SAS Do-Loop and IF Statement to compare Current and previous row values

I’m pretty new with do loops in SAS and I know that I am trying to make this loop work like a MATLAB script. I haven’t found many helpful tips online as most of the do-loop examples are just for calculations, not actually checking to see if the row before the current one has the same value.

Here is my issue that I need to solve:

I want to look at each policy numbers below and see if the one before is the same, if it is, I want to flag it.

I would consider i=1 to be the first policy(26X0118907) and i=2 to be the second policy (26X0375309).

In this case according to the code (that doesn't work) below this increment would be flagged as ‘B’. Do you know how to properly code a situation like this?

data AF_Inforce_&thestate.;
set AF_Inforce_&thestate.;
by Rating_St;

if first.Rating_St then counter=0;
counter+1;

myloop:
do i=2 to counter;
P2(i)=Policy(i);
P1(i)=Policy(i-1);

if P1(i)=P2(i) then flag='A';
else flag='B';

end;
return;

run;

Solution

The first thing you need to learn coming from MATLAB or a similar language is that SAS is different. In particular, the DATA step is its own DO loop, looping over records.

Second, it's a bit complicated to access data accross rows. However, there are a few tricks.

Vasja showed you one (lag, which doesn't actually go to a previous record, but sort of acts like it does). dif does the same thing except it compares, so if your policynum had been numeric, Vasja's code could be rewritten as dif(policy)=0 instead of policy=lag(policy)(though this is only for numerics).

A better trick in my opinion in your case is to use by group processing. Normally this works with sorted fields, but here it doesn't matter if it's sorted: you just want to know if two consecutive rows are identical, right?

data want;
  set have;
  by rating_st policy notsorted;
  if first.policy and last.policy then recflag='A';
  else if first.rating_st then recflag='A';
  else recflag='B';
run;

I don't know that I understand your rules entirely, but they're probably going to be some form of this. I put the two possibilities there, you might just want the second one (ie, you don't care if it's singular or just the first). The first would flag only singular policies.