Is there a way to perform a field calculate function based on attributes in another feature?

[IsaacRemboldt1]

I have two fields that describe a points placement on a line. Each point has a unique ID, and a field describing the next ID "downstream". My goal is to compare a third field (field3) where the unique ID = the downstream ID.

The downstream (DownstreamID) value of the third field (Field3_2) should be less than the third field (Field3_1) of the upstream (UniqueID) feature.

I also included a section of visual basic code for performing a similar function in excel.

For (Each Feature) 
   if UniqeID /*(from feature1)*/ = DownstreamID /*(from feature2)*/ & Field3_1 /*(from feature1)*/ > Field3_2 /*(from feature2)*/: 
      return(1) 
   else: 
      return(0)

__________________________________________________________________________________________________

 If NextDownID <> -1 Then
 
    NextDownRow = Application.Match(Cells(rw, 2), Columns(1), 0)
 
    For i = 0 To 7 'create array with US Flows
       If Cells(rw, USColumn + i).Value <> "" Then 'check if flow value exists
          tmpUS(i) = Cells(rw, USColumn + i).Value 'if yes, add to array
       Else
          tmpUS(i) = 0 'else, put in 0 (null causes issues)
       End If
    Next i
 
    For j = 0 To 7 'create array with DS Flows
       tmpq = Application.Index(Columns(3 + j), NextDownRow)
       If tmpq <> "" Then
          tmpDS(j) = tmpq
       Else
          tmpDS(j) = 0
       End If
    Next j
 
    For k = 0 To 7
       tmpcmp(k) = tmpDS(k) - tmpUS(k) 'tmpCmp will be negative if US flows are > DS flows (i.e. flows are not increasing in the DS direction)
       If tmpcmp(k) < 0 Then
          Worksheets(outputsheet).Cells(outrw, 1).Value = NextDownID 'creates a row in the output sheet for the NextDownID. Only occurs if a DS flow need sto be replaced
          NextDownNEW = Application.VLookup(NextDownID, Worksheets(inputsheet).Range("A:B"), 2, False) 'Determines the NextDownID associated with the Output Sheet HydroID i.e. InputSheet HydroID's NextDownID's NextDownID
          Worksheets(outputsheet).Cells(outrw, 2).Value = NextDownNEW 'Prints NextDownNEW. Useful for iterating macro
          Worksheets(outputsheet).Cells(outrw, k + 3).Value = tmpUS(k) 'Prints flows that need to be revised
          FailCount = FailCount + 1
       Else
 
       End If
    Next k
 Else
 End If
 
 If Sheets(outputsheet).Cells(outrw, 1).Value <> 0 Then
    outrw = outrw + 1 'If the Loop printed a HydroID, add 1 to outrow so on the next iteration the macro prints to the next row
 Else
 End If
 
    rw = rw + 1
Loop

[DarrenWiens2]

Make a join, then calculate.

edit: using Python, you may be able to read all features of one feature class into a dictionary (depending on feature class size), then go through the features of the other feature class using an Update Cursor to change values.

[IsaacRemboldt1]

I think I phrased my question a bit incorrectly, by feature I meant a single row in the same shapefile. So I would compare a Unique ID in field one with all the downstream IDs in field 2 for every feature to find a match within the same shapefile. I have a visual basic example of this code for excel, but I was trying to get a working python example for doing it directly in ArcMap.

[DarrenWiens2 · Accepted solution]

Ah, in that case, I would read all your features into a dictionary, using Downstream ID as the key, then loop through all the features with an Update Cursor and look up the values in the dictionary to populate the current feature in the cursor, matching the current Upstream ID to the key in the dictionary.

[IsaacRemboldt1]

Thanks for this tip, I had started messing around with the "table to numpy array" tool but using a dictionary instead of an array might work better.

[DarrenWiens2]

You could do that, too. In any case, the pattern is similar: make a copy, join/match, calculate.