True when Sorted can be unified with a list holding the
element of
List. Key determines which part of each element in
List is used for comparing two term and Order
describes the relation between each set of consecutive elements in
Sorted.141The
definition of this predicate was established after discussion with
Joachim Schimpf from the ECLiPSe team. ECLiPSe currently only accepts <=<>>=
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 below:142For
compatibility with ECLiPSe, the values <=<>>=
| Order | Ordering | Duplicate handling |
@< | ascending | remove |
@=< | ascending | keep |
@> | descending | remove |
@>= | descending | keep |
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).
See also sort/2
(ISO), msort/2, keysort/2, predsort/3
and order_by/2.
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