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8.3  Term comparison

8.3.1  Standard total ordering of terms

The built-in predicates described in this section allows the user to compare Prolog terms. Prolog terms are totally ordered according to the standard total ordering of terms which is as follows (from the smallest term to the greatest):

A list is treated as a compound term (whose principal functor is ’.’/2).

The portability of the order of variables is not guaranteed (in the ISO reference the order of variables is system dependent).

8.3.2  (==)/2 - term identical, (\==)/2 - term not identical,
(@<)/2 - term less than, (@=<)/2 - term less than or equal to,
(@>)/2 - term greater than, (@>=)/2 - term greater than or equal to

Templates

==(?term, ?term)
\==(?term, ?term)
@<(?term, ?term)
@=<(?term, ?term)
@>(?term, ?term)
@>=(?term, ?term)

Description

These predicates compare two terms according to the standard total ordering of terms (section 8.3.1).

Term1 == Term2 succeeds if Term1 and Term2 are equal.

Term1 \== Term2 succeeds if Term1 and Term2 are different.

Term1 @< Term2 succeeds if Term1 is less than Term2.

Term1 @=< Term2 succeeds if Term1 is less than or equal to Term2.

Term1 @> Term2 succeeds if Term1 is greater than Term2.

Term1 @>= Term2 succeeds if Term1 is greater than or equal to Term2.

==, \==, @<, @=<, @> and @>= are predefined infix operators (section 8.14.10).

Errors

None.

Portability

ISO predicates.

8.3.3  compare/3

Templates

compare(?atom, +term, +term)

Description

compare(Order, Term1, Term2) compares Term1 and Term2 according to the standard (section 8.3.1) and unifies Order with:

Errors

Order is neither a variable nor an atom  type_error(atom, Order)
Order is an atom but not <, = or >  domain_error(order, Order)

Portability

ISO predicate.


Copyright (C) 1999-2013 Daniel Diaz Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved. More about the copyright
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