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   <section id="reading-and-writing-datasets">
 <h1><a class="toc-backref" href="#id7" role="doc-backlink">Reading and Writing Datasets</a><a class="headerlink" href="#reading-and-writing-datasets" title="Link to this heading">¶</a></h1>
 <p>This section contains a number of recipes for reading and writing
 datasets.  Datasets are a collection of one or more files containing
 tabular data.</p>
 <nav class="contents" id="contents">
 <p class="topic-title">Contents</p>
 <ul class="simple">
 <li><p><a class="reference internal" href="#reading-and-writing-datasets" id="id7">Reading and Writing Datasets</a></p>
 <ul>
 <li><p><a class="reference internal" href="#read-a-partitioned-dataset" id="id8">Read a Partitioned Dataset</a></p></li>
 </ul>
 </li>
 </ul>
 </nav>
 <section id="read-a-partitioned-dataset">
 <h2><a class="toc-backref" href="#id8" role="doc-backlink">Read a Partitioned Dataset</a><a class="headerlink" href="#read-a-partitioned-dataset" title="Link to this heading">¶</a></h2>
 <p>The individual data files that make up a dataset will often be
 distributed across several different directories according to some
 kind of partitioning scheme.</p>
 <p>This simplifies management of the data and also allows for partial
 reads of the dataset by inspecting the file paths and utilizing the
 guarantees provided by the partitioning scheme.</p>
 <p>This recipe demonstrates the basics of reading a partitioned dataset.
 First let us inspect our data:</p>
 <div class="literal-block-wrapper docutils container" id="id1">
 <div class="code-block-caption"><span class="caption-text">A listing of files in our dataset</span><a class="headerlink" href="#id1" title="Link to this code">¶</a></div>
 <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span><span class="w"> </span><span class="n">directory_base</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">airquality_basedir</span><span class="p">;</span>

 <span class="c1">// Create a filesystem</span>
 <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">fs</span><span class="o">::</span><span class="n">LocalFileSystem</span><span class="o">&gt;</span><span class="w"> </span><span class="n">fs</span><span class="w"> </span><span class="o">=</span>
 <span class="w">    </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">fs</span><span class="o">::</span><span class="n">LocalFileSystem</span><span class="o">&gt;</span><span class="p">();</span>

 <span class="c1">// Create a file selector which describes which files are part of</span>
 <span class="c1">// the dataset.  This selector performs a recursive search of a base</span>
 <span class="c1">// directory which is typical with partitioned datasets.  You can also</span>
 <span class="c1">// create a dataset from a list of one or more paths.</span>
 <span class="n">arrow</span><span class="o">::</span><span class="n">fs</span><span class="o">::</span><span class="n">FileSelector</span><span class="w"> </span><span class="n">selector</span><span class="p">;</span>
 <span class="n">selector</span><span class="p">.</span><span class="n">base_dir</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">directory_base</span><span class="p">;</span>
 <span class="n">selector</span><span class="p">.</span><span class="n">recursive</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">true</span><span class="p">;</span>

