Sequential attribute field calculations with null values using Arcpy

Question

I have a series of x,y co-ordinates converted from a .ply file. I was able to extract the file into a suitable x,y format for arcmap, the issue I have is that they are co-ordinate points representing 90 polygons. So in arcmap I have the coordinates for polygon 1, then a series of null rows, then co-ordinates for poly 2 etc. I have been manually entering the polygon number using field calculator but with 91 it is taking me a long time, so I thought I could use the sequential calculations script. Each null row is the same number of rows between each poly (7). so what I need is a script that will update a field with polygon 1, skips 7 null field rows, poly 2, etc etc.

The other issue is that the number of points in each polygon is different, so the only way to differentiate each polygon is the null rows in between.

I meant this script in particular

http://help.arcgis.com/EN/ARCGISDESKTOP/10.0/HELP/index.html#//005s0000002m000000

Accumulative and sequential calculations

Answer 1

Here's one way to number the polygons:

import arcpy

fc = "C:\\temp\\data.gdb\\points"

polygon = 1
nulls = 0

with arcpy.da.UpdateCursor(fc, ("x", "Polygon")) as cursor:
    for row in cursor:

        if row[0] is not None:
            row[1] = polygon
            cursor.updateRow(row)

        else:
            nulls += 1

            if nulls == 7:
                polygon += 1
                nulls = 0

Change the fc variable to the location of your point feature class. You can run this right in the Python window in ArcMap.

Answer 2

You can do this with an arcpy update cursor. Below is a little script that should take care of it. It assumes Polygon is a numerical field, like an integer field. Basically, it assigns a fixed number whenever it sees x data, and increments that number when it sees the next Null value for x, waiting to use that new number until it sees x data again.

import arcpy

featureClass = "C:\\path\\to\\your\\geodatabase_and\\featureclass"

fields = ("x", "y", "Polygon")

with arcpy.da.UpdateCursor(featureClass, fields) as cursor:

    aUniqueID = 1
    frozen = False

    for row in cursor:

        if row[0]:
            if frozen:
                frozen = False
            row[2] = aUniqueID     

        if not row[0]:
            if not frozen:
                aUniqueID += 1
                frozen = True

        cursor.updateRow(row)