headers.
+#' \itemize{
+#'   \item attributeName (required)
+#'   \item code (required)
+#'   \item definition (required)
+#' }
 #' 
-#' - attributeName (required)
-#' - code (required)
-#' - definition (required)
-#'
 #' @return an eml "attributeList" object
 #' @export
 set_attributes <-
@@ -130,6 +141,9 @@ set_attribute <- function(row, factors = NULL, missingValues = NULL) {
           "' is not a recognized standard unit; treating as custom unit. ",
           "Please be sure you also define a custom unit in your EML record, ",
           "or replace with a recognized standard unit. ",
+          ifelse(is.na(row[["unit"]]), 
+                 'For unitless values, use "dimensionless" as the unit. ',
+                 NULL),
           "See set_unitList() for details."
       )
     } else {