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Date:   Wed, 27 Sep 2006 16:47:52 -0400
Reply-To:   Richard Ristow <wrristow@mindspring.com>
Sender:   "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:   Richard Ristow <wrristow@mindspring.com>
Subject:   Re: quick question
Comments:   To: "Hoover, Matthew" <mhoover@donahue.umassp.edu>
Comments:   cc: Justin Black <justin.black@gmail.com>, Gary Rosin <grosin@stcl.edu>, Hal 9000 <9000.hal@gmail.com>
In-Reply-To:   <5290ED5BE539294F9C1534FBC9C28E9D049A1239@EXCHANGE.ad.umass p.edu>
Content-Type:   text/plain; charset="iso-8859-1"; format=flowed

OK, you knew you were waving a red flag at THIS bull (grin).

At 10:42 AM 9/27/2006, Hoover, Matthew wrote:

>I'm unsure of what procedures require an execute >command. For example, for code like this: > > >DO IF (SYSMIS(gradef03)). >. COMPUTE gradef03=d_grf03. >END IF. >EXECUTE. > >DO IF (SYSMIS(grades04)). >. COMPUTE grades04=d_grs04. >END IF. >EXECUTE. > >Could I re-write this [without either EXECUTE]?

Yes, you could; it would work fine, and faster.

By way of a short tutorial, and skipping Python (it's just in the way, for this discussion), the standard order of SPSS programs is a 'transformation program' followed by a 'procedure', then another transformation program and procedure, etc.

(Before this starts, there needs to be a file, so there needs to be a command that defines one: DATA LIST, GET FILE, MATCH FILES, ...)

The transformation program is made up of transformation commands: COMPUTE, IF, RECODE, DO IF, LOOP, ... They explicitly define the values, or properties, of variables.

The 'procedures' are the statistical and reporting commands: FREQUENCIES, REGRESSION, all the others. EXECUTE is a procedure. Procedures use the whole file, and require that the whole file be read. Transformation programs work on a case (or record) at a time; the program is 'handed' one case at a time, does its work, and 'hands' the result to the procedure, until the procedure has 'seen' the whole file.

If you write

COMPUTE C = A + B. EXECUTE. COMPUTE D = C * E.

then the first COMPUTE statement is a transformation program by itself. For each record of the file, it computes the new value of C, then passes the record to EXECUTE (which does nothing with it). The new values of C remain in the file, and the second COMPUTE, which is also a transformation program, 'sees' the new values and them to compute D.

If you leave out the EXECUTE, and write

COMPUTE C = A + B. COMPUTE D = C * E.

then the two COMPUTEs together are a single transformation program. For each record, the program first computes the new value of C; then, using that new value, it computes the new value of D; then, it passes that record to whatever procedure follows.

Put in EXECUTE, and SPSS runs through the file doing the first computation, then again doing the second. Leave it out, and it runs through once, doing both computations.

(Think of the file as a table, with the variables being columns, and the cases or records being rows. If you have an EXECUTE between those COMPUTEs, SPSS moves down the rows making the first computation; then, starts back at the top and makes the second. If you have no EXECUTE, SPSS moves *once* down the rows, making all the computations row by row.)

Now, to comment on comments:

>using the syntax below will reduce the number of >required transformations and should give you the >same results as the DO IF's you're using currently. > >IF (SYSMIS(gradef03)) gradef03=d_grf03 . >IF (SYSMIS(grades04)) grades04=d_grs04 . >EXE .

It will give the same results. It is fewer transformation lines than using the DO IFs, but which you prefer is mainly taste; you'll never see any difference in the speed. But this "EXE." is not needed, either; it, too, reads the whole file to no purpose. The next procedure, or SAVE FILE, will cause the file to be read and the computations performed.

>>Will SPSS automatically run execute commands >>when it needs to calculate a value for a new >>variable (therefore, you can eliminate them >>completely in syntax except perhaps at the end)? > >I believe SPSS executes commands when necessary >to generate a variable so it can be used, but not until then.

That last is it. It isn't that SPSS runs 'EXECUTE' commands; it's that it doesn't need them. SPSS runs the whole transformation program when it 'needs' the results: for a procedure, or a SAVE.

>My mental checklist for using .exe is: > >a.) immediate after the lag function is used to compute a var

No; almost never needed. If you use the lagged value of a variable WHICH IS MODIFIED IN THE SAME TRANSFORMATION PROGRAM, then EXECUTE may be needed; see section "Use EXECUTE sparingly" in Raynald Levesque's book.

>b.) after writing out to an .sps file and before >inserting that file within the same syntax

Yes, but it may not be for the reason you're thinking. If you're using PRINT or WRITE or XSAVE in a transformation program, they won't be executed until the transformation program is run, which is concurrently with the next procedure. You do need the do-nothing procedure "EXECUTE" to cause the transformation program to be run and the results made available.

>c.) before using the delete vars command

That's needed; DELETE VARS needs to be at the head of a transformation program. However, you can almost always rearrange your logic to avoid the EXECUTE; for example, put DELETE VARS directly after the next procedure you were going to run anyway.

Finally, do see "Use EXECUTE Sparingly" in Raynald Levesque's book: Levesque, Raynald, "SPSSŪ Programming and Data Management, 3rd Edition/A Guide for SPSSŪ and SASŪ Users". SPSS, Inc., Chicago, IL, 2005.

You can download it free as a PDF file, from http://www.spss.com/spss/SPSS_programming_data_mgmt.pdf.

(The third edition includes a lot on Python programming. For any SPSS version earlier 14, the second edition will be at least as good.)

....... Cheers, and onward. My apologies for any place I'm not clear - a tutorial may not come out right, the first time you write it.


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