New ILF Statements

Application designers gain more control through these new ILF statements.

PUSH and POP

The new PUSH and POP statements allow designers to load (PUSH) field or record data onto an internally maintained stack, and then retrieve (POP) them for use later. The structure of the statement is:

      PUSH     AAA FILE OR FIELD NAME   OCC      TYPE

and

      POP      AAA FILE OR FIELD NAME   OCC      TYPE

(AAA is the application ID, OCC is the [optional] occurrence number.)

The TYPE field is used to specify either FIELD or RECORD, and load/retrieve accordingly. Scanning on the TYPE field displays six possible values (FIELD, DEFAULT FIELD, ORIGINAL FIELD, RECORD, DEFAULT RECORD, ORIGINAL RECORD), apparently because the structure of the statement was taken from the STORE/RESTORE statements. However, it is important to note that there is only one stack for each FIELD, and one for each RECORD; therefore specifying DEFAULT FIELD or ORIGINAL FIELD is no different than specifying FIELD. Note, though, that an error will occur if you specify ORIGINAL FIELD on something other than the Process Control File.

The POP statement retrieves values in the reverse order that they were PUSHED, i.e. a LIFO function, and returns a TRUE flag if a value was successfully POPped, and a FALSE flag if the stack was empty. There is no automatic way to empty a stack, other than to repeat the POP statement in a loop until it returns a FALSE. An ideal use for PUSH and POP is to load virtual keystrokes (user options) into a stack, and have them executed automatically and sequentially. See the section on SELECT below for an example of this usage.

SELECT

The SELECT statement allows the access path of in input process to be changed dynamically. The structure of the statement is:

      SELECT  AAA FILENAME                        KEY IS  _____________________________

The application and file name must be specified, even though this statement is relevant only for the Process Control File (PCF). SCAN is available for the key field name, and only defined keys are presented as options.

One reasonable use for the SELECT statement would be to facilitate access path switching by users, without them having to use F3 (SELECT ACCESS PATH), and perhaps in that way limiting which keys they can select. Note that the SELECT statement does not force KEY ENTRY phase, so, without further coding, a user not in KEY ENTRY would see no immediate change in view. It would seem that a valuable use would be to redisplay a scrolling screen using an alternate key, and to do so, the above defined PUSH and POP statements come in very handy. Here's an example:

The file is called TEST42. The primary key is TEST42 ID NO.. Alternate keys are defined as TEST42 CATEGORY and TEST42 REGION. The screen has three buttons defined, linked to User Option 1, 2, and 3. The Option Intercept Event Point contains the following code:

      *     First, we disable the SELECT ACCESS PATH option (F3)
      IF      --- OPTION                    EQ     SELECT ACCESS PATH
T     SET     --- OPTION                     =
      *     Next, use the SELECT statement to set the desired path
      IF      --- OPTION                    EQ     USER 1

T     SELECT  TST TEST42                       KEY IS  TEST42 CATEGORY
F     IF      --- OPTION                    EQ     USER 2
FT    SELECT  TST TEST42                       KEY IS  TEST42 REGION
FF    IF      --- OPTION                    EQ     USER 3
FFT   SELECT  TST TEST42                       KEY IS  TEST42 ID NO
FFF   END
      *     Ignore these buttons if pushed in ADD mode
      IF      --- MODE                      EQ     ADD
T     END
      *     Now, push the options into the stack to switch into KEY ENTRY
      *     and redisplay the records (remember, last in, first out)
      SET     --- OPTION                     =     RETURN
      PUSH    --- OPTION                    FIELD
      *
      IF      --- MODE                      EQ     DELETE
T     SET     --- OPTION                    =      DELETE MODE
F     IF      --- MODE                      EQ     CHANGE
FT    SET     --- OPTION                    =      CHANGE MODE
FF    SET     --- OPTION                    =      INQUIRE MODE
      PUSH    --- OPTION                    FIELD

Finally, just add one line of code to Pre-Display:

      POP     --- OPTION                    FIELD

Also related to this new statement is a new Pre-Defined Field called --- ACCESS PATH. As one might assume, the field contains the value of the current access path for the Process Control File. The field may be viewed, but not changed via ILF code (that is to say, an ILF statement to SET --- ACCESS PATH to a specified value will be ignored).

Open Issues, Bugs, Suggestions

OPEN - Note that PUSHing a field after PUSHing a record containing that field will not cause the field value on the PUSHed record to change. The stack for the RECORD is independent from that of the field. This is a different behavior than designers may be used to from the STORE/RESTORE statements.

OPEN - It appears that fields with multiple occurrences ARE NOT supported by multiple stacks. For example, PUSH value 1 into field occurrence 1, then PUSH value 2 into field occurrence 2. If you then execute a POP on the field occurrence 1, it will retrieve the last value PUSHED into the field stack, ignoring occurrence, thus returning the value 2. However, that POP statement will place that value of 2 into occurrence 1. So the occurrence value is relevant for the source field and the destination field, but a single field name will support only one merged stack.

Some enhanced functionality that might be considered for down the road would be the following statements:

• POPCLEAR - Empty the stack for the specified field or record
• POPFIFO - Have the specified field or record get POPped in a FIFO (first in, first out) manner, rather than LIFO
• POPLIFO - Reverse the above, and return to standard defined behavior

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-- AlKalter - 04 Apr 2008