Arcpy how to conduct addition in a script, function needed?

Question

I'm trying to work on my first arcpy script and am running into some troubles. The objective is to create a new field, check several existing fields (Cnt1,2,3 and Pop1,2,3) for missing data (-99 value) and if find missing data populate new field with a 0, if not then populate the field with the result from a simple calculation. From a separate related post I got some feedback that the problem was that I am concatenating the values, not adding them and this if-elif statement might need to be within a function and then called. Any advice greatly appreciated!

Answer 1

This was a fun python problem to illustrate the workflow on.

I might update this later to cleanup the exception handling when you calculate the field. The comments show what I am doing at each step of the script.

import arcpy, sys

#Let's start with using a workspace. This will make later work easier
arcpy.env.workspace = "C:\\gislab2\\Python\\take_home\\"
shapefile = "USCancer2000.shp"
field_name = "Henry1"
#Validate field name
field_name = arcpy.ValidateFieldName(field_name)
#Make sure our shapefile exists
if not arcpy.Exists(shapefile):
    sys.exit("Input shapefile " + shapefile + " does not exist in workspace " + arcpy.env.workspace + ". Cannot continue.")
#Check if field exists already, and delete it if it does
if (len(arcpy.ListFields(shapefile, field_name)) == 1):
    arcpy.DeleteField_Management(shapefile,field_name)
#Add the field
arpcy.AddField_management(shapefile,field_name,"LONG", "","","","","NULLABLE","NON_REQUIRED","")
#Rather than use CalculateField, the logic involved makes this better suited for a cursor
cursor = arcpy.UpdateCursor(shapefile)

for row in cursor:
    #Get the value of each of our fields, and put them into variables
    c1 = row.getValue("Cnt1")
    c2 = row.getValue("Cnt2")
    c3 = row.getValue("Cnt3")
    p1 = row.getValue("Pop1")
    p2 = row.getValue("Pop2")
    p3 = row.getValue("Pop3")
    #check for missing values, assuming that non-missing values are all greater than or equal to zero
    #also make sure we are not going to divide by zero
    try: #The try allows us to catch errors so we can bail out and still set the field to zero
        if((min(c1,c2,c3,p1,p2,p3) < 0) or ((p1+p2+p3) == 0) ):
            field_value = 0
        else:
            print "Calculating (" + "+".join([str(c1),str(c2),str(c3)]) + ")/(" + "+".join([str(p1),str(p2),str(p3)]) + ")"  #This is a debugging line, can remove once works properly.
            field_value = 1.0 * (c1+c2+c3)/(p1+p2+p3)
    except:
        #If something goes wrong, set the value to 0 still
        field_value = 0
    #Set the value of Henry1 for just this row
    print "Setting '" + field_name + "' to " + str(field_value) #This is a debugging line, can remove once works properly.
    row.setValue(field_name,field_value)
    cursor.updateRow(row)
    #At this point, we look back to the type of the for-in statement and do the next row in the shapefil

#Do cleanup at the end
del row
del cursor

Answer 2

It looks like your script is attempting to rename the column name rather than the field value. Given that the column is named "Henry1" and your Cnt or Pop values are -99, then your Henry1 would change to 0, else the name would be converted to another numeric value.. or if there are strings in the Cnt or Pop values Python would be expected to add and divide them. You need to define a variable other than "Field_name" for your field values, and then focus on a function to calculate those field values.