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|Accessing a table|
This section describes the predicates to read data from a table.
Records are addressed by their offset in the table (file). As records have generally non-fixed length, searching is often required. The predicates below allow for finding records in the file.
|Unify value with the name of the file with which the table is associated.|
|Unify value with declaration of n-th (1-based) field.|
|Unify value with the field separator character.|
|Unify value with the record separator character.|
|Unify value with the 1-based index of the field that is sorted or fails if the table contains no sorted fields.|
|Unify value with the total number of columns in the table.|
|Unify value with the number of characters in the table-file, not the number of records.|
|Unify value with a term Start|
recordand arity the number of not-skipped columns), each of the arguments containing the converted data. An error is raised if the data could not be converted. Next is unified with the start position for the next record.
Fields is a list of field specifiers. Each specifier is of the format:
FieldName(Value [, Options])
Options is a list of options to specify the search. By
default, the package will search for an exact match, possibly using the
ordering table associated with the field (see
|Uses prefix search with the default table.|
|Uses prefix search with the specified ordering table.|
|Searches for a substring in the field. This requires linear search of the table.|
|Searches for a substring, using the table information for determining the equivalence of characters.|
|Equivalence using the given table.|
If Value is unbound (i.e. a variable), the record is considered not specified. The possible option list is ignored. If a match is found on the remaining fields, the variable is unified with the value found in the field.
First, the system checks whether there is an ordered field that is specified. In this case, binary search is employed to find the matching record(s). Otherwise, linear search is used.
If the match contains a specified field that has the property
unique set (see new_table/4), in_table/3
succeeds deterministically. Otherwise it will create a backtrack-point
and backtracking will yield further solutions to the query.
may be comfortable used to bind the table transparently to a predicate.
For example, we have a file with lines of the format.1This
disproot.dat table from the AAT
database used in GRASP
C1C2 is a two-character identifier used in the other tables, and FullName is the description of the identifier. We want to have a predicate identifier_name(?Id, ?FullName) to reflect this table. The code below does the trick:
:- dynamic stored_idtable_handle/1. idtable(Handle) :- stored_idtable_handle(Handle). idtable(Handle) :- new_table('disproot.dat', [ id(atom, [downcase, sorted, unique]), name(atom) ], [ field_separator(0',) ], Handle), assert(stored_idtable_handle(Handle)). identifier_name(Id, Name) :- idtable(Handle), in_table(Handle, [id(Id), name(Name)], _).