feat: mp_prevobs.sas macro

base/mp_prevobs.sas

@@ -0,0 +1,88 @@
+/**
+  @file
+  @brief Enables previous observations to be re-instated
+  @details Remembers the last X observations by storing them in a hash table.
+  Is a convenience over the use of lag() or retain, when an entire observation
+  needs to be restored.
+
+  This macro will also restore automatic variables (such as _n_ and _error_).
+
+  Example Usage:
+
+      data example;
+        set sashelp.class;
+        calc_var=_n_*3;
+        * initialise hash and save from PDV ;
+        %mp_prevobs(INIT,history=2)
+        if _n_ =10 then do;
+          * fetch previous but 1 record;
+          %mp_prevobs(FETCH,-2) 
+          put _n_= name= age= calc_var=; 
+          * fetch previous record;
+          %mp_prevobs(FETCH,-1) 
+          put _n_= name= age= calc_var=; 
+          * reinstate current record ;
+          %mp_prevobs(FETCH,0) 
+          put _n_= name= age= calc_var=;
+        end;
+      run;
+
+  Result:
+
+  <img src="https://imgur.com/PSjHoET.png" alt="mp_prevobs sas" width="400"/>
+
+  Credit is made to `data _null_` for authoring this very helpful paper:
+  https://www.lexjansen.com/pharmasug/2008/cc/CC08.pdf
+
+  @param action Either FETCH a current or previous record, or INITialise.
+  @param record The relative (to current) position of the previous observation 
+   to return.  
+  @param history= The number of records to retain in the hash table. Default=5
+  @param prefix= the prefix to give to the variables used to store the hash name
+   and index. Default=mp_prevobs
+
+  @version 9.2
+  @author Allan Bowe
+
+**/
+
+%macro mp_prevobs(action,record,history=5,prefix=mp_prevobs
+)/*/STORE SOURCE*/;
+%let action=%upcase(&action);
+%let prefix=%upcase(&prefix);
+%let record=%eval((&record+0) * -1);
+
+%if &action=INIT %then %do;
+    
+  if _n_ eq 1 then do; 
+    attrib &prefix._VAR length=$64; 
+    dcl hash &prefix._HASH(ordered:'Y');
+    &prefix._KEY=0;
+    &prefix._HASH.defineKey("&prefix._KEY"); 
+    do while(1); 
+      call vnext(&prefix._VAR); 
+      if &prefix._VAR='' then leave;
+      if &prefix._VAR eq "&prefix._VAR" then continue; 
+      else if &prefix._VAR eq "&prefix._KEY" then continue; 
+      &prefix._HASH.defineData(&prefix._VAR);
+    end; 
+    &prefix._HASH.defineDone(); 
+  end;
+  /* this part has to happen before FETCHing */
+  &prefix._KEY+1;
+  &prefix._rc=&prefix._HASH.add();
+  if &prefix._rc then putlog 'adding' &prefix._rc=;
+  %if &history>0 %then %do;
+    if &prefix._key>&history+1 then 
+      &prefix._HASH.remove(key: &prefix._KEY - &history - 1);
+    if &prefix._rc then putlog 'removing' &prefix._rc=;
+  %end;
+%end;
+%else %if &action=FETCH %then %do;
+  if &record > &prefix._key then putlog "Not enough records in &Prefix._hash yet";
+  else &prefix._rc=&prefix._HASH.find(key: &prefix._KEY - &record);
+  if &prefix._rc then putlog &prefix._rc= " when fetching " &prefix._KEY=
+    "with record &record and " _n_=;
+%end;
+
+%mend;