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<h1><span class="section-number">7.8.1. </span>Summary<a class="headerlink" href="#summary" title="Permalink to this headline">¶</a></h1>
<p>Groonga has tokenizer module that tokenizes text. It is used when
the following cases:</p>
<blockquote>
<div><ul>
<li><p>Indexing text</p>
<div class="figure align-center" id="id1">
<a class="reference internal image-reference" href="../../_images/used-when-indexing.png"><img alt="../../_images/used-when-indexing.png" src="../../_images/used-when-indexing.png" style="width: 80%;" /></a>
<p class="caption"><span class="caption-text">Tokenizer is used when indexing text.</span><a class="headerlink" href="#id1" title="Permalink to this image">¶</a></p>
</div>
</li>
<li><p>Searching by query</p>
<div class="figure align-center" id="id2">
<a class="reference internal image-reference" href="../../_images/used-when-searching.png"><img alt="../../_images/used-when-searching.png" src="../../_images/used-when-searching.png" style="width: 80%;" /></a>
<p class="caption"><span class="caption-text">Tokenizer is used when searching by query.</span><a class="headerlink" href="#id2" title="Permalink to this image">¶</a></p>
</div>
</li>
</ul>
</div></blockquote>
<p>Tokenizer is an important module for full-text search. You can change
trade-off between <a class="reference external" href="http://en.wikipedia.org/wiki/Precision_and_recall">precision and recall</a> by changing
tokenizer.</p>
<p>Normally, <a class="reference internal" href="../tokenizers/token_bigram.html#token-bigram"><span class="std std-ref">TokenBigram</span></a> is a suitable tokenizer. If you don’t
know much about tokenizer, it’s recommended that you choose
<a class="reference internal" href="../tokenizers/token_bigram.html#token-bigram"><span class="std std-ref">TokenBigram</span></a>.</p>
<p>You can try a tokenizer by <a class="reference internal" href="../commands/tokenize.html"><span class="doc">tokenize</span></a> and
<a class="reference internal" href="../commands/table_tokenize.html"><span class="doc">table_tokenize</span></a>. Here is an example to
try <a class="reference internal" href="../tokenizers/token_bigram.html#token-bigram"><span class="std std-ref">TokenBigram</span></a> tokenizer by
<a class="reference internal" href="../commands/tokenize.html"><span class="doc">tokenize</span></a>:</p>
<p>Execution example:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>tokenize TokenBigram "Hello World"
# [
# [
# 0,
# 1337566253.89858,
# 0.000355720520019531
# ],
# [
# {
# "position": 0,
# "force_prefix": false,
# "value": "He"
# },
# {
# "position": 1,
# "force_prefix": false,
# "value": "el"
# },
# {
# "position": 2,
# "force_prefix": false,
# "value": "ll"
# },
# {
# "position": 3,
# "force_prefix": false,
# "value": "lo"
# },
# {
# "position": 4,
# "force_prefix": false,
# "value": "o "
# },
# {
# "position": 5,
# "force_prefix": false,
# "value": " W"
# },
# {
# "position": 6,
# "force_prefix": false,
# "value": "Wo"
# },
# {
# "position": 7,
# "force_prefix": false,
# "value": "or"
# },
# {
# "position": 8,
# "force_prefix": false,
# "value": "rl"
# },
# {
# "position": 9,
# "force_prefix": false,
# "value": "ld"
# },
# {
# "position": 10,
# "force_prefix": false,
# "value": "d"
# }
# ]
# ]
</pre></div>
</div>
<p>“tokenize” is the process that extracts zero or more tokens from a
text. There are some “tokenize” methods.</p>
<p>For example, <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">World</span></code> is tokenized to the following tokens by
bigram tokenize method:</p>
<blockquote>
<div><ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">He</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">el</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ll</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">lo</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">o_</span></code> (<code class="docutils literal notranslate"><span class="pre">_</span></code> means a white-space)</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">_W</span></code> (<code class="docutils literal notranslate"><span class="pre">_</span></code> means a white-space)</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Wo</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">or</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">rl</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ld</span></code></p></li>
</ul>
</div></blockquote>
<p>In the above example, 10 tokens are extracted from one text <code class="docutils literal notranslate"><span class="pre">Hello</span>
<span class="pre">World</span></code>.</p>
<p>For example, <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">World</span></code> is tokenized to the following tokens by
white-space-separate tokenize method:</p>
<blockquote>
<div><ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">Hello</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">World</span></code></p></li>
</ul>
</div></blockquote>
<p>In the above example, 2 tokens are extracted from one text <code class="docutils literal notranslate"><span class="pre">Hello</span>
<span class="pre">World</span></code>.</p>
<p>Token is used as search key. You can find indexed documents only by
tokens that are extracted by used tokenize method. For example, you
can find <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">World</span></code> by <code class="docutils literal notranslate"><span class="pre">ll</span></code> with bigram tokenize method but you
can’t find <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">World</span></code> by <code class="docutils literal notranslate"><span class="pre">ll</span></code> with white-space-separate tokenize
method. Because white-space-separate tokenize method doesn’t extract
<code class="docutils literal notranslate"><span class="pre">ll</span></code> token. It just extracts <code class="docutils literal notranslate"><span class="pre">Hello</span></code> and <code class="docutils literal notranslate"><span class="pre">World</span></code> tokens.</p>
<p>In general, tokenize method that generates small tokens increases
recall but decreases precision. Tokenize method that generates large
tokens increases precision but decreases recall.</p>
<p>For example, we can find <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">World</span></code> and <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">or</span> <span class="pre">B</span></code> by <code class="docutils literal notranslate"><span class="pre">or</span></code> with
bigram tokenize method. <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">World</span></code> is a noise for people who
wants to search “logical and”. It means that precision is
decreased. But recall is increased.</p>
<p>We can find only <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">or</span> <span class="pre">B</span></code> by <code class="docutils literal notranslate"><span class="pre">or</span></code> with white-space-separate
tokenize method. Because <code class="docutils literal notranslate"><span class="pre">World</span></code> is tokenized to one token <code class="docutils literal notranslate"><span class="pre">World</span></code>
with white-space-separate tokenize method. It means that precision is
increased for people who wants to search “logical and”. But recall is
decreased because <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">World</span></code> that contains <code class="docutils literal notranslate"><span class="pre">or</span></code> isn’t found.</p>
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