Insert (internally existing) column headers as first row to a table - sas

Question

Assume that we have a table INPUT_TABLE which has four columns name, lat, lon, and z, filled with many data sets. In the SAS Explorer it would e.g. look like this:
name lat lon z
1 Germany 49.420469 8.7269178 17
2 England 51.5540693 -0.8249039 16
...
I handover a PREPROCESSED_TABLE based on this INPUT_TABLE to a macro %tabl:
data V42.PREPROCESSED_TABLE;
set V21.INPUT_TABLE;
drop NAME;
run;
%tabl(libin=V42, file=PREPROCESSED_TABLE);
The macro itself I am not allowed to modify.
Among other things, %tabl also writes a plain text file PREPROCESSED_TABLE.txt:
49.420469|8.7269178|17
51.5540693|-0.8249039|16
I would like to have the header names written out as well, e.g.:
lat|lon|z
49.420469|8.7269178|17
51.5540693|-0.8249039|16
My idea is to expand the PREPROCESSED_TABLE somewhere in the data step - could somebody help me with that, please? How can I read out the header names which are internally stored?

Answer 1

If the goal is to make a file with one line with the variable names then just write the file yourself. First get the names into a dataset (in order) and then write them. For example you could use PROC TRANSPOSE with OBS=0 dataset option to generate a file with one observation per variable.
proc transpose data=V42.PREPROCESSED_TABLE(obs=0) out=NAMES ;
var _all_ ;
run;
Which you can then use to write to a file.
data _null_;
set names ;
file 'preprocessed.txt' dsd dlm='|';
put _name_ # ;
run;
If you also want to add the data to that same file just use a second data step. Make sure to use the MOD option on the FILE statement so that data lines are appended to the existing file.
data _null_;
set V42.PREPROCESSED_TABLE;
file 'preprocessed.txt' dsd dlm='|' mod;
put (_all_) (+0);
run;
If you need to call the existing macro for other reasons you could either ignore the file it creates. Or if for some reason the content is different than just the simple dump of the file then you could just concatenate the file with the the headers with the file the macro generates. Say the macro generated 'PREPROCESSED_TABLE.txt' and your code generated the one line file 'headers.txt'. Then this step will read both and write 'PREPROCESSED_TABLE_w_headers.txt';
data _null_;
file 'PREPROCESSED_TABLE_w_headers.txt';
if _n_=1 then do;
infile 'headers.txt';
input;
put _infile_;
end;
infile 'PREPROCESSED_TABLE.txt';
input;
put _infile_;
run;

Answer 2

Given Reeza's and Tom's hints, I figured out a workaround myself: We simple call out macro %tabl twice, once with a 1-row-table with column-names and once with the data. This approach essentially corresponds to attaching to the file first the headers and then then data to the file (except that I have to worry about additional things added by %tabl further down in the process chain).
The technical difficulty I had was how to extract this 1-row-table with column names from the meta-info of the table input table V21.INPUT_TABLE.
My team mate showed me how that is done. To make it testable for everybody, I will show this step for the test data table sashelp.class:
proc contents data=sashelp.class out=meta (keep=NAME VARNUM) noprint;
run;
proc sort data=meta out=meta2;
by VARNUM;
run;
proc transpose data=meta2 out=colheaders (drop=_NAME_ _LABEL_);
var name;
run;
As a result, we will have a table colheaders with exactly one line containing the table headers, sorted by VARNUM which is the order in which they appear in the original table:
COL1 COL2 COL3 COL4 COL5
1 NAME SEX AGE HEIGHT WEIGHT
Problem solved, at least theoretically.