Apple Newton Utilities User's Guide download pdf (Page 446)

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CHAPTER 11

Data Storage and Retrieval

11-14 About Data Storage on Newton Devices

cursor is positioned on the next valid entry in index key order. Similarly, if a valid

entry is not found at the key value speciﬁed for an

endKey or endExclKey value,

the cursor is positioned on the previous valid entry in index key order. (The cursor

is never positioned beyond the

endKey value or before the beginKey value.)

For information on using index queries, see “Querying on Single-Slot Indexes”

beginning on page 11-39 and “Querying on Multiple-Slot Indexes” beginning on

page 11-47.

Tag-based Queries 11

Index queries can also include or exclude entries according to the presence of one

or more tags. A tag is an optional developer-deﬁned symbol that resides in a

speciﬁed slot in the soup entry.

The symbols used as tags are stored as the key values in the soup’s tags index. As

with any other index, the system maintains the tags index automatically and queries

can test values in this index without reading soup entries into memory. Thus, tag-

based queries are quick and efﬁcient.

Unlike other indexes, the tags index alone cannot be used as the basis of an index

query—it does not sort entries (as other indexes do), it only selects or eliminates

entries according to their associated tag values. However, you need not specify an

additional index in order to query on tag values; when a separate index is not

speciﬁed, queries on tags test all entries in the soup.

The tags for which the query tests are speciﬁed by a tags query speciﬁcation

frame or tags query spec supplied as part of the query spec. The tags query spec

can specify set operators such as

not, any, equal, and all to create complex

ﬁlters based on tag values. For example, you could use these operators to query for

entries having the

'USA or 'west tags that do not have the 'California tag.

The set operators used by tags query specs are described in greater detail in “Tag-

based Queries” beginning on page 11-14 of this book and “Tags Query

Speciﬁcation Frame” (page 9-13) in Newton Programmer’s Reference.

Customized Tests 11

The use of indexes, begin keys, end keys, and tags provides sufﬁcient control over

query results for most uses; however, you can specify additional customized tests

when necessary. These tests take the form of an

indexValidTest or

validTest function that you define in the query spec.

The

indexValidTest function tests the index key values associated with each

entry in the range of values over which the cursor iterates. This function returns

nil for an entry that is to be rejected, and returns any non-nil value for an entry

that is to be included in the results of the query. Like all tests that manipulate index

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