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<p>A <span><i>cons</i></span> is a compound data <span><i>object</i></span> having two components called the <span><i>car</i></span> and the <span><i>cdr</i></span> .</p>
<p>|</p><p><strong>car cons rplacd</strong> </p><p><strong>cdr rplaca</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 14–1. Some defined names relating to conses.</strong></p>
<p>Depending on context, a group of connected <em>conses</em> can be viewed in a variety of different ways. A variety of operations is provided to support each of these various views.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="1411-conses-as-trees">14.1.1 Conses as Trees<a href="be-b-cons-concepts.html#1411-conses-as-trees" class="hash-link" aria-label="Direct link to 14.1.1 Conses as Trees" title="Direct link to 14.1.1 Conses as Trees">​</a></h2>
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<p>A <span><i>tree</i></span> is a binary recursive data structure made up of <em>conses</em> and <span><i>atoms</i></span>: the <em>conses</em> are themselves also <span><i>trees</i></span> (sometimes called “subtrees” or “branches”), and the <span><i>atoms</i></span> are terminal nodes (sometimes called <em>leaves</em>). Typically, the <em>leaves</em> represent data while the branches establish some relationship among that data.</p>
<p>|</p><p><strong>caaaar caddar cdar nsubst</strong> </p><p><strong>caaadr cadddr cddaar nsubst-if</strong> </p><p><strong>caaar caddr cddadr nsubst-if-not caadar cadr cddar nthcdr</strong> </p><p><strong>caaddr cdaaar cdddar sublis</strong> </p><p><strong>caadr cdaadr cddddr subst</strong> </p><p><strong>caar cdaar cdddr subst-if</strong> </p><p><strong>cadaar cdadar cddr subst-if-not cadadr cdaddr copy-tree tree-equal cadar cdadr nsublis</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 14–2. Some defined names relating to trees.</strong></p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14111-general-restrictions-on-parameters-that-must-be-trees">14.1.1.1 General Restrictions on Parameters that must be Trees<a href="be-b-cons-concepts.html#14111-general-restrictions-on-parameters-that-must-be-trees" class="hash-link" aria-label="Direct link to 14.1.1.1 General Restrictions on Parameters that must be Trees" title="Direct link to 14.1.1.1 General Restrictions on Parameters that must be Trees">​</a></h3>
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<p>Except as explicitly stated otherwise, for any <em>standardized function</em> that takes a <span><i>parameter</i></span> that is required to be a <span><i>tree</i></span>, the consequences are undefined if that <span><i>tree</i></span> is circular.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="1412-conses-as-lists">14.1.2 Conses as Lists<a href="be-b-cons-concepts.html#1412-conses-as-lists" class="hash-link" aria-label="Direct link to 14.1.2 Conses as Lists" title="Direct link to 14.1.2 Conses as Lists">​</a></h2>
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<p>A <span><i>list</i></span> is a chain of <em>conses</em> in which the <span><i>car</i></span> of each <span><i>cons</i></span> is an <span><i>element</i></span> of the <span><i>list</i></span>, and the <span><i>cdr</i></span> of each <span><i>cons</i></span> is either the next link in the chain or a terminating <span><i>atom</i></span>.</p>
<p>A <span><i>proper list</i></span> is a <span><i>list</i></span> terminated by the <span><i>empty list</i></span>. The <span><i>empty list</i></span> is a <span><i>proper list</i></span>, but is not a <span><i>cons</i></span>.</p>
<p>An <em>improper list</em> is a <span><i>list</i></span> that is not a <span><i>proper list</i></span>; that is, it is a <span><i>circular list</i></span> or a <span><i>dotted list</i></span>.</p>
<p>A <span><i>dotted list</i></span> is a <span><i>list</i></span> that has a terminating <span><i>atom</i></span> that is not the <span><i>empty list</i></span>. A <em>non-nil atom</em> by itself is not considered to be a <span><i>list</i></span> of any kind—not even a <span><i>dotted list</i></span>.</p>
<p>A <span><i>circular list</i></span> is a chain of <em>conses</em> that has no termination because some <span><i>cons</i></span> in the chain is the <span><i>cdr</i></span> of a later <span><i>cons</i></span>.</p>
<p>|</p><p><strong>append last nbutlast rest</strong> </p><p><strong>butlast ldiff nconc revappend copy-alist list ninth second copy-list list* nreconc seventh eighth list-length nth sixth endp make-list nthcdr tailp</strong> </p><p><strong>fifth member pop tenth first member-if push third fourth member-if-not pushnew</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 14–3. Some defined names relating to lists.</strong></p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14121-lists-as-association-lists">14.1.2.1 Lists as Association Lists<a href="be-b-cons-concepts.html#14121-lists-as-association-lists" class="hash-link" aria-label="Direct link to 14.1.2.1 Lists as Association Lists" title="Direct link to 14.1.2.1 Lists as Association Lists">​</a></h3>
<!-- -->
<p>An <span><i>association list</i></span> is a <span><i>list</i></span> of <em>conses</em> representing an association of <span><i>keys</i></span> with <span><i>values</i></span>, where the <span><i>car</i></span> of each <span><i>cons</i></span> is the <span><i>key</i></span> and the <span><i>cdr</i></span> is the <span><i>value</i></span> associated with that <span><i>key</i></span>.</p>
