logic behind lag() when using with first. ??

[atul_desh]

I want the difference of latest amount & previous amount according to loan_id, In Try1 I'm getting not correct output then I tried Try2

which worked but i'm not able to understand why Try1 Got failed ??

Try1:

data loan1;
input loan_id date ddmmyy10. amount;
format date ddmmyy10.;
datalines;
1 10/11/2015 800
1 12/11/2015 400
2 09/11/2015 800
2 10/11/2015 0
3 01/11/2015 2000
1 11/11/2015 600
3 02/11/2015 800
3 03/11/2015 600
2 08/11/2015 1000
;
run;

proc sort data=loan1;
by loan_id descending date;
run;

data want;
set loan1;
by loan_id;
if first.loan_id then n=1;
else n+1;
if n=2 then diff_amt=amount-lag1(amount);
run;

proc print data=want;
run;

Output : 

SAS Output

Obs loan_id date amount n diff_amt 1 2 3 4 5 6 7 8 9

1	12/11/2015	400	1	.
1	11/11/2015	600	2	.
1	10/11/2015	800	3	.
2	10/11/2015	0	1	.
2	09/11/2015	800	2	200
2	08/11/2015	1000	3	.
3	03/11/2015	600	1	.
3	02/11/2015	800	2	0
3	01/11/2015	2000	3	.

Try 2:

This gave me correc o/p : 

data want;
set loan1;
by loan_id;
if first.loan_id then n=1;
else n+1;
prev_value=lag1(amount);
if n=2 then diff_amt=amount-prev_value;
run;

proc print data=want;
run;

[Tom]

LAG() returns values based on the previous calls. It knows nothing about observations or data step iterations.  The reason the first one does not do what you want is because use did not call LAG() for every value.  Try this little data step and look at the results.

data _null_;
  do i=1 to 10 ;
    lagall = lag(i);
    if mod(i,2)=0 then lageven=lag(i);
    else lagodd = lag(i);
    put (i lag: ) (=);
  end;
run;

[mkeintz]

This is a good opportunity to benefit from multiperior lags, and arrays with lower bounds of 0:

Regards

Mark

data _null_;
  array L{0:1} lageven  lagodd;
  do I=1 to 10;
    L{mod(i,2)} = lag2(i);
    put (i lag:) (=);
  end;
run;

[mkeintz]

This program does not require sorting.  Instead it holds the "lagged values" in a hash table.  No need for the lag function:

regards,

Mark

data loan1;
input loan_id date ddmmyy10. amount;
format date ddmmyy10.;
datalines;
1 10/11/2015 800
1 12/11/2015 400
2 09/11/2015 800
2 10/11/2015 0
3 01/11/2015 2000
1 11/11/2015 600
3 02/11/2015 800
3 03/11/2015 600
2 08/11/2015 1000
;
run;

data want;
  set loan1;
  attrib prior_amt length=8;
  if _n_=1 then do;
    declare hash h();
	 h.definekey('loan_id');
	 h.definedata('prior_amt');
	 h.definedone();
  end;
  rc=h.find();

  if rc^=0 then diff_amt=.;
  else diff_amt=amount-prior_amt;

  rc=h.replace(key:loan_id,data:amount);
run;