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 <meta property="og:title" content="Schema class — Schema" />
 <meta property="og:description" content="A Schema is a list of Fields, which map names to
 Arrow data types. Create a Schema when you
 want to convert an R data.frame to Arrow but don't want to rely on the
 default mapping of R types to Arrow types, such as when you want to choose a
 specific numeric precision, or when creating a Dataset and you want to
 ensure a specific schema rather than inferring it from the various files.
 Many Arrow objects, including Table and Dataset, have a $schema method
 (active binding) that lets you access their schema." />


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 <div class="row">
   <div class="col-md-9 contents">
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     <h1>Schema class</h1>
     <small class="dont-index">Source: <a href='https://github.com/apache/arrow/blob/master/r/R/schema.R'><code>R/schema.R</code></a></small>
     <div class="hidden name"><code>Schema.Rd</code></div>
     </div>

     <div class="ref-description">
     <p>A <code>Schema</code> is a list of <a href='Field.html'>Field</a>s, which map names to
 Arrow <a href='data-type.html'>data types</a>. Create a <code>Schema</code> when you
 want to convert an R <code>data.frame</code> to Arrow but don't want to rely on the
 default mapping of R types to Arrow types, such as when you want to choose a
 specific numeric precision, or when creating a <a href='Dataset.html'>Dataset</a> and you want to
 ensure a specific schema rather than inferring it from the various files.</p>
 <p>Many Arrow objects, including <a href='Table.html'>Table</a> and <a href='Dataset.html'>Dataset</a>, have a <code>$schema</code> method
 (active binding) that lets you access their schema.</p>
     </div>

     <pre class="usage"><span class='fu'>schema</span><span class='op'>(</span><span class='va'>...</span><span class='op'>)</span></pre>

     <h2 class="hasAnchor" id="arguments"><a class="anchor" href="#arguments"></a>Arguments</h2>
     <table class="ref-arguments">
     <colgroup><col class="name" /><col class="desc" /></colgroup>
     <tr>
       <th>...</th>
       <td><p>named list of <a href='data-type.html'>data types</a></p></td>
     </tr>
     </table>

     <h2 class="hasAnchor" id="methods"><a class="anchor" href="#methods"></a>Methods</h2>


 <ul>
 <li><p><code>$ToString()</code>: convert to a string</p></li>
 <li><p><code>$field(i)</code>: returns the field at index <code>i</code> (0-based)</p></li>
 <li><p><code>$GetFieldByName(x)</code>: returns the field with name <code>x</code></p></li>
 <li><p><code>$WithMetadata(metadata)</code>: returns a new <code>Schema</code> with the key-value
 <code>metadata</code> set. Note that all list elements in <code>metadata</code> will be coerced
 to <code>character</code>.</p></li>
 </ul>

     <h2 class="hasAnchor" id="active-bindings"><a class="anchor" href="#active-bindings"></a>Active bindings</h2>


 <ul>
 <li><p><code>$names</code>: returns the field names (called in <code><a href='https://rdrr.io/r/base/names.html'>names(Schema)</a></code>)</p></li>
 <li><p><code>$num_fields</code>: returns the number of fields (called in <code><a href='https://rdrr.io/r/base/length.html'>length(Schema)</a></code>)</p></li>
 <li><p><code>$fields</code>: returns the list of <code>Field</code>s in the <code>Schema</code>, suitable for
 iterating over</p></li>
 <li><p><code>$HasMetadata</code>: logical: does this <code>Schema</code> have extra metadata?</p></li>
 <li><p><code>$metadata</code>: returns the key-value metadata as a named list.
 Modify or replace by assigning in (<code>sch$metadata &lt;- new_metadata</code>).
 All list elements are coerced to string.</p></li>
 </ul>

     <h2 class="hasAnchor" id="r-metadata"><a class="anchor" href="#r-metadata"></a>R Metadata</h2>


