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Select the top 20% of records.

RickeyFight

I have converted a raster into points. I have then clipped the raster with my buildings layer.

All buildings have a unique id and all the points in the same building have that id. (ex. in the image 65379 is the unique id for that building)

What I want to do is select the top 20% of points based on an attribute value for each building. Another issue is that the number of points per building changes based on the area.

I know how many points are in each building.

I am thinking I need a for each statement but I am not sure.

Any help is greatly appreciated.

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Accepted answers

RichardFairhurst

If you want to select the top 20% of the highest values this code should work:

import arcpy

"""Customize the layer name, IDField and valueField"""
lyrName = "sourceLayer"
IDField = "RID"
valueField = "MEAS"
Top20Field = "TOP20PERCENT"

"""Get the current map layers."""
mxd = arcpy.mapping.MapDocument("CURRENT")

"""Find the layer name"""
lyr = arcpy.mapping.ListLayers(mxd, lyrName)[0]

"""Create a dictionary of keys, values and record counts"""
valueDict = {}    
with arcpy.da.SearchCursor(lyr, [IDField, valueField, "OID@"]) as searchRows:
   for searchRow in searchRows:
       keyValue = searchRow[0]
       if not keyValue in valueDict:
           valueDict[keyValue] = [[(searchRow[1],searchRow[2])], 1]
       else:
           valueDict[keyValue][0].append((searchRow[1], searchRow[2]))
           valueDict[keyValue][1] += 1
print "Dictionary Read"

"""Create an OID List of records that are in the Top 20%"""
OIDList = []
for keyValue in sorted(valueDict.keys()):
    valueDict[keyValue][0] = sorted(valueDict[keyValue][0], reverse=True)
    top20Percent = int(round(valueDict[keyValue][1] * .2, 0))
    for n in range(0, top20Percent):
        OIDList.append(valueDict[keyValue][0][1])

"""Write a flag value to the Top20Field to indicate whether or not each record is in the Top 20%"""
with arcpy.da.UpdateCursor(lyr,["OID@", Top20Field]) as updateRows:
    for updateRow in updateRows:
        if updateRow[0] in OIDList:
            updateRow[1] = "Yes"
        else:
            updateRow[1] = "No"
        updateRows.updateRow(updateRow)

If you want the top 20% with the lowest values change line 30 to valueDict[keyValue][0] = sorted(valueDict[keyValue][0])

All comments

RichardFairhurst

If you want to select the top 20% of the highest values this code should work:

import arcpy

"""Customize the layer name, IDField and valueField"""
lyrName = "sourceLayer"
IDField = "RID"
valueField = "MEAS"
Top20Field = "TOP20PERCENT"

"""Get the current map layers."""
mxd = arcpy.mapping.MapDocument("CURRENT")

"""Find the layer name"""
lyr = arcpy.mapping.ListLayers(mxd, lyrName)[0]

"""Create a dictionary of keys, values and record counts"""
valueDict = {}    
with arcpy.da.SearchCursor(lyr, [IDField, valueField, "OID@"]) as searchRows:
   for searchRow in searchRows:
       keyValue = searchRow[0]
       if not keyValue in valueDict:
           valueDict[keyValue] = [[(searchRow[1],searchRow[2])], 1]
       else:
           valueDict[keyValue][0].append((searchRow[1], searchRow[2]))
           valueDict[keyValue][1] += 1
print "Dictionary Read"

"""Create an OID List of records that are in the Top 20%"""
OIDList = []
for keyValue in sorted(valueDict.keys()):
    valueDict[keyValue][0] = sorted(valueDict[keyValue][0], reverse=True)
    top20Percent = int(round(valueDict[keyValue][1] * .2, 0))
    for n in range(0, top20Percent):
        OIDList.append(valueDict[keyValue][0][1])

"""Write a flag value to the Top20Field to indicate whether or not each record is in the Top 20%"""
with arcpy.da.UpdateCursor(lyr,["OID@", Top20Field]) as updateRows:
    for updateRow in updateRows:
        if updateRow[0] in OIDList:
            updateRow[1] = "Yes"
        else:
            updateRow[1] = "No"
        updateRows.updateRow(updateRow)

If you want the top 20% with the lowest values change line 30 to valueDict[keyValue][0] = sorted(valueDict[keyValue][0])

RickeyFight

Thank you for your quick response!

It works as expected. I had a few issues at first, I was getting the error:

Runtime error

Traceback (most recent call last):

File "<string>", line 12, in <module>

IndexError: list index out of range

But I just ran it again and it started running!

The issue now is I have 12000 Buildings. At 30 sec per building it would take over 4 days to run. I have that kind of time but I believe it would be faster if it could run outside of arcmap.

Is there any way to sent an environment and instead of selecting the points create a new layer out of the top 20%

RichardFairhurst

An Insert cursor would probably be the best for creating a new feature class and would finish in less than 2 minutes most likely. I would not do this, unless you create a unique key for each point that is not the ObjectID, since without that you have no way to reliably relate the new feature class to the old. So I won't do this based on what you have shown.

Select by Attribute, especially using the Add to Selection option is time consuming, but it was necessary if I was to do what you originally asked and not alter the schema of the table. The only way to avoid days of processing is to create a new flag field indicating if the record was in the Top 20% or not and use an update cursor to populate it. Then you could use a simple SQL selection on that field. That could complete in under 3 minutes. I will assume that is what you will do, since it is the easiest for me to recode and the most flexible for exporting and processing.

RickeyFight

I see your point about the ObjectID and matching up. Yes a flag field would work just as well!

Thank you!

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