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> Here's my concise definition: The LAG function returns the value
> passed as an argument in the previous call to the same occurrence of
> that same LAG function If this is the first time it has been
> called, it returns a missing value.
Jack,
Maybe a visual FIFO picture could be better memorized than a definition: The
value of an argument inserted in the back of the queue "displaces" the rest
of the items in the queue up by 1 position, ejecting the value in the front
of the queue as a result.
%let nlag = 4 ;
data _null_ ;
array q ( 99 ) _temporary_ ;
front = hbound (q) ;
link showqueue ;
do arg = 1 to 9, . ;
return = lag&nlag (arg) ;
front +- 1 ;
q (front - &nlag + 1) = arg ;
link showqueue ;
end ;
stop ;
showqueue: put arg 3. ' ---> ' @ ;
do pos = front - &nlag + 1 to front ;
put q (pos) 3. @ ;
end ;
put +2 ' ---> ' return 3. / @12 10*'-' ;
run ;
. ---> . . . . ---> .
----------
1 ---> 1 . . . ---> .
----------
2 ---> 2 1 . . ---> .
----------
3 ---> 3 2 1 . ---> .
----------
4 ---> 4 3 2 1 ---> .
----------
5 ---> 5 4 3 2 ---> 1
----------
6 ---> 6 5 4 3 ---> 2
----------
7 ---> 7 6 5 4 ---> 3
----------
8 ---> 8 7 6 5 ---> 4
----------
9 ---> 9 8 7 6 ---> 5
----------
. ---> . 9 8 7 ---> 6
----------
From the utilitarian programmatic standpoint, the difference between a queue
and array of the same size is thus: The queue provides an efficient internal
mechanism of moving all the elements up by one position via a single O(1)
isntruction, while providing an O(1) access only to the front item. The
array provides an O(1) access to any item, but moving all of the array items
up by 1 takes O(n) time.
Kind regards,
=====================
Paul M. Dorfman
Jacksonville, FL
=====================
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