title: Getting a slot's value of S4 objects?
body: <p>So I have a spatialpolygons object in R; but I am not sure why I am unable to retrieve the "area" slot from it.</p>
  
  <p>Here's my R session:</p>
  
  <pre><code>&gt; spatialpolygons
  An object of class "SpatialPolygons"
  Slot "polygons":
  [[1]]
  An object of class "Polygons"
  Slot "Polygons":
  [[1]]
  An object of class "Polygon"
  Slot "labpt":
  [1] 20.50516 57.72918
  
  Slot "area":
  [1] 36.85484
  
  Slot "hole":
  [1] FALSE
  
  Slot "ringDir":
  [1] 1
  
  Slot "coords":
           [,1]     [,2]
  [1,] 16.48438 59.73633
  [2,] 22.59277 61.14258
  [3,] 24.74609 55.03418
  [4,] 17.49512 55.12207
  [5,] 16.48438 59.73633
  
  
  
  Slot "plotOrder":
  [1] 1
  
  Slot "labpt":
  [1] 20.50516 57.72918
  
  Slot "ID":
  [1] "myMultiPolygons"
  
  Slot "area":
  [1] 36.85484
  
  
  
  Slot "plotOrder":
  [1] 1
  
  Slot "bbox":
         min      max
  x 16.48438 24.74609
  y 55.03418 61.14258
  
  Slot "proj4string":
  CRS arguments:
   +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0 
  
  &gt; spatialpolygons@bbox
         min      max
  x 16.48438 24.74609
  y 55.03418 61.14258
  &gt; spatialpolygons@area
  Error: no slot of name "area" for this object of class "SpatialPolygons"
  &gt; slotNames(spatialpolygons)
  [1] "polygons"    "plotOrder"   "bbox"        "proj4string"
  &gt; names(spatialpolygons)
  [1] "myMultiPolygons"
  </code></pre>