Math oddity between data step and proc fcmp

[JimHarrison]

SAS 9.4M5
EG 7.14 x64

Data Step used for testing and debugging:

data _null_;

length szName $ 12;

szCourse = 'PHIL ';

szNmbr1 = '112 ';

szNmbr2 = '112LAB';

intLen = lengthn(cats(szCourse, szNmbr1));
intInsert = 11 - intLen;
szName = cat(trim(szCourse), repeat(' ', intInsert), trim(szNmbr1));
intNameLen = lengthn(szName);
intNameLen = length(szName);

intLen = lengthn(cats(szCourse, szNmbr2));
intInsert = 11 - intLen;
szName = cat(trim(szCourse), repeat(' ', intInsert), trim(szNmbr2));
intNameLen = lengthn(szName);
intNameLen = length(szName);

run;

The data step works as expected; it concatenates the course and number, separated by a derived number of spaces to produce a left-aligned course name, x spaces, right-aligned course number that is always 12 total character long. This comports with the SAS documentation for the repeat() function as an n+1 result.

When I move the core code into a proc FCMP function, repeat() behaves differently for some reason - it produces an n+2 result, requiring me to adjust my math.

PROC FCMP OUTLIB=&IE_Functions;

FUNCTION Build_Course_ID(szSubj $, szNumber $) $ 12;
length szName $ 12;

intInsert = 10 - lengthn(cats(szSubj, szNumber));
szName = cat(trim(szSubj), repeat(' ', intInsert), trim(szNumber));

return(szName);

ENDSUB;

RUN;

[ChrisNZ · Accepted solution]

Works for me.
Careful how you manage the trailing spaces.

proc fcmp outlib=WORK.FUNCS.GEN;
  function Build_Course_ID(STR1 $, STR2 $) $12 ;
    LEN = 12 - length(cats(STR1, STR2));
    OUT = catx(repeat(' ', LEN-1), STR1, STR2);
    return( OUT );
  endsub;
run;
                                    
options cmplib=WORK.FUNCS;

data t; 
  length A $12;
  A=Build_Course_ID('xxxxx','yyyyy');
  put A=;
run;

A=xxxxx  yyyyy