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|Built-in list operations|
Most list operations are defined in the library
described in section A.15.
Some that are implemented with more low-level primitives are built-in
and described here.
) or a term with functor `
'[|]''122The traditional list functor is the dot (
'.'). This is still the case of the command line option --traditional is given. See also section 5.1. and arity 2 and the second argument is a list.123In versions before 5.0.1, is_list/1 just checked for
[_|_]and proper_list/1 had the role of the current is_list/1. The current definition conforms to the de facto standard. Assuming proper coding standards, there should only be very few cases where a quick-and-dirty is_list/1 is a good choice. Richard O'Keefe pointed at this issue. This predicate acts as if defined by the definition below on acyclic terms. The implementation fails safely if Term represents a cyclic list.
is_list(X) :- var(X), !, fail. is_list(). is_list([_|T]) :- is_list(T).
typeerror if scanning List encounters a non-list. Note that memberchk/2 does not perform a full list typecheck. For example,
memberchk(a, [a|b])succeeds without error. If List is cyclic and Elem is not a member of List, memberchk/2 eventually raises a
typeerror.124Eventually here means it will scan as many elements as the longest list that may exist given the current stack usage before raising the exception.
This predicate fails if the tail of List is equivalent to
is logically correct. An exception would be more appropriate, but to our
best knowledge, current practice in Prolog does not describe a suitable
candidate exception term.
Note that List may contain non-ground terms. If Sorted
is unbound at call-time, for each consequtive pair of elements in
Sorted, the relation
E1 @< E2 will hold.
However, unifying a variable in Sorted may cause this
relation to become invalid,
even unifying a variable in Sorted with another
(older) variable. See also section
for the Order argument but this is likely to change. SWI-Prolog extends this predicate to deal with dicts.
If Key is the integer zero (0), the entire term is used to compare two elements. Using Key=0 can be used to sort arbitrary Prolog terms. Other values for Key can only be used with compound terms or dicts (see section 5.4). An integer key extracts the Key-th argument from a compound term. An integer or atom key extracts the value from a dict that is associated with the given key. A type_error is raised if the list element is of the wrong type and an existence_error is raised if the compound has not enough argument or the dict does not contain the requested key.
Deeper nested elements of structures can be selected by using a list of keys for the Key argument.
The Order argument is described in the table below129For
compatibility with ECLiPSe, the values
are allowed as synonyms.
The sort is stable, which implies that, if duplicates are kept, the order of duplicates is not changed. If duplicates are removed, only the first element of a sequence of duplicates appears in Sorted.
This predicate supersedes most of the other sorting primitives, for example:
sort(List, Sorted) :- sort(0, @<, List, Sorted). msort(List, Sorted) :- sort(0, @=<, List, Sorted). keysort(Pairs, Sorted) :- sort(1, @=<, Pairs, Sorted).
The following example sorts a list of rows, for example resulting from csv_read_file/2) ascending on the 3th column and descending on the 4th column:
sort(4, @>=, Rows0, Rows1), sort(3, @=<, Rows1, Sorted).
type_errorif List is a cyclic list or not a list.
Key-Valuepairs, terms whose principal functor is (-)/2. List is sorted on Key according to the standard order of terms (see section 4.7.1). Duplicates are not removed. Sorting is stable with regard to the order of the Values, i.e., the order of multiple elements that have the same Key is not changed.
predicate is often used together with library
library(pairs). It can be used to sort lists on different
or multiple criteria. For example, the following predicates sorts a list
of atoms according to their length, maintaining the initial order for
atoms that have the same length.
:- use_module(library(pairs)). sort_atoms_by_length(Atoms, ByLength) :- map_list_to_pairs(atom_length, Atoms, Pairs), keysort(Pairs, Sorted), pairs_values(Sorted, ByLength).
. If the built-in predicate compare/3 is used, the result is the same as sort/2. See also keysort/2.