calculatefield formula

ElaineKuo

System: ArcGIS 9.3

Problem:
I want to calculate the ratio of cut area to all area but did not know how to make the correct expression.
Please kindly help and thank you.

The following code has an error message:
ExecuteError: ERROR 000539: Error running expression: cutarea_km/ allarea_km <type 'exceptions.NameError'>: name 'cutarea_km' is not defined

##Script Name: Calculate area ratio ##Description: of polygons of a shapefile ##Created By: Elaine Kuo ##Date: 24/06/2012   #Import standard library modules import arcgisscripting import os  #Create the Geoprocessor object gp = arcgisscripting.create(9.3)  #Set the workspace. gp.Workspace= "H:/temp/test"  #Set the workspace. List all of the feature classes in the dataset outWorkspace= "H:/temp"  #Get a list of the featureclasses in the input folder fcs = gp.ListFeatureClasses()  # Loop through every item in the list that was just generated for fc in fcs:      # Break out the name, no path or extension, using the describe object.     desc = gp.describe(fc)     featureName = desc.name      # Add a field to this shapefile, of type LONG     gp.AddField (fc, "ratio", "double", 2,2)         #   Get a list of the fields in the featureclass     fields = gp.ListFields(fc, "C*", "String")               # Loop through every item in the list that was just generated      for field in fields:          gp.toolbox = "Data Management"          # get areas of all and cut         allarea = "allarea_km"         cutarea = "cutarea_km"          expression = "cutarea_km/ allarea_km"                  gp.CalculateField_management(fc, "ratio", expression, "PYTHON")                  #Validate the new feature class name for the output workspace.     OutFeatureClass = outWorkspace + os.sep + gp.ValidateTableName(fc,outWorkspace)       gp.AddMessage(gp.GetMessages()) print gp.GetMessages() 

MarcinGasior

ad 1.
Indeed, there was a problem with Python expression - when dividing two numbers without decimal part, the result is a number without decimal part.
That's why decimal part was cut and only 0 left.
To correct this just multiply expression by 1.0 (I also corrected previous answer).

ad 2.
SHORT and LONG data types allows to store in a field only integer numbers (without decimal part). The difference is how big/small number can be.
FLOAT and DOUBLE dat types allows to store real numeric values (with decimal part). The difference is roughly how many digits can be stored.
It seem that when you created DOUBLE field with specific precision and scale, application recognised it also fits in FLOAT type which occupies less space.

You can read about precision and scale for example in this old thread.