topic Field calculator, sum of field. in Python Questions

Field calculator, sum of field.

BenjaminMittler — Wed, 07 Jan 2015 18:22:31 GMT

I am having a bit of a problem with what seems like should be a simple field calculator expression. I have a table with about 50 rows and a column with acres for each row. I've created a new column called Percent_Acres (Double) and im looking to calculate the percent each feature makes up of the total acres.

I want an expression equal to this Percent_Acres = Acres / Total Acres.

Normally this would be easy to do and i would just look up the field statistics for the sum. However this calculation is part of a model that will be automated so i need an expression that will automatically calculate the columns sum while in the field calculator.

It appears that i am having a problem with iterating through each row to add up to the total. I have tried numerous expressions and each time i get the same result telling me that "typeerror float object is not iterable"
"

Any advise on this topic would be greatly appreciated

Thanks, Ben

Re: Field calculator, sum of field.

JakeSkinner — Sat, 11 Dec 2021 07:48:50 GMT

Hi Benjamin,

Here is a way you can do this:

def update(acres):
  import arcpy
  list = []
  with arcpy.da.SearchCursor(r"C:\temp\python\test.gdb\Sample", ["Acres"]) as cursor:
    for row in cursor:
      list.append(row[0])
  S = sum(list)
  return acres / S * 100

Re: Field calculator, sum of field.

BenjaminMittler — Mon, 12 Jan 2015 19:45:55 GMT

Thanks Jake

Re: Field calculator, sum of field.

Anonymous User — Thu, 12 Feb 2015 21:00:43 GMT

If I don't have "C:\temp\python\test.gdb\Sample" should I re-install Python?

Re: Field calculator, sum of field.

RichardFairhurst — Sat, 11 Dec 2021 07:48:53 GMT

That is not the value you should use, it is only a sample value in the correct format for you to imitate. You need to change the directory and feature class name to be the one you are actually calculating.

I would also change the code to make the list a global variable so that you do not run the cursor for each record (it only needs to be done once).

list = []
def update(acres):  
  import arcpy  
  global list  
  if len(list) == 0:
    with arcpy.da.SearchCursor(r"C:\yourPath\yourGDB.gdb\yourFC", ["Acres"]) as cursor:  
      for row in cursor:  
        list.append(row[0]) 
    del cursor, row 
  S = sum(list)  
  return acres / S * 100

Re: Field calculator, sum of field.

BlakeTerhune — Thu, 12 Feb 2015 23:42:07 GMT

Cool trick about using the global list. I didn't know you could do that in a field calc!

Re: Field calculator, sum of field.

RichardFairhurst — Thu, 12 Feb 2015 23:59:50 GMT

As the number of records being calculated grows, that trick quickly turns into a performance necessity. The same technique works for building a label expression function. See this blog on that subject. I wrote the blog the same day the light dawned on me that a global variable was essential to using a cursor inside of a loop in order to avoid doing time consuming query repetitions.

You can run a cursor against your entire feature class or related FC/table and load value lists or summaries into dictionaries/lists for the entire record set, and then build labels using the in memory data not contained in the current record, without the performance hit of building SQL statements and running the cursor on every record.

Re: Field calculator, sum of field.

DXXX — Fri, 13 Feb 2015 07:42:55 GMT

If you are stuck for time, export the table to excel and do it in excel.

Re: Field calculator, sum of field.

warrenroberts — Tue, 06 Nov 2018 03:40:07 GMT

LOL, yep

Re: Field calculator, sum of field.

JuneSkeeter — Tue, 18 Oct 2022 14:45:38 GMT

This is absurd ... is this really the easiest way to sum a field in the field calculator? Why doesn't something like this work:

!Shape_Area!/!Shape_Area!.sum()

Re: Field calculator, sum of field.

FrankyAdair — Wed, 06 Sep 2023 11:18:45 GMT

Thanks for this revised script, it's worked perfectly.

Since I'm making a tool to carry this out in ArcGISs ModelBuilder I was able to use inline variable substitution to replace r"C:\yourPath\yourGDB.gdb\yourFC" with "%Input Table%"

Right-click on the 'Calculate Field' tool within ModelBuilder, then Create Variable > From Parameter > Inpute Table. Right-click the Inpute Table and set it as a Parameter to allow user input for the tool.