<p>|</p><p><strong>acons assoc-if pairlis rassoc-if</strong> </p><p><strong>assoc assoc-if-not rassoc rassoc-if-not</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 14–4. Some defined names related to assocation lists.</strong></p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14122-lists-as-sets">14.1.2.2 Lists as Sets<a href="be-b-cons-concepts.html#14122-lists-as-sets" class="hash-link" aria-label="Direct link to 14.1.2.2 Lists as Sets" title="Direct link to 14.1.2.2 Lists as Sets">​</a></h3>
<!-- -->
<p><span><i>Lists</i></span> are sometimes viewed as sets by considering their elements unordered and by assuming there is no duplication of elements.</p>
<p>|</p><p><strong>adjoin nset-difference set-difference union intersection nset-exclusive-or set-exclusive-or</strong> </p><p><strong>nintersection nunion subsetp</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 14–5. Some defined names related to sets.</strong></p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14123-general-restrictions-on-parameters-that-must-be-lists">14.1.2.3 General Restrictions on Parameters that must be Lists<a href="be-b-cons-concepts.html#14123-general-restrictions-on-parameters-that-must-be-lists" class="hash-link" aria-label="Direct link to 14.1.2.3 General Restrictions on Parameters that must be Lists" title="Direct link to 14.1.2.3 General Restrictions on Parameters that must be Lists">​</a></h3>
<!-- -->
<!-- -->
<p>Except as explicitly specified otherwise, any <em>standardized function</em> that takes a <span><i>parameter</i></span> that is required to be a <span><i>list</i></span> should be prepared to signal an error of <span><i>type</i></span> <span><b>type-error</b></span> if the <span><i>value</i></span> received is a <span><i>dotted list</i></span>.</p>
<p>Except as explicitly specified otherwise, for any <em>standardized function</em> that takes a <span><i>parameter</i></span> that is required to be a <span><i>list</i></span>, the consequences are undefined if that <span><i>list</i></span> is <span><i>circular</i></span> .</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/lisp-docs/cl-language-reference/tree/main/docs/chap-14/be-b-cons-concepts.md" target="_blank" rel="noopener noreferrer" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"><span class="theme-last-updated">Last updated<!-- --> by <b>daninus14</b></span></div></div></footer></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="../chap-13/bd-c-dictionary/upper-case-p_lower-case-p_both-case-p_function.html"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">upper-case-p, lower-case-p, both-case-p</div></a><a class="pagination-nav__link pagination-nav__link--next" href="../category/142-conses-dictionary.html"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">14.2 Conses Dictionary</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="be-b-cons-concepts.html#1411-conses-as-trees" class="table-of-contents__link toc-highlight">14.1.1 Conses as Trees</a><ul><li><a href="be-b-cons-concepts.html#14111-general-restrictions-on-parameters-that-must-be-trees" class="table-of-contents__link toc-highlight">14.1.1.1 General Restrictions on Parameters that must be Trees</a></li></ul></li><li><a href="be-b-cons-concepts.html#1412-conses-as-lists" class="table-of-contents__link toc-highlight">14.1.2 Conses as Lists</a><ul><li><a href="be-b-cons-concepts.html#14121-lists-as-association-lists" class="table-of-contents__link toc-highlight">14.1.2.1 Lists as Association Lists</a></li><li><a href="be-b-cons-concepts.html#14122-lists-as-sets" class="table-of-contents__link toc-highlight">14.1.2.2 Lists as Sets</a></li><li><a href="be-b-cons-concepts.html#14123-general-restrictions-on-parameters-that-must-be-lists" class="table-of-contents__link toc-highlight">14.1.2.3 General Restrictions on Parameters that must be Lists</a></li></ul></li></ul></div></div></div></div></main></div></div></div><footer class="footer footer--dark"><div class="container container-fluid"><div class="row footer__links"><div class="col footer__col"><div class="footer__title">Docs</div><ul class="footer__items clean-list"><li class="footer__item"><a href="../../docs/tutorial.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Common Lisp Tutorial</a></li><li class="footer__item"><a href="../index.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Common Lisp Technical Reference</a></li><li class="footer__item"><a href="../../docs/help.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Help</a></li></ul></div><div class="col footer__col"><div class="footer__title">Community</div><ul class="footer__items clean-list"><li class="footer__item"><a href="https://stackoverflow.com/questions/tagged/common-lisp" target="_blank" rel="noopener noreferrer" class="footer__link-item">Stack Overflow<svg width="13.5" height="13.5" aria-hidden="true" viewBox="0 0 24 24" class="iconExternalLink_nPIU"><path fill="currentColor" d="M21 13v10h-21v-19h12v2h-10v15h17v-8h2zm3-12h-10.988l4.035 4-6.977 7.07 2.828 2.828 6.977-7.07 4.125 4.172v-11z"></path></svg></a></li><li class="footer__item"><a href="https:
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