Reset count for one column while checking another to trigger reset

[ispyridis]

I have three columns articulated as below:
Cetegory | SubCategory ID | Subcategory

Cat 01
0001 SubCat1



0002 SubCat2

Cat 02
0001

SubCat3





0002 Subcat4


I need to generate column 2 numbers.


I want to use ROW or ROWS command to find position of category in 1st column then define a COUNTA staring from that ROW of 1st column and on until it reaches next record in 1st column.​

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