Assign to last known value

[Stillmatic]

I'm not well versed in SAS language, just enough to get by with basic logic in regards to data and proc sql steps, maybe a sort here and there.

Hoping for some direction on a problem I'm facing.

Dataset

DATE	CUST_ID	SYSTEM	COLOR
3/31/2021	15	ORA	BLUE
4/30/2021	13	ORA	BLUE
5/31/2021	10	ORA	BLUE
6/30/2021	19	ORA	BLUE
7/31/2021	11	ORA	BLUE
8/31/2021	14	ORA	BLUE
9/30/2021	21	ORA	BLUE
10/31/2021	16	ORA	BLUE
11/30/2021	12	ORA	BLUE
12/31/2021	17	ORA	BLUE
1/31/2022	22	ORA	BLUE
2/28/2022	18	ORA	BLUE
3/31/2022	20	SIS	UNK
4/30/2022	23	SIS	UNK

The issue: When a customer switches to another system their color data goes missing. Hence the return value of unknown. What I need assistance in is coming up with the right logic to assign the unknown fields to the last known value. Now it doesn't have to be the last known value as theoretically the color should never change once set, until it loads into another system.

So the dataset I want to return is:

DATE	CUST_ID	SYSTEM	COLOR
3/31/2021	15	ORA	BLUE
4/30/2021	13	ORA	BLUE
5/31/2021	10	ORA	BLUE
6/30/2021	19	ORA	BLUE
7/31/2021	11	ORA	BLUE
8/31/2021	14	ORA	BLUE
9/30/2021	21	ORA	BLUE
10/31/2021	16	ORA	BLUE
11/30/2021	12	ORA	BLUE
12/31/2021	17	ORA	BLUE
1/31/2022	22	ORA	BLUE
2/28/2022	18	ORA	BLUE
3/31/2022	20	SIS	BLUE
4/30/2022	23	SIS	BLUE

Thanks for your help in advance!

[mkeintz]

In the case of unknown color you want to "assign the unknown fields to the last known value."   Apparently, you are ok with assigning a color to CUST_ID 20 with a known value from CUST_ID 18.  Is that correct?

Assuming the answer is yes, then the code below does what you need (untested in the absence of sample data in the form of a working data step).

data want (drop=_:);
  set have;
  length _lastknowncolor $4;
  retain _lastknowncolor ;
  if   color^= 'UNK' then _lastknowncolor=color;
  else color=_lastknowncolor;
run;

The key here is the RETAIN statement which tells SAS not to reset the retained variable to missing with each new iteration of the data step (i.e. each incoming obs in the above case).