Apple Newton Utilities Guia do Utilizador descarregar pdf (Página 443)

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CHAPTER 11

Data Storage and Retrieval

About Data Storage on Newton Devices 11-11

Querying for Indexed Values 11

Queries can retrieve items according to the presence of one or more index keys and

can test key values as well. A query that tests for the presence or value of an index

key is called an index query.

Soups that have single-slot indexes allow queries to use a single index key to select

soup entries. Detailed information is provided in “Querying on Single-Slot

Indexes” beginning on page 11-39.

Soups that have multiple-slot indexes allow queries to use multiple index keys to

select soup entries. Detailed information is provided in “Querying on Multiple-Slot

Indexes” beginning on page 11-47.

Index queries can be based only on slot names for which an index has been generated.

For example, to select entries according to the presence of the

foo slot, the soup

that receives the

Query message must be indexed on the foo slot. Entries not

having a

foo slot are not included in the set of entries referenced by the foo index.

Although the entries in the soup are not actually in any particular order themselves,

the index keys associated with them can be sorted in a speciﬁc order that is deﬁned

for each NewtonScript data type. Thus, you can envision the contents of an index

as a sequence of entries arranged in key order, as shown in Figure 11-2.

Figure 11-2 An index provides random access and imposes order

The aSoup soup shown in Figure 11-2 is indexed on the foo slot, which means

that the value of each entry’s

foo slot is used as its index key. Only those entries

containing a

foo slot are included in this index. By sorting key values, the index

imposes order on their corresponding soup entries, which are otherwise unordered.

2 3 4 5

Ascending key order

Descending key order

Index on

foo slot

Store

Soup

{foo: , ...}

{bar: , ...}

1 2 ... 438 439 440 441 442 443 444 445 446 447 448 ... 941 942

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