Get or calculate the attribute of a list-column containing nested dataframes.

For example, self_union produces a SpatialPolygonsDataFrame that has a column called union_df, which contains a data.frame for each polygon with the attributes from the constituent polygons.

get_poly_attribute(x, col, fun, ...)

Arguments

x

the list-column in the (SpatialPolygons)DataFrame that contains nested data.frames

col

the column in the nested data frames from which to retrieve/calculate attributes

fun

function to determine the resulting single attribute from overlapping polygons

...

other parameters passed on to fun

Value

An atomic vector of the same length as x

Examples

if (require(sp)) {
  p1 <- Polygon(cbind(c(2,4,4,1,2),c(2,3,5,4,2)))
  p2 <- Polygon(cbind(c(5,4,3,2,5),c(2,3,3,2,2)))
  ps1 <- Polygons(list(p1), "s1")
  ps2 <- Polygons(list(p2), "s2")
  spp <- SpatialPolygons(list(ps1,ps2), 1:2)
  df <- data.frame(a = c(1, 2), b = c("foo", "bar"),
                   c = factor(c("high", "low"), ordered = TRUE,
                              levels = c("low", "high")),
                   stringsAsFactors = FALSE)
  spdf <- SpatialPolygonsDataFrame(spp, df, match.ID = FALSE)
  plot(spdf, col = c(rgb(1, 0, 0,0.5), rgb(0, 0, 1,0.5)))
  unioned_spdf <- self_union(spdf)
  get_poly_attribute(unioned_spdf$union_df, "a", sum)
  get_poly_attribute(unioned_spdf$union_df, "c", max)
}
#> Loading required package: sp
#> Loading required namespace: rgeos

#> [1] high low  high
#> Levels: low < high