Creating variables based on other variables in SAS - sas

Question

I'm looking to create a variable based on this data sample:
Video Subject Pre_post Pre_Post_ID
1 1 0 1
1 2 0 1
1 2 0 1
1 3 0 1
1 3 0 1
2 1 1 1
2 1 1 1
2 2 1 1
2 2 1 1
2 3 1 1
4 1 0 2
4 2 0 2
4 2 0 2
4 3 0 2
4 3 0 2
5 1 1 2
5 1 1 2
5 2 1 2
5 2 1 2
5 3 1 2
The goal of the variable will be to create an ID that links the pre_post variable to the subject on the condition that the pre_post_id is the same:
Video Subject Pre_post Pre_Post_ID Subject_P_P_ID
1 1 0 1 1
1 2 0 1 2
1 2 0 1 2
1 3 0 1 3
1 3 0 1 3
2 1 1 1 1
2 1 1 1 1
2 2 1 1 2
2 2 1 1 2
2 3 1 1 3
4 1 0 2 4
4 2 0 2 5
4 2 0 2 5
4 3 0 2 6
4 3 0 2 6
5 1 1 2 4
5 1 1 2 4
5 2 1 2 5
5 2 1 2 5
5 3 1 2 6
Thank you in advance for the help!

Answer 1

You will want to track the pairs (<pre_post_id>,<subject>) as a composite key and increment the Subject_P_P_ID every time a new pair (or key) is encountered.
To simplify the discussion, call the two items in the pair item1 and item2
Here are two ways:
Sort by item1 item2, step through BY item1 item2 and track pair count using logic based on an automatic first. variable -- pair_id + (first.item2), or
Track pairs as keys of a hash and assign new id as <hash>.num_items + 1 when key lookup fails.
Sort + Data Step + Revert Sort
proc sort data=have out=have_sorted;
by item1 item2;
run;
data have_sequenced;
set have_sorted;
by item1 item2;
item1_item2_pair_id + (first.item2);
run;
proc sort data=have_sequenced out=want;
by video subject pre_post pre_post_id item1_item2_pair_id;
run;
Hash
data want;
set have;
if _n_=1 then do;
declare hash lookup();
lookup.defineKeys('item1', 'item2');
lookup.defineData('item1_item2_pair_id');
lookup.defineDone();
end;
if lookup.find() ne 0 then do;
item1_item2_pair_id = lookup.num_items+1;
lookup.add();
end;
end;