arcpy.da.UpdateCursor use variable as field index?

ErikMartin

I'm working with an arcpy data access update cursor with a lot of fields that have an old/new pattern (e.g. oldField1, newField1, oldField2, newField2, etc.  I have a very simple operation I'd like to do -- calculate the newField = oldField for each pair.   I know I could use CalculateField_management, which is actually what I had been doing previously.  But there are conflicting reports as to which is faster -- da.UpdateCursor or CalculateField.  I am trying to optimize speed, so I want to try this out. 

The root of my question: is there a way to use a variable instead of the list index in a da cursor.  Specifically:

import arcpy FC = "C:\\data\\test.gdb\\test"  fields = ["newField1", "oldField1", "newField2", "oldField2", "newField3", "oldField3", .... "newFieldn", "oldFieldn"]  #There are actually a lot more fields in this same new/old pattern  with arcpy.da.UpdateCursor(FC, fields) as rows:     i = 0     for row in rows:         j = i + 1         row = row  #simply setting the new value = old value for each old/new pair, but get "list index out of range" error  i = i + 1

I know I could take the actual index for each (eg row[0] = row[1], then row[2] = row[3] the next time, etc) but that would be messy with so many fields.

With the old cursors, I could have done something like the following, but the old cursors are definitely slower than CalculateField_management, so this is not worth pursuing: 

baseFieldNameList = ["Field1", "Field2", "Field3", ...  "Fieldn") rows = arcpy.UpdateCursor for field in baseFieldNameList:     newField = "new" + field     oldField = "old" + field     oldValue = row.getValue(oldField)     row.setValue(newField, oldValue)     rows.updateRow del rows, row

Any suggestions, workarounds, or input would be much appreciated.

Thanks,
-Erik

MathewCoyle

You are getting an index error because you never reset your index back to 0 for the next row. This example should get you a little further.

with arcpy.da.UpdateCursor(FC, fields) as rows:     for row in rows:         i = 0         while i < len(fields):  #for field in fields:             row = row[i + 1]             i += 2         rows.updateRow(row)