There are so many APIs available in RPG IV that no one can remember all of them, and several are overlooked. One such API is CEEDYWK. This API is an "ILE built-in." ILE built-in APIs are APIs whose object code is inserted into the compiled object directly.

Normally, a call to an API inserts a call to a routine in a service program or a standalone program. ILE built-ins work a little differently. Rather than insert a call to a subprocedure or program, the actual object code for the API itself is inserted directly inline, into the compiled object. This avoids the overhead of the call.

The prototype for the CEEDYWK API used by the GETDAYOFWEEK subprocedure is listed below:

The first parameter of CEEDYWK is the number of days since October 14, 1582. This value may seem a bit odd, but it gives the API a known duration to do its calculation.

The second parameter of CEEDYWK must be a variable that is defined as a 4-byte integer (10i0) value. This variable will receive the day of the week.

To calculate the days since October 14, 1582, use either the SUBDUR opcode or the %DIFF built-in function. For example:

The number of days since October 14, 1582, is calculated by passing the date whose day of the week we need to the %DIFF built-in function. The second parameter is the October 14, 1582, date hard-coded.

The third parameter indicates the type of duration being calculated. In this case, the number of days is desired, so *DAYS is specified.

The value returned from this calculation is passed to the CEEDYWK API. The API calculates the day of the week from that value and returns it as a value of 1 to 7 on the second parameter.

The following is a nice little subprocedure called DayOfWeek() that simplifies calling CEEDYWK. Pass in the desired date, and it returns the day of the week.

Here's the prototype:

Here's the procedure implementation:

Certainly the need for the GetDayOfWeek() wrapper subprocedure is debatable. However, it does make for a much cleaner interface. Compare the two ways to call CEEDYWK, first calling it directly and then calling it via the wrapper, as follows:

In this example, the CEEDYWK prototype is /INCLUDEd into the source, the BASEDATE constant needs to be declared, and then CEEDYWK is called directly, passing the results from the %DIFF() built-in function as the first parameter.

In this example, the GetDayOfWeek() subprocedure prototype is /COPYed into the source; then, the GetDayOfWeek() subprocedure is called.

I want to illustrate this technique without all the supporting /COPYs or work fields. Here is the code again, to calculate the day of the week using only the CEEDYWK API without the wrapper subprocedure:

And here is the version that uses the API wrapper subprocedure GetDayOfWeek():

You have to decide whether you want to start creating a library of usefully subprocedures and API wrappers or whether you want to use the raw API technique. Either method works fine.