% Copyright The Numerical Algorithms Group Limited 1992-94. All rights reserved.
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The domain \spadtype{KeyedAccessFile(S)} provides files which can be used as
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associative tables.
Data values are stored in these files and can be retrieved according to
their keys.
The keys must be strings so this type behaves very much like the
\spadtype{StringTable(S)} domain.
The difference is that keyed access files reside in secondary storage while
string tables are kept in memory.
For more information on table-oriented operations, see the description of
\spadtype{Table}.

\xtc{
Before a keyed access file can be used, it must first be opened.
A new file can be created by opening it for output.
}{
\spadpaste{ey: KeyedAccessFile(Integer) := open("/tmp/editor.year", "output")  \bound{ey}}
}
\xtc{
Just as for vectors, tables or lists, values are saved in a keyed access file
by setting elements.
}{
\spadpaste{ey."Char"     := 1986 \free{ey}\bound{eya}}
}
\xtc{
}{
\spadpaste{ey."Caviness" := 1985 \free{ey}\bound{eyb}}
}
\xtc{
}{
\spadpaste{ey."Fitch"    := 1984 \free{ey}\bound{eyc}}
}
\xtc{
Values are retrieved using application, in any of its syntactic forms.
}{
\spadpaste{ey."Char"     \free{eya}}
}
\xtc{
}{
\spadpaste{ey("Char")    \free{eya}}
}
\xtc{
}{
\spadpaste{ey "Char"     \free{eya}}
}
\xtc{
Attempting to retrieve a non-existent element in this way causes an error.
If it is not known whether a key exists, you should use the
\spadfunFrom{search}{KeyedAccessFile} operation.
}{
\spadpaste{search("Char", ey)   \free{eya,eyb,eyc}\bound{eyaa}}
}
\xtc{
}{
\spadpaste{search("Smith", ey)  \free{eyaa}}
}
\xtc{
When an entry is no longer needed, it can be removed from the file.
}{
\spadpaste{remove!("Char", ey)  \free{eyaa}\bound{eybb}}
}
\xtc{
The \spadfunFrom{keys}{KeyedAccessFile} operation returns a list of all the
keys for a given file.
}{
\spadpaste{keys ey  \free{eybb}}
}
\xtc{
The \spadfunFrom{\#}{KeyedAccessFile} operation gives the
number of entries.
}{
\spadpaste{\#ey     \free{eybb}}
}

The table view of keyed access files provides safe operations.
That is, if the \Language{} program is terminated between file operations,
the file is left in a consistent, current state.
This means, however, that the operations are somewhat costly.
For example, after each update the file is closed.
\xtc{
Here we add several more items to the file, then check its contents.
}{
\spadpaste{KE := Record(key: String, entry: Integer)  \bound{KE}}
}
\xtc{
}{
\spadpaste{reopen!(ey, "output")  \free{eybb,KE}\bound{eycc}}
}
\xtc{
If many items are to be added to a file at the same time, then
it is more efficient to use the \spadfunFromX{write}{KeyedAccessFile} operation.
}{
\spadpaste{write!(ey, ["van Hulzen", 1983]\$KE)  \bound{eyccA}\free{eycc}}
}
\xtc{
}{
\spadpaste{write!(ey, ["Calmet", 1982]\$KE)      \bound{eyccB}\free{eycc}}
}
\xtc{
}{
\spadpaste{write!(ey, ["Wang", 1981]\$KE)        \bound{eyccC}\free{eycc}}
}
\xtc{
}{
\spadpaste{close! ey   \free{eyccA,eyccB,eyccC}\bound{eydd}}
}
\xtc{
The \spadfunFromX{read}{KeyedAccessFile} operation is also available from
the file view, but it returns elements in a random order.
It is generally clearer and more efficient to use the
\spadfunFrom{keys}{KeyedAccessFile} operation and to extract elements by
key.
}{
\spadpaste{keys ey     \free{eydd}}
}
\xtc{
}{
\spadpaste{members ey  \free{eydd}}
}
\noOutputXtc{
}{
\spadpaste{)system rm -r /tmp/editor.year  \free{ey}}
}

For more information on related topics,  see
\downlink{`File'}{FileXmpPage}\ignore{File},
\downlink{`TextFile'}{TextFileXmpPage}\ignore{TextFile}, and
\downlink{`Library'}{LibraryXmpPage}\ignore{Library}.
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