 <p>When converting a data.frame to an Arrow Table or RecordBatch, attributes
 from the <code>data.frame</code> are saved alongside tables so that the object can be
 reconstructed faithfully in R (e.g. with <code><a href='https://rdrr.io/r/base/as.data.frame.html'>as.data.frame()</a></code>). This metadata
 can be both at the top-level of the <code>data.frame</code> (e.g. <code><a href='https://rdrr.io/r/base/attributes.html'>attributes(df)</a></code>) or
 at the column (e.g. <code><a href='https://rdrr.io/r/base/attributes.html'>attributes(df$col_a)</a></code>) or for list columns only:
 element level (e.g. <code><a href='https://rdrr.io/r/base/attributes.html'>attributes(df[1, "col_a"])</a></code>). For example, this allows
 for storing <code>haven</code> columns in a table and being able to faithfully
 re-create them when pulled back into R. This metadata is separate from the
 schema (column names and types) which is compatible with other Arrow
 clients. The R metadata is only read by R and is ignored by other clients
 (e.g. Pandas has its own custom metadata). This metadata is stored in
 <code>$metadata$r</code>.</p>
 <p>Since Schema metadata keys and values must be strings, this metadata is
 saved by serializing R's attribute list structure to a string. If the
 serialized metadata exceeds 100Kb in size, by default it is compressed
 starting in version 3.0.0. To disable this compression (e.g. for tables
 that are compatible with Arrow versions before 3.0.0 and include large
 amounts of metadata), set the option <code>arrow.compress_metadata</code> to <code>FALSE</code>.
 Files with compressed metadata are readable by older versions of arrow, but
 the metadata is dropped.</p>

     <h2 class="hasAnchor" id="examples"><a class="anchor" href="#examples"></a>Examples</h2>
     <pre class="examples"><div class='input'><span class='co'># \donttest{</span>
 <span class='va'>df</span> <span class='op'>&lt;-</span> <span class='fu'><a href='https://rdrr.io/r/base/data.frame.html'>data.frame</a></span><span class='op'>(</span>col1 <span class='op'>=</span> <span class='fl'>2</span><span class='op'>:</span><span class='fl'>4</span>, col2 <span class='op'>=</span> <span class='fu'><a href='https://rdrr.io/r/base/c.html'>c</a></span><span class='op'>(</span><span class='fl'>0.1</span>, <span class='fl'>0.3</span>, <span class='fl'>0.5</span><span class='op'>)</span><span class='op'>)</span>
 <span class='va'>tab1</span> <span class='op'>&lt;-</span> <span class='va'>Table</span><span class='op'>$</span><span class='fu'>create</span><span class='op'>(</span><span class='va'>df</span><span class='op'>)</span>
 <span class='va'>tab1</span><span class='op'>$</span><span class='va'>schema</span>
 </div><div class='output co'>#&gt; Schema
 #&gt; col1: int32
 #&gt; col2: double
 #&gt;
 #&gt; See $metadata for additional Schema metadata</div><div class='input'><span class='va'>tab2</span> <span class='op'>&lt;-</span> <span class='va'>Table</span><span class='op'>$</span><span class='fu'>create</span><span class='op'>(</span><span class='va'>df</span>, schema <span class='op'>=</span> <span class='fu'>schema</span><span class='op'>(</span>col1 <span class='op'>=</span> <span class='fu'><a href='data-type.html'>int8</a></span><span class='op'>(</span><span class='op'>)</span>, col2 <span class='op'>=</span> <span class='fu'><a href='data-type.html'>float32</a></span><span class='op'>(</span><span class='op'>)</span><span class='op'>)</span><span class='op'>)</span>
 <span class='va'>tab2</span><span class='op'>$</span><span class='va'>schema</span>
 </div><div class='output co'>#&gt; Schema
 #&gt; col1: int8
 #&gt; col2: float
 #&gt;
 #&gt; See $metadata for additional Schema metadata</div><div class='input'><span class='co'># }</span>
 </div></pre>
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	<meta property="og:title" content="Schema class — Schema" />
	<meta property="og:description" content="A Schema is a list of Fields, which map names to
	Arrow data types. Create a Schema when you
	want to convert an R data.frame to Arrow but don't want to rely on the
	default mapping of R types to Arrow types, such as when you want to choose a
	specific numeric precision, or when creating a Dataset and you want to
	ensure a specific schema rather than inferring it from the various files.
	Many Arrow objects, including Table and Dataset, have a $schema method
	(active binding) that lets you access their schema." />




