R-experts, I have a bunch of files (by date) that I can read into
dataframes as below.

df$today:							
identifier	rtgmdy	rtgmdy_dt	rtgmdy_watch	rtgmdy_nowatch
rtgmdy_watch_dt	rtgsp	rtgsp_dt
310000031	Aa3	20050701		Aa3	NA	AA-
20050510
310000086	B1	20070920		B1	NA	B+
20070828
310000106	Baa2	20040326		Baa2	NA	BBB
20051003
310000170	Baa3	20070601		Baa3	NA	BBB+
20051024
310000225	Ba2	20070601		Ba2	NA	BB+
20070423
310000386	Baa3	20070320		Baa3	NA	BBB-
20040331
							
df$20050701:							
identifier	rtgmdy	rtgmdy_dt	rtgmdy_watch	rtgmdy_nowatch
rtgmdy_watch_dt	rtgsp	rtgsp_dt
310000031	Aa2	20030501		Aa3	NA	AA-
20050510

I want to re-cast the combination of df$today and df$20050701 as
follows:

PART A: Ideal result: Where I can have just identifiers and dates and
see what changed for that date(s), and also what it changed from. I
realize that it takes a combo of some nifty melt and cast work that my
iterations have thus far not resulted.

identifier	Chg Date	Var1	Var2	Var3=(Previous Var2)

310000031	20050701	rtgmdy	Aa3	Aa2
310000031	20050510	rtgsp	AA-				
310000086	20070920	rtgmdy	B1				
310000086	20070828	rtgsp	B+				

PART B: Less than ideal result (where we don't the lookup to get the
previous value). I'll live with this type of result also. I realize the
lookups can be expensive computationally.

identifier	Chg Date	Var1	Var2
310000031	20050701	rtgmdy	Aa3
310000031	20050510	rtgsp	AA-
310000086	20070920	rtgmdy	B1
310000086	20070828	rtgsp	B+

Any pointers? Thanks!
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