Comparing value with value from the next row

Question

I have a table with two elements in this format:
a b
b a
b c
c b
c d
....
f g
g f

I have to analize if the second element is equal to the first element of the following row. If it's true, 1 is set in a new field. If not the marker is added by 1 +1. This is my code so far:

with arcpy.da.SearchCursor(tab_selektiert, ["field1"],) as n1_cur:
    with arcpy.da.UpdateCursor(tab_selektiert, ["field2", "markerfield"]) as n_cur:
        filler_row = next(n1_cur)
        for n_row, n1_row in izip_longest(n_cur, n1_cur, fillvalue=filler_row):
            if n1_row[0] == n_row[0]:

                n_row1 = counter
                n_cur.updateRow(n_row)
            else:
                numcounter = + 1
                n_row1 = counter
                n_cur.updateRow(n_row)


del n_cur
del n1_cur

My problem is that beside the search cursor the n_cursor also skip one line. As result I only get b=b, c=c and missing a=b [..]

---

Edit
My results should be something like this:

the field_2[0] and field_1[0](next row value) or field_1[0] and field_2[0](next row value) have to be equal (blue arrows).
If so a marker (started with 1) has to set in the markerfield. In the picuture from line 6 to 7, the continuity makes a step forward and the marker has to update by 1 in line 7. Line 6 has '1' as marker even while the continuity breaks, because 'c' is in linie before.

---

Edit 2

counter = 1
numberlist = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0"]


with arcpy.da.UpdateCursor(tab_selektiert, (ufield1, ufield2)) as u_cur:
    with arcpy.da.SearchCursor(tab_selektiert, sfield1) as s_cur:
        next(s_cur)
        for sfield1, in s_cur:
            ufield1, ufield2 = next(u_cur)

        print("field 1  " + sfield1 + "---" + "  field 2   " + ufield1)
        print("field 2 + 1   " + str(int(ufield1[-1:]) + int(1)))
        print("fiel 1   " + str(sfield1[-1:]))
        print("---")

        if sfield1 == ufield1:
            ufield2 = counter
        elif sfield1[-1] in numberlist and ufield1[-1] in numberlist:
            if int(ufield1[-1:]) + int(1) == int(sfield1[-1]):
                ufield2 = counter
            elif ufield1[-1] == "9" and sfield1[-1] == "0":
                ufield2 = counter
        else:
            counter+=1
            ufield2 = counter

        u_cur.updateRow((ufield1, ufield2))

This gets me to the Point that all fields with a continuity get the marker 1. But a break in the continuity like 'd' to 'f' don't activate the else clause. This fields are just empty, also the last row and the following rows still gets the 1 as marker.

Answer 1

Like @PolyGeo said, list comprehensions will help. I notice, too, that you don't define counter in your code block. Without understanding what it is, I've included it as I think you meant to have it.

    field1_list = [row[0] for row in arcpy.da.SearchCursor(tab_selektiert, ["field1"])]
    field2_list = [row[0] for row in arcpy.da.SearchCursor(tab_selektiert, ["field2"])]

    #Assuming here that counter must initialize at 1. If if should initialize at 0,
    # just change it.
    counter = 1 
    index = 0

    with arcpy.da.UpdateCursor(tab_selektiert, ["markerfield"]) as updater:
        for row in updater:
            #The value of index should never exceed len-2, since len is always
            # 1 more than the highest index value, and your function will only
            # check up to the next-to-last item in the list (the last item has
            # nothing to compare to
            if index<=len(field1_list)-2:
                n0 = field1_list[index] #The field1 value for current index
                n1 = field2_list[index] #The field2 value for the current index
                n2 = field1_list[index+1] #The field1 value for the next index
                n3 = field2_list[index+1] #The field2 value for the next index

                if n1==n2 or n3==n0 or n0==n2:
                    #EDIT: Index only increases if none of the numbers
                    # match. The markerfield is always updated with
                    # counter. 
                else:
                    #This is the condition where counter is increased
                    counter+=1

                updater.updaterRow([counter])
                index+=1

            else:
                #If you've reached the last item, update the last row with counter,
                # then break out of the loop
                updater.updateRow([counter])
                break

It's worth noting that you said counter= +1, but in python the way to do what you're trying to do is as I have it: counter+=1. I hope this helps!

Answer 2

Iterate your table and assign your value and the oid to variables. Create a set of OIDs with values equal to the next value by comparing the last oid to the current. Iterate one more time and update the field by checking OIDs.

oids = set ()

#perform check
with arcpy.da.SearchCursor(tab_selektiert, ["field1", "field2", "OID@"]) as curs:
    for fld1, fld2, oid in curs:
        try: lastVal
        except NameError:
            lastVal = fld2
            lastOid = oid
            continue
        if fld1 == lastVal:
            oids.add (lastOid)
        lastVal = fld2
        lastOid = oid


#update
counter = 0
with arcpy.da.UpdateCursor(tab_selektiert, ["OID@", "markerfield"]) as curs:
    for oid, mark in curs:
        if not oid in oids:
            counter += 1
        row = (oid, counter)
        curs.updateRow (row)

Answer 3

This is another way you can do it:

import arcpy
fc = r'C:\Default.gdb\adress1'
field_to_read = 'Textfield'
field_to_update = 'Integerfield'

all_letters = ''.join([i[0] for i in arcpy.da.SearchCursor(fc,field_to_read)]).replace(' ','') #Create one long string, 'abbabccbcd'
cursor = arcpy.da.UpdateCursor(fc,field_to_update)
count = 1
for pair,row in zip(zip(all_letters[1::2],all_letters[2::2]),cursor): #Iterate over pairs of letters (last letter, first letter) -> (b,b), (a,b), (c,c) etc. and the cursor
    if len(set(pair))==1: #If the letters are the same, length of set will be 1
        row[0] = count
        cursor.updateRow(row)
    else:
        count+=1
del cursor