如何選擇每個類別的最近距離

Question

我正試圖獲得每個類別中最近的位置。任何人都可以幫我做到這一點？

var xml = new XElement("Locations", 
        locations.OrderBy(n => n.CategoryID) 
         .ThenBy(n => distance(lat, lon, (double)n.Latitude, (double)n.Longitude)) 
         .Where(n => (distance(lat, lon, (double)n.Latitude, (double)n.Longitude) <= 5)) 
         .Select(location => 
         new XElement("Location", 
          new XAttribute("CategoryID", location.CategoryID), 
          new XElement("Category", location.Category), 
          new XElement("LocationID", location.LocationID), 
          new XElement("LocationName", location.LocationName), 
          new XElement("Latitude", location.Latitude), 
          new XElement("Longitude", location.Longitude), 
          new XElement("Distance", distance(lat, lon, (double)location.Latitude, (double)location.Longitude)), 
          new XElement("Status", (location.HasManagedHours ? "Managed Hours" : "Open")) 
          ))); 

Answer 1

我沒有測試過，但我會建議分組是要走的路，是這樣的：

var xml = new XElement("Locations",        
    locations 
    .GroupBy(n => n.CategoryID) 
    .SelectMany(g => g 
     .OrderBy(n => distance(lat, lon, (double)n.Latitude, (double)n.Longitude)) 
     .Take(1)) 
    .Select(location => 
     new XElement("Location", 
      new XAttribute("CategoryID", location.CategoryID), 
      new XElement("Category", location.Category), 
      new XElement("LocationID", location.LocationID), 
      new XElement("LocationName", location.LocationName), 
      new XElement("Latitude", location.Latitude), 
      new XElement("Longitude", location.Longitude), 
      new XElement("Distance", distance(lat, lon, (double)location.Latitude, (double)location.Longitude)), 
      new XElement("Status", (location.HasManagedHours ? "Managed Hours" : "Open")) 
     ))); 

更多信息，請參見Projection Operators和Grouping Operators。