Remove unused variables from many files

* QUESTION:Given files (year1972, year1973, year1974) with unused variables, how can I delete all unused variables. 
* Note: A variable with a constant value for all cases is considered an unused variable.

* SOLUTION.
* (was tested with version 10.07).
* Raynald Levesque 2000/09/23.

* assumptions: 	the number of variables is the same in all files.
* 		Variables must all be numeric.
* Comments: This syntax is not completely general.
* You will have to replace all paths, then.
* enter the total number of variables in the file (see *### (note 2) below).
* enter the name of the first and last variables in the file ((see *### (note 3) below).


*****************************************.
* 3 data files are created for illustration purposes.
*****************************************.
* first data file contains a missing value (for variable d) to show that this var is also deleted by syntax.
* The missing value generates a warning message which is to be ignored.
DATA LIST LIST /a b c d z.
BEGIN DATA
1 2 2 -1 2
1 1 2 . 1
1 2 2 -1 3
1 1 2 -1 5
END DATA.
LIST.
* we will want to keep only variables b and z.
SAVE OUTFILE='c:\\data\\year1972.sav'.

DATA LIST LIST /a b c d z.
BEGIN DATA
2 2 2 4 2
2 1 2 5 2
2 2 2 -1 2
2 1 2 -1 2
END DATA.
LIST.
* we will want to keep only variables b and d.
SAVE OUTFILE='c:\\data\\year1973.sav'.

DATA LIST LIST /a b c d z.
BEGIN DATA
3 2 2 4 5
3 2 2 4 2
3 2 2 -1 2
3 2 2 -1 2
END DATA.
LIST.
* we will want to keep only variables d and z.
SAVE OUTFILE='c:\\data\\year1974.sav'.

*****************************************.
* this is the end of preparatory work.
*****************************************.

SET MPRINT=yes.

* Define a macro to do one file (year) at a time.

*////////////////////////////////////////////////.
DEFINE !delete(year=!TOKENS(1)).

GET FILE=!QUOTE(!CONCAT('c:\\data\\year',!year,'.sav.')).

* Save the variable names in a file.
N OF CASES 1.
FLIP.
COMPUTE varnum=$casenum.
SAVE OUTFILE=!QUOTE(!CONCAT('c:\\data\\vn',!year,'.sav.')).
CACHE.
EXE.

GET FILE=!QUOTE(!CONCAT('c:\\data\\year',!year,'.sav.')).

*### (note 2) replace 5 by number of variables in original file. 
*### (note 3) replace a and z by name of first and last variable in file.
RENAME VARIABLE (a TO z = var1 TO var5).
COMPUTE dum=1.

* If the SD of a variable is zero, it means all cases have same value.
* since variables have been renamed with consecutive names (var1, var2 etc), the aggregate
* command does not require us to list each variable name.

*### (note 2) replace 5 by number of variables in original file. 
AGGREGATE
  /OUTFILE=*
  /BREAK=dum
  /var1 TO var5 = SD(var1 TO var5).
FLIP.
* Get rid of the dummy case.
SELECT IF SUBSTR(case_lbl,1,3)<>"DUM".

*Retrieve the original variable names.
MATCH FILES FILE=* /DROP=case_lbl.
COMPUTE varnum=$casenum.
EXECUTE.

* Keep only name of unused variables.
SELECT IF var001=0.
MATCH FILES /FILE=*
 /TABLE=!QUOTE(!CONCAT('c:\\data\\vn',!year,'.sav.'))
 /RENAME (var001 = d0)
 /BY varnum
 /DROP= d0.

*write a syntax file to delete these variables.
WRITE OUTFILE='c:\\data\\delete vars.sps' /"MATCH FILES FILE=* /DROP=" case_lbl ".".
EXE.

* Apply syntax to original file.
GET FILE=!QUOTE(!CONCAT('c:\\data\\year',!year,'.sav.')).
INCLUDE 'c:\\data\\delete vars.sps'.
SAVE OUTFILE=!QUOTE(!CONCAT('c:\\data\\y',!year,'.sav.')).
!ENDDEFINE.
*////////////////////////////////////////////////.


*### (note 4) in next macro, replace 1974 by the last year you have.

*////////////////////////////////////////////////.
DEFINE !main().
!DO !y=1972 !TO 1974.
!delete year=!y.
!DOEND.
!ENDDEFINE.
*////////////////////////////////////////////////.

* Call main macro to do the entire job.
!main.