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title="up to "3 Built-In Datatypes"" data-pltdoc="x">up</a> <a href="boxes.html" title="forward to "3.11 Boxes"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""hash-tables"">3.10<tt> </tt><a name="(part._hash-tables)"></a>Hash Tables</h4><p>A <a name="(tech._hash._table)"></a><span style="font-style: italic">hash table</span> implements a mapping from keys to values, where
both keys and values can be arbitrary Racket values, and access and
update to the table are normally constant-time operations. Keys are
compared using <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">equal?</a></span>, <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eqv%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">eqv?</a></span>, or <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eq%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">eq?</a></span>, depending on whether
the hash table is created with <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-hash%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-hash</a></span>,
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-hasheqv%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-hasheqv</a></span>, or <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-hasheq%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-hasheq</a></span>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-hash%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-hash</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-set%2521%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-set!</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"apple"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">red</span><span class="hspace"> </span><span class="RktVal">round</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-set%2521%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-set!</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"banana"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">yellow</span><span class="hspace"> </span><span class="RktVal">long</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-ref%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"apple"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'(red round)</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-ref%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"coconut"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">hash-ref: no value found for key</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">key: "coconut"</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-ref%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"coconut"</span><span class="hspace"> </span><span class="RktVal">"not there"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"not there"</span></p></td></tr></table></blockquote></div></p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash</a></span>, <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hasheqv%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hasheqv</a></span>, and <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hasheq%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hasheq</a></span> functions
create immutable hash tables from an initial set of keys and values,
in which each value is provided as an argument after its key. Immutable
hash tables can be extended with <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-set%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-set</a></span>, which produces a
new immutable hash table in constant time.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash</a></span><span class="hspace"> </span><span class="RktVal">"apple"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">red</span><span class="hspace"> </span><span class="RktVal">"banana"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">yellow</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-ref%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"apple"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'red</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">ht2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-set%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-set</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"coconut"</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">brown</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-ref%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"coconut"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">hash-ref: no value found for key</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">key: "coconut"</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-ref%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">ht2</span><span class="hspace"> </span><span class="RktVal">"coconut"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'brown</span></p></td></tr></table></blockquote></div></p><p>A literal immutable hash table can be written as an expression by using
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hash</span><span class="hspace"></span></span> (for an <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">equal?</a></span>-based table),
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hasheqv</span><span class="hspace"></span></span> (for an <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eqv%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">eqv?</a></span>-based table), or
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hasheq</span><span class="hspace"></span></span> (for an <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eq%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">eq?</a></span>-based table). A parenthesized
sequence must immediately follow <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hash</span><span class="hspace"></span></span>, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hasheq</span><span class="hspace"></span></span>,
or <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hasheqv</span><span class="hspace"></span></span>, where each element is a dotted
key–value pair. The <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hash</span><span class="hspace"></span></span>, etc. forms implicitly
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=quote.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._quote%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">quote</a></span> their key and value sub-forms.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">#hash</span><span class="RktVal">(</span><span class="RktVal">(</span><span class="RktVal">"apple"</span><span class="hspace"> </span><span class="RktVal">. </span><span class="RktVal">red</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal">"banana"</span><span class="hspace"> </span><span class="RktVal">. </span><span class="RktVal">yellow</span><span class="RktVal">)</span><span class="RktVal">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-ref%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-ref</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktVal">"apple"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'red</span></p></td></tr></table></blockquote></div></p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=reader.html%23%2528part._parse-hashtable%2529&version=6.7" class="Sq" data-pltdoc="x">Reading Hash Tables</a> in <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.7" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Reference</span></a> documents the fine points of the syntax of hash table literals.</p></blockquote></blockquote></blockquote><p>Both mutable and immutable hash tables print like immutable hash
tables, using a quoted <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hash</span><span class="hspace"></span></span>, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hasheqv</span><span class="hspace"></span></span>, or
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#hasheq</span><span class="hspace"></span></span> form if all keys and values can be expressed with
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=quote.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._quote%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">quote</a></span> or using <span class="RktRes">hash</span>, <span class="RktRes">hasheq</span>, or
<span class="RktRes">hasheqv</span> otherwise:</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktVal">#hash</span><span class="RktVal">(</span><span class="RktVal">(</span><span class="RktVal">"apple"</span><span class="hspace"> </span><span class="RktVal">. </span><span class="RktVal">red</span><span class="RktVal">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">(</span><span class="RktVal">"banana"</span><span class="hspace"> </span><span class="RktVal">. </span><span class="RktVal">yellow</span><span class="RktVal">)</span><span class="RktVal">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'#hash(("apple" . red) ("banana" . yellow))</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._srcloc%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">srcloc</a></span><span class="hspace"> </span><span class="RktVal">"file.rkt"</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">0</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252B%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">4</span><span class="hspace"> </span><span class="RktVal">4</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">(hash 1 (srcloc "file.rkt" 1 0 1 8))</span></p></td></tr></table></blockquote></div></p><p>A mutable hash table can optionally retain its keys
<span style="font-style: italic">weakly</span>, so each mapping is retained only so long as the key
is retained elsewhere.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-weak-hasheq%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-weak-hasheq</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-set%2521%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-set!</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=symbols.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._gensym%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">gensym</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">"can you see me?"</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=garbagecollection.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._collect-garbage%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">collect-garbage</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-count%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-count</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">0</span></p></td></tr></table></blockquote></div></p><p>Beware that even a weak hash table retains its values strongly, as
long as the corresponding key is accessible. This creates a catch-22
dependency when a value refers back to its key, so that the mapping is
retained permanently. To break the cycle, map the key to an <span style="font-style: italic">ephemeron</span>
that pairs the value with its key (in addition to the implicit pairing
of the hash table).</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=ephemerons.html&version=6.7" class="Sq" data-pltdoc="x">Ephemerons</a> in <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.7" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Reference</span></a> documents the fine points of using ephemerons.</p></blockquote></blockquote></blockquote><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-weak-hasheq%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-weak-hasheq</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">g</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=symbols.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._gensym%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">gensym</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-set%2521%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-set!</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktSym">g</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">g</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=garbagecollection.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._collect-garbage%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">collect-garbage</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-count%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-count</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">1</span></p></td></tr></table></blockquote></div></p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-weak-hasheq%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-weak-hasheq</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">g</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=symbols.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._gensym%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">gensym</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-set%2521%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-set!</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="hspace"> </span><span class="RktSym">g</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=ephemerons.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-ephemeron%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-ephemeron</a></span><span class="hspace"> </span><span class="RktSym">g</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktSym">g</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=garbagecollection.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._collect-garbage%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">collect-garbage</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._hash-count%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">hash-count</a></span><span class="hspace"> </span><span class="RktSym">ht</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">0</span></p></td></tr></table></blockquote><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=hashtables.html&version=6.7" class="Sq" data-pltdoc="x">Hash Tables</a> in <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.7" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Reference</span></a> provides more on hash tables and hash-table procedures.</p></blockquote></blockquote></blockquote><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="vectors.html" title="backward to "3.9 Vectors"" data-pltdoc="x">← prev</a> <a href="datatypes.html" title="up to "3 Built-In Datatypes"" data-pltdoc="x">up</a> <a href="boxes.html" title="forward to "3.11 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