 <span class="c1">// List out the files so we can see how our data is partitioned.</span>
 <span class="c1">// This step is not necessary for reading a dataset</span>
 <span class="n">ARROW_ASSIGN_OR_RAISE</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">fs</span><span class="o">::</span><span class="n">FileInfo</span><span class="o">&gt;</span><span class="w"> </span><span class="n">file_infos</span><span class="p">,</span>
 <span class="w">                      </span><span class="n">fs</span><span class="o">-&gt;</span><span class="n">GetFileInfo</span><span class="p">(</span><span class="n">selector</span><span class="p">));</span>
 <span class="kt">int</span><span class="w"> </span><span class="n">num_printed</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
 <span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="k">auto</span><span class="o">&amp;</span><span class="w"> </span><span class="n">path</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">file_infos</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
 <span class="w">  </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">path</span><span class="p">.</span><span class="n">IsFile</span><span class="p">())</span><span class="w"> </span><span class="p">{</span>
 <span class="w">    </span><span class="n">rout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">path</span><span class="p">.</span><span class="n">path</span><span class="p">().</span><span class="n">substr</span><span class="p">(</span><span class="n">directory_base</span><span class="p">.</span><span class="n">size</span><span class="p">())</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
 <span class="w">    </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">++</span><span class="n">num_printed</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">10</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
 <span class="w">      </span><span class="n">rout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;...&quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
 <span class="w">      </span><span class="k">break</span><span class="p">;</span>
 <span class="w">    </span><span class="p">}</span>
 <span class="w">  </span><span class="p">}</span>
 <span class="p">}</span>
 </pre></div>
 </div>
 </div>
 <div class="literal-block-wrapper docutils container" id="id2">
 <div class="code-block-caption"><span class="caption-text">Code Output</span><a class="headerlink" href="#id2" title="Link to this code">¶</a></div>
 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">15</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">20</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">24</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">23</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">16</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">13</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">25</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">18</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">1</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">/</span><span class="n">Month</span><span class="o">=</span><span class="mi">8</span><span class="o">/</span><span class="n">Day</span><span class="o">=</span><span class="mi">17</span><span class="o">/</span><span class="n">chunk</span><span class="o">-</span><span class="mf">0.</span><span class="n">parquet</span>
 <span class="o">...</span>
 </pre></div>
 </div>
 </div>
 <div class="admonition note">
 <p class="admonition-title">Note</p>
 <p>This partitioning scheme of key=value is referred to as “hive”
 partitioning within Arrow.</p>
 </div>
 <p>Now that we have a filesystem and a selector we can go ahead and create
 a dataset.  To do this we need to pick a format and a partitioning
 scheme.  Once we have all of the pieces we need we can create an
 arrow::dataset::Dataset instance.</p>
 <div class="literal-block-wrapper docutils container" id="id3">
 <div class="code-block-caption"><span class="caption-text">Creating an arrow::dataset::Dataset instance</span><a class="headerlink" href="#id3" title="Link to this code">¶</a></div>
 <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// Create a file format which describes the format of the files.</span>
 <span class="c1">// Here we specify we are reading parquet files.  We could pick a different format</span>
 <span class="c1">// such as Arrow-IPC files or CSV files or we could customize the parquet format with</span>
 <span class="c1">// additional reading &amp; parsing options.</span>
 <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">ParquetFileFormat</span><span class="o">&gt;</span><span class="w"> </span><span class="n">format</span><span class="w"> </span><span class="o">=</span>
 <span class="w">    </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">ParquetFileFormat</span><span class="o">&gt;</span><span class="p">();</span>

 <span class="c1">// Create a partitioning factory.  A partitioning factory will be used by a dataset</span>
 <span class="c1">// factory to infer the partitioning schema from the filenames.  All we need to</span>
 <span class="c1">// specify is the flavor of partitioning which, in our case, is &quot;hive&quot;.</span>
 <span class="c1">//</span>
 <span class="c1">// Alternatively, we could manually create a partitioning scheme from a schema.  This</span>
 <span class="c1">// is typically not necessary for hive partitioning as inference works well.</span>
 <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">PartitioningFactory</span><span class="o">&gt;</span><span class="w"> </span><span class="n">partitioning_factory</span><span class="w"> </span><span class="o">=</span>
 <span class="w">    </span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">HivePartitioning</span><span class="o">::</span><span class="n">MakeFactory</span><span class="p">();</span>

 <span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">FileSystemFactoryOptions</span><span class="w"> </span><span class="n">options</span><span class="p">;</span>
 <span class="n">options</span><span class="p">.</span><span class="n">partitioning</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">partitioning_factory</span><span class="p">;</span>

 <span class="c1">// Create a dataset factory</span>
 <span class="n">ARROW_ASSIGN_OR_RAISE</span><span class="p">(</span>
 <span class="w">    </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">DatasetFactory</span><span class="o">&gt;</span><span class="w"> </span><span class="n">dataset_factory</span><span class="p">,</span>
 <span class="w">    </span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">FileSystemDatasetFactory</span><span class="o">::</span><span class="n">Make</span><span class="p">(</span><span class="n">fs</span><span class="p">,</span><span class="w"> </span><span class="n">selector</span><span class="p">,</span><span class="w"> </span><span class="n">format</span><span class="p">,</span><span class="w"> </span><span class="n">options</span><span class="p">));</span>

 <span class="c1">// Create the dataset, this will scan the dataset directory to find all the files</span>
 <span class="c1">// and may scan some file metadata in order to determine the dataset schema.</span>
 <span class="n">ARROW_ASSIGN_OR_RAISE</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">Dataset</span><span class="o">&gt;</span><span class="w"> </span><span class="n">dataset</span><span class="p">,</span>
 <span class="w">                      </span><span class="n">dataset_factory</span><span class="o">-&gt;</span><span class="n">Finish</span><span class="p">());</span>

