# Sequential attribute field calculations with null values using Arcpy

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 • I should add, I need to then convert these points into polygons based on their IDs Nov 18 '15 at 0:22
• A screenshot of the table would be helpful. Nov 18 '15 at 2:39
• I think you'd have more success using an Update Cursor rather than using the field calculator. Keep a running count and if you encounter a null value, don't update the field, but increase your count. When you hit the next row with a value, update with that count number and so on.
– Fezter
Nov 18 '15 at 4:57
• Also, converting these points into polygons should be posted as a separate question, however, I believe you'll find answers to that on this site already.
– Fezter
Nov 18 '15 at 4:58
• This seems like an unusual task. Why are there always 7 null points in your feature class between point features (future polygon vertices maybe?)? It would probably save you a lot of headache in the future if you skipped this temporary dataset (I'm assuming) and went straight to polygons from your .ply file.
– Paul
Nov 18 '15 at 7:12

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 is not None:
row = 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.

• I actually finished doing it manually. Nov 18 '15 at 5:37

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:
if frozen:
frozen = False
row = aUniqueID

if not row:
if not frozen:
aUniqueID += 1
frozen = True

cursor.updateRow(row)

• A few suggestions: Use is/is not to check for None (see here for why). Similarly, boolean checking in python means that you don't need to check with ==. Just simply if freeze and if not freeze.
– Paul
Nov 18 '15 at 7:16
• Thanks @Paul - good points. Edited. Habits carrying over from Java :) Nov 18 '15 at 23:02