Tips for Writing Minitab "Execs"

Work Envinronment

• Use a good text editor: The use of Minitab's STORE command prevents the editing of execs, and effectively makes all but the most trivial of macros unbearable to develop. An editor that supports cut and paste can often be useful, as is the ability to store keystrokes in macros. My favorite DOS editor is QEdit by Semware of Marietta, Georgia. QEdit is shareware, widely available on computer bulletin boards and publicly available ftp sites, and has a reasonable registration fee. The Windows Notepad suffers from a rather small buffer size, making the editing of larger files impossible. While this is unlikely to impact the writing of execs, it does make it unusable for viewing larger files. I have not kept current with Windows text editors, though I have used MegaEdit (also shareware) and found it acceptable. For OS/2, I recommend the Enhanced Editor that is included with OS/2. There are numerous editors for any environment (one jest, for example, is that the ratio of editors to Navy programmers is 1.25) and many are shareware. The user should easily be able to find one that suits their taste.
• Task swapping: Task swapping is the ability to temporarily suspend a program so as to use another. Multitasking, on the other hand, is the ability to give "timeslices" of the computer's CPU to background processes. For example, if you are running a Minitab macro that will take a long time to complete, it is nice to be able to do other things with your computer while this macro executes. This requires multitasking. If you want to edit a macro concurrent to running it, you may only need task swapping. This would allow you to toggle between your editor and Minitab. There are a number of ways to get your computer to swap and multitask DOS programs. Starting with version 4.0, DOS (both IBM's PC-DOS and Microsoft's MS-DOS) included the DOSSHELL, which allows for DOS programs to be swapped. It is recommended to use DOS version 5.0, if you wish to use DOSSHELL; version 4.0 was notoriously slow and buggy. DOSSHELL is strickly a task swapper; it does not give any CPU time to suspended programs.
• Multitasking: The three most common environments for multitasking DOS programs are Microsoft Windows, Quarterdeck Desqview, and IBM OS/2. While Windows is certainly the most common of these, both Desqview and OS/2 are much better at multitasking DOS programs. The major disadvantage of Desqview is that it is strictly a DOS multitasker. You cannot run Windows programs under Desqview, without running Windows itself. I have talked to people who run Windows as one of the DOS programs they run under Desqview, and they report good results. This does require learning both environments, however, and is not always easy to set up.

  A disadvantage of running OS/2 is the hardware requirements; to get satisfactory performance, you will need at least 8 megabytes of RAM and a good sized hard disk (I recommend at least 200 MB). If these conditions are met, then OS/2 may be the best of these three options for many users; it allows not only excellent DOS support, but you can also run virtually all Windows software with it as well.

  The major disadvantages of Windows itself is system stability and poor multitasking. Since Windows runs under DOS, and DOS is strictly a non-multitasking system, Windows has to do a lot of work to get DOS to play the multitasking ballgame. When using Windows to multitask DOS programs, regular system crashes are not unusual, so care must be taken to save data whenever possible.

  A common disadvantage with all of these systems is hardware requirements: a 386 processor and 4 MB of RAM are a must for running Desqview or Windows, and 8 MB RAM is nice. If running Windows applications, 8 MB RAM is essentially required for OS/2.

Macro tips

• Reading user input: You can receive user input using the READ or SET command, if you specify the special filename 'TERMINAL'. It is also helpful to specify the number of observations of be read, whenever this is known, using the NOBS subcommand. For example:

  	READ 'TERMINAL' C50;
  		NOBS 1.

• Variable columns: Column references may be indirect; for example, CK1 refers to the column specified by the value of K1. This is especially useful for generalizing a data analysis procedure to being applicable to any column of data.
• Submacros and loops: Loops can be utilized by having a macro execute multiple times: EXEC 'macro' K would execute MACRO.MTB K times. If you want to keep a counter, remember to initialize it in the calling macro e.g., LET K1=1, and increment it in the submacro e.g., LET K1=K1+1. This is a convenient way to create a column where the results of each loop are stored sequentially e.g., LET C51(K1)=K2.
• If-then-else: While Minitab does not have a formal if-then-else structure (this appears in the Release 9 macro language), this can effectively be implemented using logical arithmetic and submacros. First, there are logical operators (& for "and" and | for "or") and constants and columns can be evaluated logically (FALSE=0, TRUE=1). The usualy method of setting a logical variable is to enclose the condition in parentheses, e.g., LET K4=(K2<K3) would make K4 equal 1 if K2 is less than K3 and 0 otherwise. Similarly, LET K4=((K2<K3)&(K2<K1)) would make K4 equal 1 if K2 is less than both K3 and K1. I have had some problems with compound logical evaluations like the latter expression. You may need to do the intermediate evaluations "by hand":

  	LET K5 = (K2 < K3)
  	LET K6 = (K2 < K1)
  	LET K4 = K5 & K6

  The "not" operator is ~, e.g., LET K2=~K1; this can also be implemented as LET K2=1-K1. Note that the helpfile for Minitab Release 9 for Windows, has the "or" and "not" operators reversed; this is a typo in the helpfile. You may want to check this with Releases 7 and 8 (I no longer own these versions); it is with the information on the LET command.

• Performance improvements: Since Minitab reads macros line by line, a simulation, where hundreds of lines of macro are read and executed, can take quite a while to perform. Two ways to improve performance of macros are using disk caching software and setting up a virtual disk in RAM. Both of these perform similar function.

  Since access to a disk is slow relative to access to RAM, if Minitab can get the macro into RAM to read it line by line, then macro execution can be improved substantially. A disk cache uses RAM to buffer disk reads (and perhaps writes), holding data in RAM that has been retrieved from disk; subsequent reads from the same part of the disk then come from the buffer instead of rereading from disk. A virtual disk is an area of RAM that operates like a disk, except that the data stored on it are transient; when the computer is powered off, the data in the virtual disk is lost.

  The advantage of a disk cache is that it is an omnibus solution; it requires no special commands to utilize once installed, and it will improve general system performance. The advantage of a virtual disk is that the data on the disk cannot be unloaded due to other disk activity; since the disk cache is a finite amount of RAM, as data is stored to it, data is also dumped out of it. If this data happens to be your macro, then Minitab's request for the macro's data will result in rereading the data from disk. Which solution works best for you depends on how much memory your system has and what you use your system for. You may want to use a combination of the two methods. DOS includes both disk cache software and virtual disk software. The disk cache software is called SMARTDRV (MS-DOS) or IBMCACHE (PC-DOS) and the virtual disk software is called either RAMDRIVE (MS-DOS) or VDISK (PC-DOS). Consult your DOS manual for details on installing either of these. Another excellent disk cache is Hyperdisk by Hyperware of Pall Mall, Tennessee; there are numerous shareware implementations of both caching and virtual disk software.

• Consolidate computations: In general, it is more efficient to operate on a column all at once than to operate on each element one at a time. For example, suppose we have test statistics for a simulation in C4, so each run of the simulation is represented by a row of C4. We want C5 to be the decision of rejection; let's say rejection happens when our test statistic is less than K5. Also, assume K1 is our counter. Then we could create C5 row by row in our submacro by:

  	LET C5(K1) = (C4(K1) < K5)

  
  However, it is more efficient to evaluate C5 in the top-level macro:

  	LET C5 = (C4 < K5)

  
  Remembering considerations like this can make your simulation macros execute much faster. Only compute what you need to in submacros, and compute what you can in top-level macros.