Looping over array, retaining values - sas

Question

I have a lot of data sets that I need to have the same structure - same variables, same order. I have a data set serving as a template ("all" in the code below). Other data are given this form by listing both the template data set (with obs=0) and a particular data set ("some" in the code below) in the same set statement. This works just fine.
I then want to loop through the variables. If one of them is missing (as it will be, if it's not present in the particular data set), it should be set to the value of the previous variable. var2 should get the value from var1 etc.
This should be done within each row. This works fine if done in a separate data step, but doesn't work if done in the same data step described above.
If done in the same data step, the values inserted for missing values will always be from row 1. Why is this? Can I achieve the wanted result without using another data step?
/* All the variables a complete data set should contain.*/
data all;
format var1-var5 $20.;
run;
/* Actual data have some of these variables, but not all. var1 is never missing, all other variables might be*/
data some;
var1="Obs 1, Value 1";
var4="Obs 1, Value 4";
output;
var1="Obs 2, Value 1";
var4="Obs 2, Value 4";
output;
run;
/* Not working - The values inserted when the conditional is true are all from row 1*/
data dont_want;
set all(obs=0) some;
array chars{*} _character_;
do i=1 to dim(chars);
if missing(chars{i}) then chars{i}=chars{i-1};
end;
drop i;
run;
/* Working*/
data temp;
set all(obs=0) some;
run;
data want;
set temp;
array chars{*} _character_;
do i=1 to dim(chars);
if missing(chars{i}) then chars{i}=chars{i-1};
end;
drop i;
run;

Answer 1

The values for the "extra" variables are being RETAINed since they are sourced from the ALL dataset. Any variable that is sourced from an input dataset is NOT reset to missing at the start of the data step iteration. Since those variables are not on the SOME dataset they do not change when an observation is read from it.
Just add code to reset them to missing. If you want to do it without knowing the names of the variables you might consider re-ordering the code.
You could define and clear the array after the compiler has "seen" the ALL dataset but before the run-time has read the SOME dataset.
data dont_want;
if 0 then set all;
array chars{*} _character_;
call missing(of chars{*});
set some;
do i=2 to dim(chars);
if missing(chars{i}) then chars{i}=chars{i-1};
end;
drop i;
run;
Or add an explicit OUTPUT statement and reset them after that.
data dont_want;
set all(obs=0) some;
array chars{*} _character_;
do i=2 to dim(chars);
if missing(chars{i}) then chars{i}=chars{i-1};
end;
drop i;
output;
call missing(of _all_);
run;

Answer 2

Couple things to do if you want to use implicit OUTPUT
Prep the PDV prior to the data reading set using a non-reading set
Set up array based on prepped PDV
Clear the array
Read the data with set
Impute your data
output
Example:
data dont_want;
if 0 then set all some; * non reading set preps the PDV;
array chars{*} _character_;
call missing(of chars(*)); * clears all auto-retained data set variables;
set all(obs=0) some; * data reading set;
* shift right an array requires left to right processing;
do i=dim(chars) to 2 by -1;
if missing(chars{i}) then chars{i}=chars{i-1};
end;
*** OR COPY right into empty slots, repeating prior copy if needed;
do i=2 to dim(chars);
if missing(chars{i}) then chars{i}=chars{i-1};
end;
drop i;
* implicit output;
run;