Re: [Tiny-cobol-users] Re: a problem with EVALUATE

[John R. C. <jo...@we...>]

On Saturday 20 August 2005 02:01 pm, Jim Morcombe wrote:
> John
>
> I've got to disagree with you.
>
> The EVALUATE TRUE allows a whole heap of spagetti-coded IF, THEN, ELSEs
> to be replaced by a nice easy-to-follow slab of structured code.  Just
> because Cobol 74 programmers never learnt new tricks doesn't doesn't
> mean they weren't worth learning.
>

> The EVALUATE TRUE improves program structure and anything that improves
> program structure needs to be supported.
>
> Jim

Well I have four problems with that. 

First I don't understand EVALUATE TRUE.

Second I have four texts on my desk which
allegedly cover COBOL 85. Only the newest of these (pub date
2000) gives an example of EVALUATE TRUE and it is identical in
effect to a straight EVALUATE. The word TRUE could be omitted. 
Two of these books represent the entire COBOL library of the
local community college. 

 Third since I don't know what it does obviously I have no
knowledge of a situation where it would be necessary or even
useful. Perhaps you can offer one. 

I cannot offer an example of using EVALUATE TRUE because frankly
I don't understand it. But here is a fragment from a real
program written 25 years ago that I converted to EVALUATE form:
(example 1.)
 EVALUATE SCREEN-FLAG
            WHEN "L"        PERFORM 150-INPUT-SCREEN
            WHEN "E"        PERFORM 200-CLEAR-FIELDS
                            PERFORM 400-ENTER-DATA
            WHEN "C"        PERFORM 400-ENTER-DATA
            WHEN "O"        PERFORM 405-SINGLE-ENTRY
            WHEN "S"        PERFORM 300-SHOW-SCREEN
            WHEN "D"        PERFORM 200-CLEAR-FIELDS
           END-EVALUATE
           EXIT PROGRAM.
-----------and here is an alternative form that I think is more
readable:
(example-2.)

         IF LOAD       PERFORM 150-INPUT-SCREEN.
         IF ENTER-ALL  PERFORM 200-CLEAR FIELDS
                       PERFORM 400-ENTER DATA.
         IF CHANGE-ALL PERFORM 400-ENTER DATA.
         IF CHANGE-ONE PERFORM 405-SINGLE-ENTRY. 
         IF SHOW       PERFORM 300-SHOW-SCREEN.  
         IF CLEAR      PERFORM 200-CLEAR-FIELDS. 
         EXIT PROGRAM.

Why no ELSE IF? Because the field SCREEN-FLAG can only hold one
value at a time. Therefore fall-through logic works. This would
be true for any real world WHEN type test. 

Finally the ELSE IF logic can also be used to good effect. This is a
more general construct than example-2.  IMO it is not spaghetti code. YMMV.
(example-3.)

         IF LOAD       PERFORM 150-INPUT-SCREEN
              ELSE IF
            ENTER-ALL  PERFORM 200-CLEAR FIELDS
                       PERFORM 400-ENTER DATA
              ELSE IF
            CHANGE-ALL PERFORM 400-ENTER DATA
              ELSE IF
            CHANGE-ONE PERFORM 405-SINGLE-ENTRY 
              ELSE IF
            SHOW       PERFORM 300-SHOW-SCREEN  
       ELSE IF
            CLEAR      PERFORM 200-CLEAR-FIELDS. 
         EXIT PROGRAM.

The word WHEN has been replaced by ELSE IF. The effect is the
same.

Examples 2 and 3 use condition names. Example 1 doesn't.
Why? because condition names in a WHEN statement
gave an error in Tiny COBOL. Now that is something that needs
fixing. 

John Culleton