This behaviour is not entirely obvious. It should be given as an example.
?- sort(1, @>, [1-2, 2-1, 2-2], X)
.
X = [2-1, 1-2].
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Predicate sort/4 |
<
, =<
, >
and >=
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 below:144For
compatibility with ECLiPSe, the values
, <
, =<
and >
are allowed as synonyms.
>=
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.
This behaviour is not entirely obvious. It should be given as an example.
?- sort(1, @>, [1-2, 2-1, 2-2], X)
.
X = [2-1, 1-2].
Does this predicate come from ECLIPSE?
In the ECLIPSE manual: sort(+Key, +Order, +Random, -Sorted)
sort(4, @>=, Rows0, Rows1)
,
this should be explained with more example. I don't understand the role of the key value. And what kind of list is Rows0.