<!DOCTYPE html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <meta name="generator" content="hevea 2.05"> <meta name="Author" content="Daniel Diaz"> <meta name="Keywords" content="GNU Prolog, manual, Prolog, compiler, constraints, finite domains"> <link rel="icon" type="image/x-icon" href="/gprolog.ico"><link rel="stylesheet" type="text/css" href="gprolog.css"> <title>Term comparison</title> </head> <body TEXT=black BGCOLOR=white> <a href="gprolog026.html"><img src="previous_motif.gif" alt="Previous"></a> <a href="gprolog024.html"><img src="contents_motif.gif" alt="Up"></a> <a href="gprolog028.html"><img src="next_motif.gif" alt="Next"></a> <hr> <h3 class="subsection" id="sec77">8.3  Term comparison</h3> <ul> <li><a href="gprolog027.html#sec78">Standard total ordering of terms</a> </li><li><a href="gprolog027.html#%28%3D%3D%29%2F2"><span class="c003">(==)/2</span> - term identical, <span class="c003">(</span><span class="c003">\</span><span class="c003">==)/2</span> - term not identical, <br> <span class="c003">(@</span><span class="c003"><</span><span class="c003">)/2</span> - term less than, <span class="c003">(@=</span><span class="c003"><</span><span class="c003">)/2</span> - term less than or equal to, <br> <span class="c003">(@</span><span class="c003">></span><span class="c003">)/2</span> - term greater than, <span class="c003">(@</span><span class="c003">></span><span class="c003">=)/2</span> - term greater than or equal to</a> </li><li><a href="gprolog027.html#compare%2F3"><span class="c003">compare/3</span> </a> </li></ul> <h4 class="subsubsection" id="sec78">8.3.1  Standard total ordering of terms</h4> <p> <a id="Standard-total-ordering-of-terms"></a> 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):</p><ul class="itemize"><li class="li-itemize">variables, oldest first.</li><li class="li-itemize">finite domain variables (section <a href="gprolog055.html#Finite-Domain-variables">9.1.1</a>), oldest first.</li><li class="li-itemize">floating point numbers, in numeric order.</li><li class="li-itemize">integers, in numeric order.</li><li class="li-itemize">atoms, in alphabetical (i.e. character code) order.</li><li class="li-itemize">compound terms, ordered first by arity, then by the name of the principal functor and by the arguments in left-to-right order.</li></ul><p>A list is treated as a compound term (whose principal functor is <span class="c003">’.’/2</span>).</p><p>The portability of the order of variables is not guaranteed (in the ISO reference the order of variables is system dependent).</p> <h4 class="subsubsection" id="(==)/2">8.3.2  <a id="hevea_default241"></a><span class="c003">(==)/2</span> - term identical, <a id="hevea_default242"></a><span class="c003">(\==)/2</span> - term not identical, <br> <a id="hevea_default243"></a> <a id="hevea_default244"></a> <a id="hevea_default245"></a> <a id="hevea_default246"></a> <span class="c003">(@<)/2</span> - term less than, <span class="c003">(@=<)/2</span> - term less than or equal to, <br> <span class="c003">(@>)/2</span> - term greater than, <span class="c003">(@>=)/2</span> - term greater than or equal to</h4> <p><span class="c009">Templates</span></p><dl class="list"><dt class="dt-list"> </dt><dd class="dd-list"><span class="c003"> ==(?term, ?term)<br> \==(?term, ?term) <br> @<(?term, ?term) <br> @=<(?term, ?term)<br> @>(?term, ?term)<br> @>=(?term, ?term)</span></dd></dl><p><span class="c009">Description</span></p><p>These predicates compare two terms according to the standard total ordering of terms (section <a href="#Standard-total-ordering-of-terms">8.3.1</a>).</p><p><span class="c003">Term1 == Term2</span> succeeds if <span class="c003">Term1</span> and <span class="c003">Term2</span> are equal.</p><p><span class="c003">Term1 \== Term2</span> succeeds if <span class="c003">Term1</span> and <span class="c003">Term2</span> are different.</p><p><span class="c003">Term1 @< Term2</span> succeeds if <span class="c003">Term1</span> is less than <span class="c003">Term2</span>.</p><p><span class="c003">Term1 @=< Term2</span> succeeds if <span class="c003">Term1</span> is less than or equal to <span class="c003">Term2</span>.</p><p><span class="c003">Term1 @> Term2</span> succeeds if <span class="c003">Term1</span> is greater than <span class="c003">Term2</span>.</p><p><span class="c003">Term1 @>= Term2</span> succeeds if <span class="c003">Term1</span> is greater than or equal to <span class="c003">Term2</span>.</p><p><span class="c003">==</span>, <span class="c003">\==</span>, <span class="c003">@<</span>, <span class="c003">@=<</span>, <span class="c003">@></span> and <span class="c003">@>=</span> are predefined infix operators (section <a href="gprolog038.html#op%2F3%3A%28Term-input%2Foutput%29">8.14.10</a>).</p><p><span class="c009">Errors</span></p><p>None.</p><p><span class="c009">Portability</span></p><p>ISO predicates.</p> <h4 class="subsubsection" id="compare/3">8.3.3  <a id="hevea_default247"></a><span class="c003">compare/3</span> </h4> <p><span class="c009">Templates</span></p><dl class="list"><dt class="dt-list"> </dt><dd class="dd-list"><span class="c003"> compare(?atom, +term, +term)</span></dd></dl><p><span class="c009">Description</span></p><p><span class="c003">compare(Order, Term1, Term2)</span> compares <span class="c003">Term1</span> and <span class="c003">Term2</span> according to the standard (section <a href="#Standard-total-ordering-of-terms">8.3.1</a>) and unifies <span class="c003">Order</span> with:</p><ul class="itemize"><li class="li-itemize">the atom <span class="c003"><</span> if <span class="c003">Term1</span> is less than <span class="c003">Term2</span>.</li><li class="li-itemize">the atom <span class="c003">=</span> if <span class="c003">Term1</span> and <span class="c003">Term2</span> are equal.</li><li class="li-itemize">the atom <span class="c003">></span> if <span class="c003">Term1</span> is greater than <span class="c003">Term2</span>.</li></ul><p><span class="c009">Errors</span></p><table class="c001 cellpading0"><tr><td class="hbar" colspan=3></td></tr> <tr><td class="c018"> <span class="c003">Order</span> is neither a variable nor an atom</td><td class="c013">  </td><td class="c018"><span class="c003">type_error(atom, Order)</span> </td></tr> <tr><td class="hbar" colspan=3></td></tr> <tr><td class="c018"> <span class="c003">Order</span> is an atom but not <span class="c003"><</span>, <span class="c003">=</span> or <span class="c003">></span></td><td class="c013">  </td><td class="c018"><span class="c003">domain_error(order, Order)</span> </td></tr> <tr><td class="hbar" colspan=3></td></tr> </table><p><span class="c009">Portability</span></p><p>ISO predicate.</p> <hr class="c011"> Copyright (C) 1999-2013 Daniel Diaz Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved. <a href="index.html#copyright">More about the copyright</a> <hr> <a href="gprolog026.html"><img src="previous_motif.gif" alt="Previous"></a> <a href="gprolog024.html"><img src="contents_motif.gif" alt="Up"></a> <a href="gprolog028.html"><img src="next_motif.gif" alt="Next"></a> </body> </html>
