How to calculate a windowed rank function?

[daviewales]

Suppose I have some sorted data like this:

test
A
A
B
C
C
C
D
D

And I want to rank the items in order, but restart the rank each time the group changes:

test	rank
A	1
A	2
B	1
C	1
C	2
C	3
D	1
D	2

Can I do this with Visidata?

The closest I've gotten is using the Window function: w with the argument 1 0, then creating a new =Python column:

=1 if test != test_previous else None

This gives me the first value of each group, but doesn't give me the subsequent ranks.
I saw this issue which suggests it might not currently be possible, but thought I'd ask in case there was another way: #1399

(Note, also working around #2279 by creating a new frozen column with ' and manually setting the value of the first cell to NULL with zd.)

[midichef]

Hi, yes, you can do this with Visidata. It took me a while to remember how. I did it with your method: mark the start of the group. The two other key steps are:

1. Get the row number, using i (addcol-incr).
2. Let each row see the row number of the start of its group. Select all rows with gs, then f to fill nulls.
   
   The expression for first is w[0] != w[1] if len(w) == 2 else True.

[daviewales]

That is extremely clever. A bit too clever for me to remember it I'm sure, but it will be here for future reference now. Thanks!