DataStore Library: 6 Enumeration

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DataStoreFieldEnum(), DataStoreRecordEnum()

DataStoreFieldEnum() enumerates through fields of a record. This routine uses a Boolean callback to determine whether to continue enumeration. If the callback returns TRUE, enumeration stops.

DataStoreRecordEnum() enumerates through records of a datastore in storage order, starting at the specified RecordNum in the specified direction. This routine uses a Boolean callback routine to determine whether to continue enumeration. If the callback returns TRUE, enumeration ends; if FALSE, enumeration continues until the callback returns TRUE or until the routine reaches the last record.

There are two DataStoreRecordEnumFlags that can be passed in this routine:

• DSREF_BACKWARDS
  Enumerate backwards through the datastore.
• DSREF_START_AT_END
  Enumerate from first record (going forward) or from last record if DSREF_BACKWARDS is also passed.

The following example enumerates through the a datastore looking for the maximum value in a particular field.

Code Display 9-7 Enumerating Through a Datastore

	/* data to be passed to the callback routine */
	FloatNum	enumData = 0;

	/* record at which to start enumeration */
	RecordNum	rec = 0;

	/* 
	 * Open the datastore. For this example, assume dsToken
	 * is a global variable.
	 */
	if(DataStoreOpen("Exchange Rates", oself, 0, &dsToken) == DSE_NO_ERROR)
	{
		/* 
		 * Enumerate through the datastore starting at the first record
		 * (so pass zero in the flags parameter);
		 * find the maximum value of the "exchange rate" field.
		 */
		if(DataStoreRecordEnum(dsToken, &rec, 0, &enumData, EnumCallback)
							== DSE_NO_MORE_RECORDS)
		{
			/* do something with the value */
		}
		DataStoreClose(dsToken);
	}

	/*
	 * The callback compares the data in the "exchange rate" field to the value
	 * passed in with enumData. If the field data is greater than that of
	 * enumData, copy the field's data to enumData.
	 */

	Boolean EnumCallback(RecordHeader *record, void *enumData)
	{
		/* parameters for getting field data */
		FloatNum	rateBuffer, *pRateBuffer;
		FieldType	type;
		word	size;

		if(DataStoreGetFieldPtr(dsToken, record, 3, (void **)&pRateBuffer,
						&type, &size) == DSDE_NO_ERROR)
		{
			if(*pRateBuffer > *((FloatNum *)enumData))
			{
				*((FloatNum *)enumData) = *pRateBuffer;
			}
		}
		return FALSE; /* FALSE to continue enumeration */
	}

If the routine returns DSE_NO_MORE_RECORDS, it has reached the last record in the file. If it returns DSE_NO_ERROR, it writes the RecordNum of the last record examined in the startRecord parameter.

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