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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../unordered_map.html" title="cpp/container/unordered map"><span class="t-lines"><span>unordered_map</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../unordered_multimap.html" title="cpp/container/unordered multimap"><span class="t-lines"><span>unordered_multimap</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../stack.html" title="cpp/container/stack"><span class="t-lines"><span>stack</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../queue.html" title="cpp/container/queue"><span class="t-lines"><span>queue</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../priority_queue.html" title="cpp/container/priority queue"><span class="t-lines"><span>priority_queue</span></span></a></div></div></td></tr>
</table></div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../unordered_map.html" title="cpp/container/unordered map"><tt>std::unordered_map</tt></a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h1"><td colspan="5"> Member functions</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="unordered_map.html" title="cpp/container/unordered map/unordered map"><span class="t-lines"><span>unordered_map::unordered_map</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="~unordered_map.html" title="cpp/container/unordered map/~unordered map"><span class="t-lines"><span>unordered_map::~unordered_map</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="operator=.html" title="cpp/container/unordered map/operator="><span class="t-lines"><span>unordered_map::operator=</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="get_allocator.html" title="cpp/container/unordered map/get allocator"><span class="t-lines"><span>unordered_map::get_allocator</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Iterators</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="begin.html" title="cpp/container/unordered map/begin"><span class="t-lines"><span>unordered_map::begin</span><span>unordered_map::cbegin</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="end.html" title="cpp/container/unordered map/end"><span class="t-lines"><span>unordered_map::end</span><span>unordered_map::cend</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Capacity</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="erase.html" title="cpp/container/unordered map/erase"><span class="t-lines"><span>unordered_map::erase</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="size.html" title="cpp/container/unordered map/size"><span class="t-lines"><span>unordered_map::size</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="max_size.html" title="cpp/container/unordered map/max size"><span class="t-lines"><span>unordered_map::max_size</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Modifiers</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="clear.html" title="cpp/container/unordered map/clear"><span class="t-lines"><span>unordered_map::clear</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="insert.html" title="cpp/container/unordered map/insert"><span class="t-lines"><span>unordered_map::insert</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="insert_or_assign.html" title="cpp/container/unordered map/insert or assign"><span class="t-lines"><span>unordered_map::insert_or_assign</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="emplace.html" title="cpp/container/unordered map/emplace"><span class="t-lines"><span>unordered_map::emplace</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="emplace_hint.html" title="cpp/container/unordered map/emplace hint"><span class="t-lines"><span>unordered_map::emplace_hint</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="try_emplace.html" title="cpp/container/unordered map/try emplace"><span class="t-lines"><span>unordered_map::try_emplace</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="erase.html" title="cpp/container/unordered map/erase"><span class="t-lines"><span>unordered_map::erase</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="swap.html" title="cpp/container/unordered map/swap"><span class="t-lines"><span>unordered_map::swap</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Lookup</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="count.html" title="cpp/container/unordered map/count"><span class="t-lines"><span>unordered_map::count</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="find.html" title="cpp/container/unordered map/find"><span class="t-lines"><span>unordered_map::find</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="equal_range.html" title="cpp/container/unordered map/equal range"><span class="t-lines"><span>unordered_map::equal_range</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Bucket interface</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="begin2.html" title="cpp/container/unordered map/begin2"><span class="t-lines"><span>unordered_map::begin2</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="end2.html" title="cpp/container/unordered map/end2"><span class="t-lines"><span>unordered_map::end2</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="bucket_count.html" title="cpp/container/unordered map/bucket count"><span class="t-lines"><span>unordered_map::bucket_count</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="max_bucket_count.html" title="cpp/container/unordered map/max bucket count"><span class="t-lines"><span>unordered_map::max_bucket_count</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="bucket_size.html" title="cpp/container/unordered map/bucket size"><span class="t-lines"><span>unordered_map::bucket_size</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="bucket.html" title="cpp/container/unordered map/bucket"><span class="t-lines"><span>unordered_map::bucket</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Hash policy</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="load_factor.html" title="cpp/container/unordered map/load factor"><span class="t-lines"><span>unordered_map::load_factor</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="max_load_factor.html" title="cpp/container/unordered map/max load factor"><span class="t-lines"><span>unordered_map::max_load_factor</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="rehash.html" title="cpp/container/unordered map/rehash"><span class="t-lines"><span>unordered_map::rehash</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="reserve.html" title="cpp/container/unordered map/reserve"><span class="t-lines"><span>unordered_map::reserve</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Observers</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="hash_function.html" title="cpp/container/unordered map/hash function"><span class="t-lines"><span>unordered_map::hash_function</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="key_eq.html" title="cpp/container/unordered map/key eq"><span class="t-lines"><span>unordered_map::key_eq</span></span></a></div></div></td></tr>
</table></div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/unordered_map/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> Key<span class="sy3">&</span> key <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td> (1) </td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td> <div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> Key<span class="sy3">&&</span> key <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td> (2) </td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Returns a reference to the value that is mapped to a key equivalent to <code>key</code>, performing an insertion if such key does not already exist.
