This section describes the predicates required for creating and
destroying the access to external database tables.
- new_table(+File,
+Columns, +Options, -Handle)
- Create a description of a new table, stored in File. Columns
is a list of descriptions for each column. A column description is of
the form
ColumnName(Type [, ColumnOptions])
Type denotes the Prolog type to which the field should be
converted and is one of:
integer | Convert to a Prolog integer. The
input is treated as a decimal number. |
hexadecimal | Convert to a Prolog integer.
The input is treated as a hex number. |
float | Convert to a Prolog floating point
number. The input is handled by the C-library function
strtod(). |
atom | Convert to a Prolog atom. |
string | Convert to a SWI-Prolog string
object. |
code_list | Convert to a list of ASCII
codes. |
ColumnOptions is a list of additional properties of the
column. Supported values are:
sorted | The field is strictly sorted, but
may have (adjacent) duplicate entries. If the field is textual, it
should be sorted alphabetically, otherwise it should be sorted
numerically. |
sorted(+Table) | The (textual)
field is sorted using the ordering declared by the named ordering
table. This option may be used to define reverse order,‘dictionary’order
or other irregular alphabetical ordering. See
new_order_table/2. |
unique | This column has distinct values for
each row in the table. |
downcase | Map all uppercase in the field to
lowercase before converting to a Prolog atom, string or code_list. |
map_space_to_underscore | Map spaces to
underscores before converting to a Prolog atom, string or code_list. |
syntax | For numerical fields. If the field
does not contain a valid number, matching the value fails. Reading the
value returns the value as an atom. |
width(+Chars) | Field has fixed
width of the specified number of characters. The column-separator is not
considered for this column. |
arg(+Index) | For read_table_record/4,
unify the field with the given argument of the record term. Further
fields will be assigned index+1, ... . |
skip | Don't convert this field to Prolog.
The field is simply skipped without checking for consistency. |
The Options argument is a list of global options for the
table. Defined options are:
record_separator(+Code) | Character
(ASCII) value of the
character separating two records. Default is the newline (ASCII
10). |
field_separator(+Code) | Character
(ASCII) value of the
character separating two fields in a record. Default is the space (ASCII
32), which also has a special meaning. Two fields separated by a space
may be separated by any non-empty sequence of spaces and tab (ASCII
9) characters. For all other separators, a single character separates
the fields. |
encoding(+Encoding) | Text
encoding of the file. Values are iso_latin_1 (default),
utf8 or native. The latter uses the native
multibyte to unicode conversion. |
escape(+Code, +ListOfMap) | Sometimes,
a table defines escape sequences to make it possible to use the
separator-characters in text-fields. This options provides a simple way
to handle some standard cases. Code is the ASCII
code of the character that leads the escape sequence. The default is
-1, and thus never matched.
ListOfMap is a list of
From = To character mappings. The
default map table is the identity map, unless Code refers to
the
\ character, in which case
\b, \e, \n, \r and \t
have their usual meaning. |
functor(+Head) | Functor used by read_table_record/4.
Default is record using the maximal argument index of the
fields as arity. |
If the options are parsed successfully, Handle is unified
with a term that may be used as a handle to the table for future
operations on it. Note that new_table/4
does not access the file system, so its success only indicates the
description could be parsed, not the presence, access or format of the
file.
- open_table(+Handle)
- Open the table. This predicate normally does not need to be called
explicitely, as all operations on the table handle will automatically
open the table if this is required. It fails if the file cannot be
accessed or some other error with the required operating-system
resources occurs. The contents of the file is not examined by this
predicate.
- close_table(+Handle)
- Close the file and other system resources, but do not remove the
description of the table, so it can be re-opened later.
- free_table(+Handle)
- Close and remove the handle. After this operation, Handle
becomes invalid and further references to it causes undefined behaviour.
