ArcGIS polyline file How to update column in attribute table

I have three columns

Before update

from node    to node    gridcode   
2            3          8
3            2          9
7            2          5

After update, I want "node column" to be same as "grid code" column, so I used field calculator in ArcGIS (from node = gridcode). My "from node" column has been changed to gridcode but not sure how to update "to node" column (that depends on values in “from node” column)

Below is the final result I want:

from node    to node    gridcode
8            9          8
9            8          9
5            8          5

So far I have tried "join", "spatial join" but I am not getting the required results. How can I do that in Python or ArcGIS?

Solution

The only way to do this I can see is with Python.

First you need to build a dictionary to understand what the relationship is between the old node values and new node values -- old_node and new_node. This can be done with a SearchCursor, matching up the "from node" (old_node) to the gridcode (new_node).

# instantiate empty dictionary
node_values = {}
# build dictionary of values
with arcpy.da.SearchCursor(feature_class, ["fromnode", "gridcode"]) as cursor:
    for row in cursor:
        old_node = row[0]
        new_node= row[1]
        node_values[old_node] = new_node

Then, you need to update all your values. This needs an UpdateCursor (somewhat like Field Calculator, but lets you do more since you're working within Python).

with arcpy.da.UpdateCursor(feature_class, ["fromnode", "tonode", "gridcode"]) as cursor:
    for row in cursor:
        # set fromnode to gridcode value
        row[0] = row[2]
        # set tonode to new equivalent node
        to_node = row[1]
        new_to_node = node_values[to_node]
        row[1] = new_to_node
        cursor.updateRow(row)

Note that there are a lot of ways this code can be "tightened up", improved, and shortened, and there are variables that need to be assigned (e.g. feature_class) before those snippets will work properly. As always, I recommend making a backup of your data before messing around in it with somebody else's Python scripts ;)