</p>
<div class="t-li1"><span class="t-li">1)</span> Inserts a <code>value_type</code> object constructed in-place from <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../utility/piecewise_construct.html"><span class="kw940">std::<span class="me2">piecewise_construct</span></span></a>, <a href="../../utility/tuple/forward_as_tuple.html"><span class="kw944">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>key<span class="br0">)</span>, <a href="../../utility/tuple.html"><span class="kw938">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span> if the key does not exist. <span class="t-rev-inl t-since-cxx17"><span> This function is equivalent to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> this<span class="sy2">-</span><span class="sy1">></span>try_emplace<span class="br0">(</span>key<span class="br0">)</span>.<span class="me1">first</span><span class="sy2">-</span><span class="sy1">></span>second<span class="sy4">;</span></span></span>.</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> <br /> When the default allocator is used, this results in the key being copy constructed from <code>key</code> and the mapped value being <a href="../../language/value_initialization.html" title="cpp/language/value initialization">value-initialized</a>.
<table class="t-par-begin">
<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div><code>value_type</code> must be <a href="../../concept/EmplaceConstructible.html" title="cpp/concept/EmplaceConstructible"><code>EmplaceConstructible</code></a> from <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../utility/piecewise_construct.html"><span class="kw940">std::<span class="me2">piecewise_construct</span></span></a>, <a href="../../utility/tuple/forward_as_tuple.html"><span class="kw944">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>key<span class="br0">)</span>, <a href="../../utility/tuple.html"><span class="kw938">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span>. When the default allocator is used, this means that <code>key_type</code> must be <a href="../../concept/CopyConstructible.html" title="cpp/concept/CopyConstructible"><code>CopyConstructible</code></a> and <code>mapped_type</code> must be <a href="../../concept/DefaultConstructible.html" title="cpp/concept/DefaultConstructible"><code>DefaultConstructible</code></a>.
</td></tr></table></div>
<div class="t-li1"><span class="t-li">2)</span> Inserts a <code>value_type</code> object constructed in-place from <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../utility/piecewise_construct.html"><span class="kw940">std::<span class="me2">piecewise_construct</span></span></a>, <a href="../../utility/tuple/forward_as_tuple.html"><span class="kw944">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>key<span class="br0">)</span><span class="br0">)</span>, <a href="../../utility/tuple.html"><span class="kw938">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span> if the key does not exist. <span class="t-rev-inl t-since-cxx17"><span> This function is equivalent to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> this<span class="sy2">-</span><span class="sy1">></span>try_emplace<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>key<span class="br0">)</span><span class="br0">)</span>.<span class="me1">first</span><span class="sy2">-</span><span class="sy1">></span>second<span class="sy4">;</span></span></span>.</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> <br /> When the default allocator is used, this results in the key being move constructed from <code>key</code> and the mapped value being <a href="../../language/value_initialization.html" title="cpp/language/value initialization">value-initialized</a>.
<table class="t-par-begin">
<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div><code>value_type</code> must be <a href="../../concept/EmplaceConstructible.html" title="cpp/concept/EmplaceConstructible"><code>EmplaceConstructible</code></a> from <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../../utility/piecewise_construct.html"><span class="kw940">std::<span class="me2">piecewise_construct</span></span></a>, <a href="../../utility/tuple/forward_as_tuple.html"><span class="kw944">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>key<span class="br0">)</span><span class="br0">)</span>, <a href="../../utility/tuple.html"><span class="kw938">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span>. When the default allocator is used, this means that <code>key_type</code> must be <a href="../../concept/MoveConstructible.html" title="cpp/concept/MoveConstructible"><code>MoveConstructible</code></a> and <code>mapped_type</code> must be <a href="../../concept/DefaultConstructible.html" title="cpp/concept/DefaultConstructible"><code>DefaultConstructible</code></a>.