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	</header>

	<div class="row">
	<div class="col-md-9 contents">
	<div class="page-header">
	<h1>Schema class</h1>
	<small class="dont-index">Source: <a href='https://github.com/apache/arrow/blob/master/r/R/schema.R'><code>R/schema.R</code></a></small>
	<div class="hidden name"><code>Schema.Rd</code></div>
	</div>

	<div class="ref-description">
	<p>A <code>Schema</code> is a list of <a href='Field.html'>Field</a>s, which map names to
	Arrow <a href='data-type.html'>data types</a>. Create a <code>Schema</code> when you
	want to convert an R <code>data.frame</code> to Arrow but don't want to rely on the
	default mapping of R types to Arrow types, such as when you want to choose a
	specific numeric precision, or when creating a <a href='Dataset.html'>Dataset</a> and you want to
	ensure a specific schema rather than inferring it from the various files.</p>
	<p>Many Arrow objects, including <a href='Table.html'>Table</a> and <a href='Dataset.html'>Dataset</a>, have a <code>$schema</code> method
	(active binding) that lets you access their schema.</p>
	</div>

	<pre class="usage"><span class='fu'>schema</span><span class='op'>(</span><span class='va'>...</span><span class='op'>)</span></pre>

	<h2 class="hasAnchor" id="arguments"><a class="anchor" href="#arguments"></a>Arguments</h2>
	<table class="ref-arguments">
	<colgroup><col class="name" /><col class="desc" /></colgroup>
	<tr>
	<th>...</th>
	<td><p>named list of <a href='data-type.html'>data types</a></p></td>
	</tr>
	</table>

	<h2 class="hasAnchor" id="methods"><a class="anchor" href="#methods"></a>Methods</h2>



	<ul>
	<li><p><code>$ToString()</code>: convert to a string</p></li>
	<li><p><code>$field(i)</code>: returns the field at index <code>i</code> (0-based)</p></li>
	<li><p><code>$GetFieldByName(x)</code>: returns the field with name <code>x</code></p></li>
	<li><p><code>$WithMetadata(metadata)</code>: returns a new <code>Schema</code> with the key-value
	<code>metadata</code> set. Note that all list elements in <code>metadata</code> will be coerced
	to <code>character</code>.</p></li>
	</ul>

	<h2 class="hasAnchor" id="active-bindings"><a class="anchor" href="#active-bindings"></a>Active bindings</h2>



	<ul>
	<li><p><code>$names</code>: returns the field names (called in <code><a href='https://rdrr.io/r/base/names.html'>names(Schema)</a></code>)</p></li>
	<li><p><code>$num_fields</code>: returns the number of fields (called in <code><a href='https://rdrr.io/r/base/length.html'>length(Schema)</a></code>)</p></li>
	<li><p><code>$fields</code>: returns the list of <code>Field</code>s in the <code>Schema</code>, suitable for
	iterating over</p></li>
	<li><p><code>$HasMetadata</code>: logical: does this <code>Schema</code> have extra metadata?</p></li>
	<li><p><code>$metadata</code>: returns the key-value metadata as a named list.
	Modify or replace by assigning in (<code>sch$metadata <- new_metadata</code>).
	All list elements are coerced to string.</p></li>
	</ul>

	<h2 class="hasAnchor" id="r-metadata"><a class="anchor" href="#r-metadata"></a>R Metadata</h2>




