Random sample/select polygons by attributes iteratively python advice

HollyCopeland

Hi all,

I need to write a script to do the following:

Randomly select polygons (parcels in my case) in a feature class using an attibute with parcel value. It needs to keep selecting parcels randomly until it reaches a total max parcel value. So, if each parcel is valued between 1-100, and I have a max value of 500, it needs to select enough parcels to reach the target value of 500, then I'll do some operations/calculations on these selected parcels. Finally, I need to create a loop to do this 100 or so times.

Is what I propose above doable and fairly straightforward. I'm relatively new at Python programming, so any tips about the logic in the code would be so helpful and appreciated.

Thanks,

Holly

HollyCopeland

This code isn't nearly as elegant as yours, Kim, but perhaps it is useful to someone else, so I'm posting it. Holly

CODE:
# ---------------------------------------------------------------------------
# Select Random Parcels.py
# Created on: 2011-5-18
# Creator: Holly Copeland (hcopeland@tnc.org)
# Requirements: Spatial Analyst Extension
# Description: This script randomly selects data from a feature class and
# stops selecting when it reaches a threshold value set by the user
# It also iterates multiple times and generates summary statistics from a
# separate field at the end
# ---------------------------------------------------------------------------

# Import arcpy module
import arcpy, os, sys, random, array
from arcpy.sa import *
from arcpy import env

# Check out the ArcGIS Spatial Analyst extension license
arcpy.CheckOutExtension("Spatial")

arcpy.env.workspace = "D:\\Work\\SageGrouseEaseStudyBackup\\Data\WYParcels_Merge.gdb"
#
#Specify Input Parameters
#
fc = "WYParcelsPrivate_AgValuesMay2011"

scenarioValue = 5000000  #this is the THRESHOLD value
iterationCount = 0
iterationNumber = 5 #number of iterations

sgSumTotal = []
sgMeanTotal = []

while iterationCount < iterationNumber:
    #Set parcel accumulation value to 0
    accumulatorValue = 0
    objectIDList = []

    #Set sage grouse accumulation value to 0
    sagegrouseValue = 0
    sagegrouseList = []
    sagegrouseListSum = []

   
    # Create a search cursor
    #


    # The variables row and rows are initially set to None, so that they
    #  can be deleted in the finally block regardless of where (or if)
    #  script fails.
    #
    row, rows = None, None
   
    #Start while statement to accumulate parcels
    #Value in while statement should be the same as the scenario
   
    while accumulatorValue < scenarioValue:
        objectID = random.randrange(1,27577) #generate random number to select parcel object id
        expression = '"OBJECTID" = ' + str(objectID) 
       
        try:
            rows = arcpy.SearchCursor(fc, expression)  #select the objectid from above

            #Iterate through the rows in the cursor of randomly selected parcels
            for row in rows:
                #assuming you are sampling without replacement
                if row.OBJECTID not in objectIDList:
                    accumulatorValue += row.AdjustedParcelValue
                    sagegrouseValue += row.SageGrouseTest
                    objectIDList.append(row.OBJECTID)
                    sagegrouseList.append(row.SageGrouseTest)
                   
        except:
            arcpy.AddError("Script Bombed")

    #calculate statistics for each iteration
    sgSum = sum(sagegrouseList)
    sgMax = max(sagegrouseList)
    sgMin = min(sagegrouseList)
    sgMean = sum(sagegrouseList)/len(sagegrouseList)
    sgSumTotal.append(sgSum)
    sgMeanTotal.append(sgMean)
    print "Sage Grouse Statistics for Run #" ,iterationCount
    print "No of parcels:" ,len(objectIDList)
    print "Max:" ,sgMax
    print "Min:" ,sgMin
    print "Sum:" ,sgSum
    print "Mean:" ,sgMean


    iterationCount += 1

sgSumTotalFinal = sum(sgSumTotal)
sgMeanTotalFinal = sum(sgMeanTotal)/len(sgSumTotal)

print "Final Sage Grouse Statistics (Sum, Mean):" ,sgSumTotalFinal, sgMeanTotalFinal