Get all possible crossproducts of pairs of variables

 

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*(Q) I need to create all possible combinations of crossproducts from a range of 
	variables. That is to say, I need to multiply each variable by all the other
	variables and save the products as new variables. The matrix is contiguous 
	so I figure I can use a range function in SPSS. I am having difficulty figuring
	out how to do this. As there are 168 column vectors in this data-set, the 
	task is too formidable to attempt one variable at a time.
*	Can anyone suggest a solution.

*(A) Posted by rlevesque@videotron.ca to SPSSX-L on 2002/01/27.

*****************.
* PART A.

* The following answers the question: new variables are created for each product.
* The problem is that it is very difficult to use the resulting variables because they do not
	have descriptive labels.
* PART B below fills that gap.
*****************.

* Create a dummy data file of 5 variables.
* The method works for up to the maximum number
	of variables supported by SPSS which is about 32, 000.
* In terms of the initial number of variables, this means around 250 variables.

* You will have to change the numbers 4,5 and 10 in the syntax to the numbers
	 corresponding to your actual data file. 

DATA LIST LIST /agecateg var2 crit19 cntry lastvarn.
BEGIN DATA
1 2 3 4 5 6
2 5 1 3 6 4 
2 5 1 4 7 3
2 5 8 9 6 1
END DATA.
LIST.
SAVE OUTFILE='c:\\temp\\mydata.sav'.

* given n variables, there are (n-1)*n/2 crossproducts.
* When n=5, there are 10 cross products, hence we need cp1 to cp10 to hold 
	the cross products.
VECTOR v=agecateg TO lastvarn /cp(10F8.0).
COMPUTE #idx=1.
LOOP #cnt1=1 TO 4.
LOOP #cnt2=#cnt1 +1 TO 5.
COMPUTE cp(#idx)=v(#cnt1)*v(#cnt2).
COMPUTE #idx=#idx+1.
END LOOP.
END LOOP.
EXECUTE.
SAVE OUTFILE='c:\\temp\\mydata2.sav'.
* That's it.

*****************.
* PART B.

* Let's see now how we can add meaningful labels to cp variables.
*****************.

GET FILE='c:\\temp\\mydata.sav'.
* at this point we only need to work with the variable names, so we 
	lmit the data file to a single case.
N OF CASES 1.
FLIP.
* The variable names are now in a string variables, let's number them.
COMPUTE id=$CASENUM.
* We need the variable names to be in the same case. Define a vector to
	hold the names.
*STRING vn1 TO vn5(A8).
VECTOR vn(5A8).

* Copy each label to its required column.
COMPUTE vn(id)=case_lbl.
COMPUTE nobreak=1.
* Then collapse all cases into a single case.
AGGREGATE
  /OUTFILE=*
  /BREAK=nobreak
  /vn1 TO vn5 = MAX(vn1 TO vn5).

* Build the variable label definitions.
VECTOR v=vn1 TO vn5.
STRING #deflab(A50).
COMPUTE #idx=1.

LOOP #cnt1=1 TO 4.
LOOP #cnt2=#cnt1 +1 TO 5.
COMPUTE #deflab=CONCAT(
	'VARIABLE LABEL cp',LTRIM(STRING(#idx,F8.0)),' "',
	v(#cnt1),' * ',v(#cnt2),'".').
* Create a syntax file which will define the variable labels.
WRITE OUTFILE='c:\\temp\\define var labels.SPS' /#deflab.
COMPUTE #idx=#idx+1.
END LOOP.
END LOOP.
EXECUTE.

GET FILE='c:\\temp\\mydata2.sav'.
INCLUDE 'c:\\temp\\define var labels.SPS'.
FREQ ALL.
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