	<p>When converting a data.frame to an Arrow Table or RecordBatch, attributes
	from the <code>data.frame</code> are saved alongside tables so that the object can be
	reconstructed faithfully in R (e.g. with <code><a href='https://rdrr.io/r/base/as.data.frame.html'>as.data.frame()</a></code>). This metadata
	can be both at the top-level of the <code>data.frame</code> (e.g. <code><a href='https://rdrr.io/r/base/attributes.html'>attributes(df)</a></code>) or
	at the column (e.g. <code><a href='https://rdrr.io/r/base/attributes.html'>attributes(df$col_a)</a></code>) or for list columns only:
	element level (e.g. <code><a href='https://rdrr.io/r/base/attributes.html'>attributes(df[1, "col_a"])</a></code>). For example, this allows
	for storing <code>haven</code> columns in a table and being able to faithfully
	re-create them when pulled back into R. This metadata is separate from the
	schema (column names and types) which is compatible with other Arrow
	clients. The R metadata is only read by R and is ignored by other clients
	(e.g. Pandas has its own custom metadata). This metadata is stored in
	<code>$metadata$r</code>.</p>
	<p>Since Schema metadata keys and values must be strings, this metadata is
	saved by serializing R's attribute list structure to a string. If the
	serialized metadata exceeds 100Kb in size, by default it is compressed
	starting in version 3.0.0. To disable this compression (e.g. for tables
	that are compatible with Arrow versions before 3.0.0 and include large
	amounts of metadata), set the option <code>arrow.compress_metadata</code> to <code>FALSE</code>.
	Files with compressed metadata are readable by older versions of arrow, but
	the metadata is dropped.</p>

	<h2 class="hasAnchor" id="examples"><a class="anchor" href="#examples"></a>Examples</h2>
	<pre class="examples"><div class='input'><span class='co'># \donttest{</span>
	<span class='va'>df</span> <span class='op'><-</span> <span class='fu'><a href='https://rdrr.io/r/base/data.frame.html'>data.frame</a></span><span class='op'>(</span>col1 <span class='op'>=</span> <span class='fl'>2</span><span class='op'>:</span><span class='fl'>4</span>, col2 <span class='op'>=</span> <span class='fu'><a href='https://rdrr.io/r/base/c.html'>c</a></span><span class='op'>(</span><span class='fl'>0.1</span>, <span class='fl'>0.3</span>, <span class='fl'>0.5</span><span class='op'>)</span><span class='op'>)</span>
	<span class='va'>tab1</span> <span class='op'><-</span> <span class='va'>Table</span><span class='op'>$</span><span class='fu'>create</span><span class='op'>(</span><span class='va'>df</span><span class='op'>)</span>
	<span class='va'>tab1</span><span class='op'>$</span><span class='va'>schema</span>
	</div><div class='output co'>#> Schema
	#> col1: int32
	#> col2: double
	#>
	#> See $metadata for additional Schema metadata</div><div class='input'><span class='va'>tab2</span> <span class='op'><-</span> <span class='va'>Table</span><span class='op'>$</span><span class='fu'>create</span><span class='op'>(</span><span class='va'>df</span>, schema <span class='op'>=</span> <span class='fu'>schema</span><span class='op'>(</span>col1 <span class='op'>=</span> <span class='fu'><a href='data-type.html'>int8</a></span><span class='op'>(</span><span class='op'>)</span>, col2 <span class='op'>=</span> <span class='fu'><a href='data-type.html'>float32</a></span><span class='op'>(</span><span class='op'>)</span><span class='op'>)</span><span class='op'>)</span>
	<span class='va'>tab2</span><span class='op'>$</span><span class='va'>schema</span>
	</div><div class='output co'>#> Schema
	#> col1: int8
	#> col2: float
	#>
	#> See $metadata for additional Schema metadata</div><div class='input'><span class='co'># }</span>
	</div></pre>
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