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I am trying to figure out a solution to an field calculation problem. What I'm trying to do is automatically (using either ArcPy or the Field Calculator) generate sequential numbers, beginning at 1, in a new blank integer field (called 'Point_ID') for every recurrence of a value in a second field (called 'Line_ID'). The sorting of the sequential values in the second field ('Line_ID') will be based on the order in a third field ('FID'). Can anyone help me do this? I am very green when it comes to advanced field calculations and ArcPy (and Python in general, for that matter). So your explicitness is much appreciated.

Another way to put it: there are duplicate values in the Line_ID field. I would like to create a new field that counts up for every duplicate occurrence in the Line_ID field, with the sort order based on the FID field. So, if there are nine values in the Line_ID field that have the value "A," the new Point_ID field will go from 1 to 9, with the order based on the sort on FID.

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I think ArcPy and the Field Calculator are both overkill for this. I would try using the Summary Statistics tool with a case_field of Line_ID first and if that does not give you a solution then perhaps add a picture of the input and expected output for your process to your question using the edit button. I'll +1 your question in case you have insufficient reputation to do that. – PolyGeo Aug 6 '13 at 5:54
up vote 3 down vote accepted

I'm not sure how green you are, so here are "pretty explicit" instructions...

  1. Open up Field Calculator on the Point_ID field
  2. At the top, choose the "Python" parser
  3. Click the checkbox beside "Show Codeblock"
  4. In the "Pre-Logic script code", paste the following code...

    prevFieldValue = ''
    counter = 1
    def GetDuplicateCounter(myFieldValue):
      global prevFieldValue
      global counter
      if myFieldValue == prevFieldValue:
        counter += 1
        counter = 1
      prevFieldValue = myFieldValue
      return counter
  5. In the "Point_ID = " box, type in GetDuplicateCounter(!Line_ID!)

Note: If your Line_ID field is not a string field, then change the first line of code to prevFieldValue = 0 or something similar...

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This worked like a charm. I'm only green when it comes to the syntax of the code itself; I understand the logic and how to use the field calculator. But thanks a lot for a perfect solution and for being extra explicit! Exactly what I was looking for. – jchapman311 Aug 6 '13 at 17:34

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