In Search of a Solution that Auto Fills Cells

Which version of Excel are you using? Have you upgraded from Excel 2010?

If you have PowerQuery (Get & Transform on the Data ribbon in versions from 2016 onwards), it's a simple fill down operation:

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Item#", Int64.Type}, {"Data", type text}}),
    #"Filled Down" = Table.FillDown(#"Changed Type",{"Item#"})
in
    #"Filled Down"

Excel 2016 (Windows) 32 bit

	F	G
3	Item#	Data
4	123	data
5	123	data
6	123	data
7	123	asdf
8	123	data
9	123	data
10	456	data
11	456	data
12	456	data
13	456	data
14	789	data
15	789	data
16	789	data
17	789	data
18	789	data
19	789	data
20	789	data
21	789	data
22	789	data
23	321	data
24	321	data
25	321	data
26	654	data
27	987	data
28	987	data

Sheet: Sheet1

Thanks Ali, I am using office 365. I am not familiar with Powerquery.

In A4, copied down:

=IF(A3=C4,A3,C4)

OK - so please update your forum user profile.

Are you interested in this solution? If so, I can explain it to you.

Then use:

=IF(A4="",C3,A4) in C4 copied down

and (if desired) copy/paste values back onto column A

You're welcome.

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