Field Calculator Sequential Numbering related to a Field

Question

Hi there,
I need to number Features sequentially, with the Python Script I found here, everything works fine and my Table was numbered from 1 to 2436. For further processing it would be nice if I could number Features in relation to a Field in the Table. I have Categories from 1 to 7 in my table and numbering should start in each Category at 1 and numbers sequentially.
 
i.e. like this:
 
Category ---  New_Number
1 ---------------1
1 ---------------2
1 ---------------3
1 ---------------4
2 ---------------1
2 ---------------2
3 ---------------1
3 ---------------2
3 ---------------3
3 ---------------4

Thanks so far

RDHarles · Answer

You could do something like this.

Assumes:
1.) The Frequency Tool was run and the results were joined to Table.dbf.
2.) The "category numbers" are in a field called 'Category'.
3.) A field called 'Seq' exists to write the sequential numbers to.

My test table looked like this:

Category  Seq   Frequency
1 --------- 1 ----- 4
1 ---------            2           ----- 4
1 ---------           3           ----- 4
1 ---------           4 -----           4
2            --------- 1           ------2
2            --------- 2           ------2
3 ---------           1           ----- 4
3 ---------           2           ----- 4
3            --------- 3           ----- 4
3            --------- 4 -----          4
import arcpy

count=0
# Loop through the table
rows = arcpy.UpdateCursor("Table.dbf")
for row in rows:    
    # Get the values in the Category & FREQUENCY fields
    num = row.getValue("Category")
    freq = row.getValue("FREQUENCY")
    # Add one to the count
    count=count+1
    # If count is <= the frequency
    if count <= freq:        
        print "Field value "+str(num)+" assigned a "+str(count)
        # Write count in field Seq
        row.Seq = count
        # Execute the new values to the table
        rows.updateRow(row)
    # Reset counter when freq is reached.
    if count == freq:
        count=0

print "
Done.
"

KimOllivier · Answer

Use a search cursor (read only) to get out the objectid and your counters into a dictionary. Then you can manipulate the list using direct tools on libraries. Finally open an update cursor and put back the changes.
It is very hard to process using a cursor otherwise because you can only step through in one direction.

DavidRahe · Answer

Hi Don,
thanks for the fast reply,
originally I was thinking about somthing like a "where" clause or somthing like this but your tip sounds very good.

I´m not (yet) very familiar with python, so I will need approx. one or two days for the cursor implementation. This is no problem, I don´t need a solution ASAP, and I think getting used to cursors will speed up my work on longtime 😉

If I get stuck somwhere I´ll cry here for new advice ^^
Thanks alot
Dave