I am trying to field calculate selected features only in ArcGIS Online. Alternatively, I suppose I could also use an if type expression to alter only blank/null fields. Is this possible, and if so how?
I know that ArcGIS Online calculations do not honor a selected set. Not sure why, but I've been burned on it. Check here: Calculate field values—ArcGIS Online Help | ArcGIS you might a solution....
The link that Joe provided says "If you have a filter on your layer, only the records that meet the filter criteria will have their values calculated." Here is some info on setting filters: Apply filters—ArcGIS Online Help | ArcGIS
FYI: the feature service layer Calculate API allows passing a where clause to filter which records will be affected by calculate. So without having a filter on the layer, you can pass a ad-hoc where clause to the calculate API. If you want to apply calculate to only null field, you can use something like myField IS NULL.