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| <main id="main" class="col-md-9"><div class="page-header"> |
| <img src="" class="logo" alt=""><h1>Schema class</h1> |
| <small class="dont-index">Source: <a href="https://github.com/apache/arrow/blob/master/r/R/schema.R" class="external-link"><code>R/schema.R</code></a></small> |
| <div class="d-none name"><code>Schema.Rd</code></div> |
| </div> |
| |
| <div class="ref-description section level2"> |
| <p>A <code>Schema</code> is an Arrow object containing <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>$schema</code> method |
| (active binding) that lets you access their schema.</p> |
| </div> |
| |
| <div class="section level2"> |
| <h2 id="ref-usage">Usage<a class="anchor" aria-label="anchor" href="#ref-usage"></a></h2> |
| <div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">schema</span><span class="op">(</span><span class="va">...</span><span class="op">)</span></span></code></pre></div> |
| </div> |
| |
| <div class="section level2"> |
| <h2 id="arguments">Arguments<a class="anchor" aria-label="anchor" href="#arguments"></a></h2> |
| <dl><dt>...</dt> |
| <dd><p><a href="Field.html">fields</a> or field name/<a href="data-type.html">data type</a> pairs</p></dd> |
| |
| </dl></div> |
| <div class="section level2"> |
| <h2 id="methods">Methods<a class="anchor" aria-label="anchor" href="#methods"></a></h2> |
| |
| |
| <ul><li><p><code>$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>$WithMetadata(metadata)</code>: returns a new <code>Schema</code> with the key-value |
| <code>metadata</code> set. Note that all list elements in <code>metadata</code> will be coerced |
| to <code>character</code>.</p></li> |
| </ul></div> |
| <div class="section level2"> |
| <h2 id="active-bindings">Active bindings<a class="anchor" aria-label="anchor" href="#active-bindings"></a></h2> |
| |
| |
| <ul><li><p><code>$names</code>: returns the field names (called in <code>names(Schema)</code>)</p></li> |
| <li><p><code>$num_fields</code>: returns the number of fields (called in <code>length(Schema)</code>)</p></li> |
| <li><p><code>$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$metadata <- new_metadata</code>). |
| All list elements are coerced to string.</p></li> |
| </ul></div> |
| <div class="section level2"> |
| <h2 id="r-metadata">R Metadata<a class="anchor" aria-label="anchor" href="#r-metadata"></a></h2> |
| |
| |
| |
| <p>When converting a data.frame to an Arrow Table or RecordBatch, attributes |
| from the <code>data.frame</code> are saved alongside tables so that the object can be |
| reconstructed faithfully in R (e.g. with <code><a href="https://rdrr.io/r/base/as.data.frame.html" class="external-link">as.data.frame()</a></code>). This metadata |
| can be both at the top-level of the <code>data.frame</code> (e.g. <code>attributes(df)</code>) or |
| at the column (e.g. <code>attributes(df$col_a)</code>) or for list columns only: |
| element level (e.g. <code>attributes(df[1, "col_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> |
| </div> |
| |
| <div class="section level2"> |
| <h2 id="ref-examples">Examples<a class="anchor" aria-label="anchor" href="#ref-examples"></a></h2> |
| <div class="sourceCode"><pre class="sourceCode r"><code><span class="r-in"><span><span class="fu">schema</span><span class="op">(</span>a <span class="op">=</span> <span class="fu"><a href="data-type.html">int32</a></span><span class="op">(</span><span class="op">)</span>, b <span class="op">=</span> <span class="fu"><a href="data-type.html">float64</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span></span> |
| <span class="r-out co"><span class="r-pr">#></span> Schema</span> |
| <span class="r-out co"><span class="r-pr">#></span> a: int32</span> |
| <span class="r-out co"><span class="r-pr">#></span> b: double</span> |
| <span class="r-in"><span></span></span> |
| <span class="r-in"><span><span class="fu">schema</span><span class="op">(</span></span></span> |
| <span class="r-in"><span> <span class="fu"><a href="Field.html">field</a></span><span class="op">(</span><span class="st">"b"</span>, <span class="fu"><a href="https://rdrr.io/r/base/double.html" class="external-link">double</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span>,</span></span> |
| <span class="r-in"><span> <span class="fu"><a href="Field.html">field</a></span><span class="op">(</span><span class="st">"c"</span>, <span class="fu"><a href="data-type.html">bool</a></span><span class="op">(</span><span class="op">)</span>, nullable <span class="op">=</span> <span class="cn">FALSE</span><span class="op">)</span>,</span></span> |
| <span class="r-in"><span> <span class="fu"><a href="Field.html">field</a></span><span class="op">(</span><span class="st">"d"</span>, <span class="fu"><a href="data-type.html">string</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span></span> |
| <span class="r-in"><span><span class="op">)</span></span></span> |
| <span class="r-out co"><span class="r-pr">#></span> Schema</span> |
| <span class="r-out co"><span class="r-pr">#></span> b: double</span> |
| <span class="r-out co"><span class="r-pr">#></span> c: bool not null</span> |
| <span class="r-out co"><span class="r-pr">#></span> d: string</span> |
| <span class="r-in"><span></span></span> |
| <span class="r-in"><span><span class="va">df</span> <span class="op"><-</span> <span class="fu"><a href="https://rdrr.io/r/base/data.frame.html" class="external-link">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" class="external-link">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></span> |
| <span class="r-in"><span><span class="va">tab1</span> <span class="op"><-</span> <span class="fu"><a href="Table.html">arrow_table</a></span><span class="op">(</span><span class="va">df</span><span class="op">)</span></span></span> |
| <span class="r-in"><span><span class="va">tab1</span><span class="op">$</span><span class="va">schema</span></span></span> |
| <span class="r-out co"><span class="r-pr">#></span> Schema</span> |
| <span class="r-out co"><span class="r-pr">#></span> col1: int32</span> |
| <span class="r-out co"><span class="r-pr">#></span> col2: double</span> |
| <span class="r-out co"><span class="r-pr">#></span> </span> |
| <span class="r-out co"><span class="r-pr">#></span> See $metadata for additional Schema metadata</span> |
| <span class="r-in"><span><span class="va">tab2</span> <span class="op"><-</span> <span class="fu"><a href="Table.html">arrow_table</a></span><span class="op">(</span><span class="va">df</span>, schema <span class="op">=</span> <span class="fu">schema</span><span class="op">(</span>col1 <span class="op">=</span> <span class="fu"><a href="data-type.html">int8</a></span><span class="op">(</span><span class="op">)</span>, col2 <span class="op">=</span> <span class="fu"><a href="data-type.html">float32</a></span><span class="op">(</span><span class="op">)</span><span class="op">)</span><span class="op">)</span></span></span> |
| <span class="r-in"><span><span class="va">tab2</span><span class="op">$</span><span class="va">schema</span></span></span> |
| <span class="r-out co"><span class="r-pr">#></span> Schema</span> |
| <span class="r-out co"><span class="r-pr">#></span> col1: int8</span> |
| <span class="r-out co"><span class="r-pr">#></span> col2: float</span> |
| <span class="r-out co"><span class="r-pr">#></span> </span> |
| <span class="r-out co"><span class="r-pr">#></span> See $metadata for additional Schema metadata</span> |
| </code></pre></div> |
| </div> |
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