Concatenate multiple equalities together

[kimih0223]

I have a SAS Datasets with multiple equalities: i.e. A=C, B=C etc. and I would like to make concatenate it into one line to A=B=C.

For example IDs in 1 and 2 represent equality,

ID1, ID2;

1,3;

2,3;

I would like to make these into 

ID1, ID2, ID3;

1, 2, 3;

Would there be a good algorithm to do this?

[ChrisNZ · Accepted solution]

The hash table is the best suited I reckon.

 Assuming data set sorted and organised so that ID1 is smaller than ID2.

data HAVE;
input ID1  ID2  ;
cards;
1 3
2 3
5 9
6 9 
;
run;
data _null_;
  retain GRP;
  if _N_=1 then do;  
    ID=.; GROUP=.;
    dcl hash H(ordered:'a');
    H.definekey('ID');
    H.definedata('ID','GROUP');
    H.definedone();
  end;
  set HAVE end=LASTOBS;;
  RC1=H.find(key:ID1);
  if RC1=0 then GRP1=GROUP;
  RC2=H.find(key:ID2);
  if RC2=0 then GRP2=GROUP;
  if RC1=0 and RC2=0 then return;
  else if RC1=0 then do;
    H.add(KEY:ID2, data:ID2, data:GRP1);
  end;
  else if RC2=0 then do;
    H.add(KEY:ID1, data:ID1, data:GRP2);
  end;
  else do;
    GRP+1;
    H.add(key:ID1, data:ID1, data:GRP);
    H.add(key:ID2, data:ID2, data:GRP);
  end;
  if LASTOBS then H.output(dataset:'WANT');
run;
proc print data=WANT;
run;

Obs	ID	GROUP
1	1	1
2	2	1
3	3	1
4	5	2
5	6	2
6	9	2