2

I'm trying to dissolve polygons using polygon neighbors in arcgis 10.1, but don't want to use duplicate rows. However, the field values will switch when they're duplicated.

If I run this:

arcpy.env.workspace = "C:\Users\Ant\Documents\ArcGIS\ITN.gdb"
fc = r"BLPUs_PolygonNeighbors"
fields = ["src_OBJECTID","nbr_OBJECTID"]
with arcpy.da.SearchCursor(fc,fields) as cursor:
   for row in cursor:
      print "{0}, {1}".format(row[0],row[1])

I get:

1, 2
1, 4
2, 1
2, 3
2, 4
3, 2
3, 4
4, 1
4, 2
4, 3

Then when I run my script to dissolve based on these values, I will end up with duplicate polygons such as 1 & 2 as well as 2 & 1. Can someone please help me with writing the UpdateCursor to go through and delete these duplicates? I don't know how to look through when the fields have then switched.

Also, what if I then have three polygons to merge? If I had three columns such as OID1, OID2 and OID3, is there an SQL expression to capture whether the three values in these have been repeated in a different order but in the same row previously? Thanks

9

This should do it with a single pass.

arcpy.env.workspace = r"C:\Users\Ant\Documents\ArcGIS\ITN.gdb"
fc = r"BLPUs_PolygonNeighbors"
fields = ["src_OBJECTID","nbr_OBJECTID"]

row_pairs = set()

with arcpy.da.UpdateCursor(fc,fields) as cursor:
    for row in cursor:
        row_pair = tuple(sorted(row))
        if row_pair in row_pairs:
            cursor.deleteRow()
        else:
            row_pairs.add(row_pair)
  • By creating an empty set, will row_pair only be added if it doesn't already exist in row_pairs? I'm trying to understand why to use that over creating an empty list. – Paul Aug 21 '13 at 22:49
  • If the row_pair is not yet in the set, it gets added to the set. But if it's already in the set, the row is a known duplicate and it's deleted. A set is useful because it will only ever have one copy of each value so it won't get excessively big and uses a hash to look up values so searching it is faster than searching a list. – Jason Scheirer Aug 21 '13 at 23:30
  • 1
    thanks, that worked - had to use UpdateCursor, not SearchCursor though. Edited the answer. Cheers – user2581350 Aug 22 '13 at 0:26
  • +1 Hmm, I need to look more into sets then, thanks! – Paul Aug 22 '13 at 0:53
  • i guess it was for the deleteRow :) – user2581350 Aug 22 '13 at 14:21
2

You can use sorted() (which is a built in Python function) in a generator expression to sort the sub-lists separately and then use sorted() on the entire list so that it's in ascending order. Using your example:

cursordata = [[1, 2], [1, 4], [2, 1], [2, 3], [2, 4], [3, 2], [3, 4], [4, 1], [4, 2], [4, 3]]
print sorted((sorted(x) for x in cursordata))

[[1, 2], [1, 2], [1, 4], [1, 4], [2, 3], [2, 3], [2, 4], [2, 4], [3, 4], [3, 4]]

This can easily be extended to sublists of length n.

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