 <span class="n">rout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;We discovered the following schema for the dataset:&quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span>
 <span class="w">     </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span>
 <span class="w">     </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">dataset</span><span class="o">-&gt;</span><span class="n">schema</span><span class="p">()</span><span class="o">-&gt;</span><span class="n">ToString</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
 </pre></div>
 </div>
 </div>
 <div class="literal-block-wrapper docutils container" id="id4">
 <div class="code-block-caption"><span class="caption-text">Code Output</span><a class="headerlink" href="#id4" title="Link to this code">¶</a></div>
 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">We</span> <span class="n">discovered</span> <span class="n">the</span> <span class="n">following</span> <span class="n">schema</span> <span class="k">for</span> <span class="n">the</span> <span class="n">dataset</span><span class="p">:</span>

 <span class="n">Ozone</span><span class="p">:</span> <span class="n">int32</span>
 <span class="n">Solar</span><span class="o">.</span><span class="n">R</span><span class="p">:</span> <span class="n">int32</span>
 <span class="n">Wind</span><span class="p">:</span> <span class="n">double</span>
 <span class="n">Temp</span><span class="p">:</span> <span class="n">int32</span>
 <span class="n">Month</span><span class="p">:</span> <span class="n">int32</span>
 <span class="n">Day</span><span class="p">:</span> <span class="n">int32</span>
 </pre></div>
 </div>
 </div>
 <p>Once we have a dataset object we can read in the data.  Reading the data
 from a dataset is sometimes called “scanning” the dataset and the object
 we use to do this is an arrow::dataset::Scanner.  The following snippet
 shows how to scan the entire dataset into an in-memory table:</p>
 <div class="literal-block-wrapper docutils container" id="id5">
 <div class="code-block-caption"><span class="caption-text">Scanning a dataset into an arrow::Table</span><a class="headerlink" href="#id5" title="Link to this code">¶</a></div>
 <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// Create a scanner</span>
 <span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">ScannerBuilder</span><span class="w"> </span><span class="nf">scanner_builder</span><span class="p">(</span><span class="n">dataset</span><span class="p">);</span>
 <span class="n">ARROW_RETURN_NOT_OK</span><span class="p">(</span><span class="n">scanner_builder</span><span class="p">.</span><span class="n">UseThreads</span><span class="p">(</span><span class="nb">true</span><span class="p">));</span>
 <span class="n">ARROW_ASSIGN_OR_RAISE</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">dataset</span><span class="o">::</span><span class="n">Scanner</span><span class="o">&gt;</span><span class="w"> </span><span class="n">scanner</span><span class="p">,</span>
 <span class="w">                      </span><span class="n">scanner_builder</span><span class="p">.</span><span class="n">Finish</span><span class="p">());</span>

 <span class="c1">// Scan the dataset.  There are a variety of other methods available on the scanner as</span>
 <span class="c1">// well</span>
 <span class="n">ARROW_ASSIGN_OR_RAISE</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">arrow</span><span class="o">::</span><span class="n">Table</span><span class="o">&gt;</span><span class="w"> </span><span class="n">table</span><span class="p">,</span><span class="w"> </span><span class="n">scanner</span><span class="o">-&gt;</span><span class="n">ToTable</span><span class="p">());</span>
 <span class="n">rout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;Read in a table with &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">table</span><span class="o">-&gt;</span><span class="n">num_rows</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot; rows and &quot;</span>
 <span class="w">     </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">table</span><span class="o">-&gt;</span><span class="n">num_columns</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot; columns&quot;</span><span class="p">;</span>
 </pre></div>
 </div>
 </div>
 <div class="literal-block-wrapper docutils container" id="id6">
 <div class="code-block-caption"><span class="caption-text">Code Output</span><a class="headerlink" href="#id6" title="Link to this code">¶</a></div>
 <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Read</span> <span class="ow">in</span> <span class="n">a</span> <span class="n">table</span> <span class="k">with</span> <span class="mi">153</span> <span class="n">rows</span> <span class="ow">and</span> <span class="mi">6</span> <span class="n">columns</span>
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