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As a rule, MFS programmers do not need to concern themselves with these esoteric aspects of select list processing. In some cases -- for instance, in SI.MFS -- the MFS deliberately assumes control over specific tasks (in this case, returning sorted key lists). However, an MFS that does not concern the return of a sorted or reduced key list does not need to include logic for these tasks.
Simple (Latent) Select
In BASIC+, a simple SELECT statement (not an extended SELECT ... BY statement) does not directly produce a list of record keys for processing. Instead, the SELECT statement simply initializes variables in the system, indicating that a select condition is active.
In addition, the SELECT is passed to the filing system so it can perform any filing system-specific activity to initialize a select condition. A filing system SELECT call returns a select mode with three possible values in the RECORD argument:
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Select Mode
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Meaning
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0
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No select list is active (disable the select)
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1
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Latent file select
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2
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Latent index select
If a select condition is active, subsequent BASIC+ READNEXT statements will return record keys until the file has been exhausted. If no select list is active, a subsequent BASIC+ READNEXT statement will fall through its ELSE branch.
The select mode value is available to an BASIC+ program in the system variable @LIST.ACTIVE. If the SELECT call fails for any reason, the RECORD argument is passed back false, indicating to the system that no select list is active. @LIST.ACTIVE will also be set to false.
This BASIC+ program fragment illustrates the typical flow of a simple SELECT:
| Code Block |
|---|
SELECT FILE ; * establish "latent" select
DONE = 0
LOOP
READNEXT @ID ELSE DONE = 1
UNTIL DONE DO
/* process the row */
REPEAT |
ReadNext Processing
Once a select condition is established (select mode is true), the system can use a READNEXT command to return record keys from the select list. In BASIC+, READNEXT is used to return a single key from the select list. In a filing system, however, a READNEXT call returns an entire block of keys.
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