Calculating several cells into one cell in same table using ArcPy

Question

I'm very new to Python and scripting in ArcGIS. I need to calculate some cells from one field in table into another one cell in another field in same table.

Here's what i need:

enter image description here

Here's what i try and it doesnt work.

A = arcpy.da.SearchCursor("S2_merge2", ["Name1"] , 'FID = 0')
B = arcpy.da.SearchCursor("S2_merge2", ["Name1"] , 'FID = 1')
C = arcpy.da.SearchCursor("S2_merge2", ["Name1"] , 'FID = 2')

cursor = arcpy.da.UpdateCursor("S2_merge2", ["Name5"] , FID = 0')

for row in cursor: 
 row[0] = A+B+C 
 cursor.updateRow(row)

• GISGe: Do you need to calculate field 'Name5' for all rows or only for the first row of your table?
• Aaron: How would you handle naming the last row in the field "Name5"? A complete example of your intended output would be helpful.
• Fox: I need result as shown in picture - only one cell in Name5 should be changed (first row).

Answer 1

first copy the column "Name1" in a python list

features = [[row[0] for row in arcpy.da.SearchCursor("S2_merge2", ("Name1") )]

then use the indices with your search cursor (note that I use a selection to avoid troubles with the last 2 rows, as you did not tell how to manage those)

cursor = arcpy.da.UpdateCursor("S2_merge2", ["Name5"], 'FID < ' + str(int(arcpy.GetCount_management("S2_merge2").getOutput(0))-2) )
i=0
for row in cursor: 
 row[0] = features[i] + features[i+1] + features[i+2]
 cursor.updateRow(row)
 i+=1

with a single row :

fid_val = 42 #42 is just an example
cursor = arcpy.da.UpdateCursor("S2_merge2", ["Name5"], 'FID = ' + str(fid_val) )
for row in cursor: 
 row[0] = features[fid_val] + features[fid_val+1] + features[fid_val+2]
 cursor.updateRow(row)

• Fox: as I understand - " .getOutput(0))-2) ) " means that ALL-2 row's will be updated - is there any way to Update only one row? (for exemple i dont know quantity of rows in table)
• radouxju: you can use any query as third argument , so FID = 0 would dot the trick for a single row. Note that if you start from another FID value than 0, you need to initialize i to this FID value. Also note that getcount will give you the quantity of rows, so you don't have to worry about not knowing it