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Min and Max

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Often, when I use an API or C runtime function in RPG IV, I need to pass the minimum of two or more values to the API or C function. So I end up doing this:

 

 

 

 

 

 

     /free
          if (%size(a) < %size(b));
             minLen = b;
          else;
             minLen = a;
          endif;
          callp myFoo(var1:var2: minLen);
     /end-free

Most other languages have a macro or built-in function that can easily return the minimum or maximum lengths of two or more values. For RPG IV, I've long abandoned this practice of conditionally calculating the minimum value and have been using my own RPG xTools' MIN and MAX subprocedures, like so:

  callp myFoo(var1:var2: min(%size(a):%size(b)));

Certainly, this is subjectively easier, although I still prefer the following:

  callp myFoo(var1:var2: %min(%size(a):%size(b)));

Until IBM provides us with a built-in function, I've decided to post the simple code behind my MIN and MAX subprocedures. They aren't rocket science, so why not?

The MIN and MAX subprocedures return the numeric minimum or maximum value, respectively. The input values are converted to Packed(30,9), which is compatible with V5R1 and early releases of OS/400. The original code is in fixed-format RPG IV, to make it pre-V5R1 compatible, but I know how much this audience prefers free-format, so I've converted it for you.

     P Min             B                   Export
     D Min             PI            30P 9
     D nValue1                       30P 9 Const
     D nValue2                       30P 9 Const
     D nValue3                       30P 9 Const OPTIONS(*NOPASS)
     D nValue4                       30P 9 Const OPTIONS(*NOPASS)
     D nValue5                       30P 9 Const OPTIONS(*NOPASS)
     D nValue6                       30P 9 Const OPTIONS(*NOPASS)
     D nValue7                       30P 9 Const OPTIONS(*NOPASS)
     D nValue8                       30P 9 Const OPTIONS(*NOPASS)

     D nMin            S             30P 9

      /free
          if (%parms() < 1);
              nMin = 0;
          endif;
          if (%parms() >= 1);
              nMin = nValue1;
          endif;

          if  (%parms()>= 2 and
               nMin > nValue2);
            nMin = nValue2
          endif;

          if  (%parms()>= 3 and
               nMin > nValue3);
            nMin = nValue3;
          endif;

          if  (%parms()>= 4 and
               nMin > nValue4);
            nMin = nValue4;
          endif;

          if  (%parms()>= 5 and
               nMin > nValue5);
            nMin = nValue5;
          endif;

          if  (%parms()>= 6 and
               nMin > nValue6);
            nMin = nValue6;
          endif;

          if  (%parms()>= 7 and
               nMin > nValue7);
            nMin = nValue7;
          endif;
          if  (%parms()>= 8 and
               nMin > nValue8);
            nMin = nValue9;
          endif;

         return    nMin;
      /end-free
     P Min             E

The MAX subprocedure is effectively the same, so I'm not going to reproduce it here. You can easily clone the MIN function and change the names. One exception is that the conditional statements in MAX would be as follows:

      /free
            if (%parms() >= 2 and
                nMax < nValue2);
              nMax = nValue2;
            endif;
      /end-free

Where to Apply Min and Max

Using these two functions is more pervasive in my code than I first thought. Since I'm a big fan of using C runtime functions in RPG IV, I use the MIN subprocedure extensively. The quintessential example is the C runtime memcpy() function. Unlike in RPG IV, in C you have to specify the length of the field being copied. You also have to worry about not overwriting the target. For example, in RPG IV this works famously:

     D name            S             25A   Inz('Robert Cozzi, Jr')
     D First           S             10A
     D nPos            S             10I 0
    
      /free
           nPos %scan(' ' : name);
           if (nPos > 0);
               first = %subst(name:1:nPos-1);
           endif;
      /end-free

At the end of the routine, the name "Robert" is copied to the field named FIRST. Using the C runtime memcpy() function, you might code this:

     D name            S             25A   Inz('Robert Cozzi, Jr')
     D First           S             10A
     D nPos            S             10I 0
    
      /free
           nPos %scan(' ' : name);
           if (nPos > 1);
               memcpy(%addr(first) : %addr(name) : nPos-1);
           endif;
      /end-free

On the surface, this might look great, but what happens when the first name is more than 10 positions long? Using the RPG IV method, it would just work, as RPG knows about field lengths. But the C runtime would cause a "learning experience." So we use our trusty MIN function to solve this problem, as follows:

     D name            S             25A   Inz('Robert Cozzi, Jr')
     D First           S             10A
     D nPos            S             10I 0
    
      /free
           nPos %scan(' ' : name);
           if (nPos > 1);
               memcpy(%addr(first) : %addr(name) : 
                           min(%size(first):nPos-1));
           endif;
      /end-free

By wrapping the length of the field named FIRST and the number of bytes to copy (i.e., nPos-1) in MIN(x:y), the lowest of the two values is returned, so you never have an integrity issue.

Of course, you now need to make sure you clear the FIRST field before calling memcpy(), as follows:

      /free
           nPos %scan(' ' : name);
           if (nPos > 1);
               first = ' ';   // Clear the target.
               memcpy(%addr(first) : %addr(name) : 
                          min(%size(first):nPos-1));
           endif;
      /end-free

A great use for memcpy() is copying an array to a field or a field to an array. This way, no data structures or tricks are needed. For example:

     D nBytes          S             10I 0
     D Libs            S             11A   Dim(255)                             
     D LibNames        S           2805A

    
      /free
           nBytes = min(%size(LibNames):%size(libs:*ALL));
           memcpy(%addr(libs) : %addr(libnames) : nBytes);
      /end-free

In the example above, the nBytes field is initialized to the lesser length of the LIBNAMES field and the total length of the LIBS array (all elements). As illustrated, both are the same length, so nothing would happen regardless of which size you used for the memcpy() function. But what if LIBNAMES was shorter? For example:

     D nBytes          S             10I 0
     D Libs            S             11A   Dim(255)            
     D LibNames        S            275A

    
      /free
           nBytes = min(%len(LibNames):%size(libs:*ALL));
           memcpy(%addr(libs) : %addr(libnames) : nBytes);
      /end-free

Suddenly, it becomes important to use the lesser of the two lengths.

Accurate programming is extremely important when using non-RPG IV interfaces, such as C and MI. While it isn't as critical as Windows apps, which may cause the computer to reboot, using poor programming practices could cause your job/session to go crazy and even end abnormally. Do you really want that?

 

Bob Cozzi is author of the best-selling The Modern RPG IV Language, Fourth Edition as well as RPG TNT: 101 Dynamite Tips 'n Techniques with RPG IV and is host of the i5 Podcast Network, which provides free video and audio podcasts to the i5 community. You can also see him in person at RPG World in May 2007.

BOB COZZI

Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.


MC Press books written by Robert Cozzi available now on the MC Press Bookstore.

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