</td></tr></table></div>
<p>If an insertion occurs and results in a rehashing of the container, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is greater than <span class="t-c"><span class="mw-geshi cpp source-cpp">max_load_factor<span class="br0">(</span><span class="br0">)</span><span class="sy2">*</span>bucket_count<span class="br0">(</span><span class="br0">)</span></span></span>.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1"><a href="operator_at.html#Parameters"><span class="tocnumber">1</span> <span class="toctext">Parameters</span></a></li>
<li class="toclevel-1"><a href="operator_at.html#Return_value"><span class="tocnumber">2</span> <span class="toctext">Return value</span></a></li>
<li class="toclevel-1"><a href="operator_at.html#Exceptions"><span class="tocnumber">3</span> <span class="toctext">Exceptions</span></a></li>
<li class="toclevel-1"><a href="operator_at.html#Complexity"><span class="tocnumber">4</span> <span class="toctext">Complexity</span></a></li>
<li class="toclevel-1"><a href="operator_at.html#Notes"><span class="tocnumber">5</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1"><a href="operator_at.html#Example"><span class="tocnumber">6</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1"><a href="operator_at.html#See_also"><span class="tocnumber">7</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/operator_at_assoc&action=edit&section=T-1" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Parameters">Parameters</span></h3>
<table class="t-par-begin">
<tr class="t-par">
<td> key
</td>
<td> -
</td>
<td> the key of the element to find
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/operator_at_assoc&action=edit&section=T-2" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Return_value">Return value</span></h3>
<p>Reference to the mapped value of the new element if no element with key <code>key</code> existed. Otherwise a reference to the mapped value of the existing element whose key is equivalent to <code>key</code>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/operator_at_assoc&action=edit&section=T-3" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Exceptions">Exceptions</span></h3>
<p>If an exception is thrown by any operation, the insertion has no effect
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/operator_at_assoc&action=edit&section=T-4" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Complexity">Complexity</span></h3>
<p>Average case: constant, worst case: linear in size.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/operator_at_assoc&action=edit&section=T-5" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>In the published C++11 and C++14 standards, this function was specified to require <code>mapped_type</code> to be <a href="../../concept/DefaultInsertable.html" title="cpp/concept/DefaultInsertable"><code>DefaultInsertable</code></a> and <code>key_type</code> to be <a href="../../concept/CopyInsertable.html" title="cpp/concept/CopyInsertable"><code>CopyInsertable</code></a> or <a href="../../concept/MoveInsertable.html" title="cpp/concept/MoveInsertable"><code>MoveInsertable</code></a> into <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span>. This specification was defective and was fixed by <a rel="nofollow" class="external text" href="http://cplusplus.github.io/LWG/lwg-defects.html#2469">LWG issue 2469</a>, and the description above incorporates the resolution of that issue.
</p><p>However, one implementation (libc++) is known to construct the <code>key_type</code> and <code>mapped_type</code> objects via two separate allocator <code>construct()</code> calls, as arguably required by the standards as published, rather than emplacing a <code>value_type</code> object.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/operator_at_assoc&action=edit&section=T-6" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><p> This example demonstrates how to modify existing values and insert new values using <code>operator[]</code>:
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <iostream></span>
<span class="co2">#include <unordered_map></span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../unordered_map.html"><span class="kw1106">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span><span class="kw4">char</span>, <span class="kw4">int</span><span class="sy1">></span> letter_counts <span class="br0">{</span><span class="br0">{</span><span class="st0">'a'</span>, <span class="nu0">27</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'b'</span>, <span class="nu0">3</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'c'</span>, <span class="nu0">1</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span>
 
<a href="../../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"initially:<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>pair <span class="sy4">:</span> letter_counts<span class="br0">)</span> <span class="br0">{</span>
<a href="../../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> pair.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">": "</span> <span class="sy1"><<</span> pair.<span class="me1">second</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
letter_counts<span class="br0">[</span><span class="st0">'b'</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="co1">// update an existing value</span>
 
letter_counts<span class="br0">[</span><span class="st0">'x'</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">9</span><span class="sy4">;</span> <span class="co1">// insert a new value</span>
 
<a href="../../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"after modifications:<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>pair <span class="sy4">:</span> letter_counts<span class="br0">)</span> <span class="br0">{</span>
<a href="../../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> pair.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">": "</span> <span class="sy1"><<</span> pair.<span class="me1">second</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">initially:
a: 27
b: 3
c: 1
after modifications:
a: 27
b: 42
c: 1
x: 9</pre></div></div>
</div>
<div class="t-example"><p> The following example counts the occurrences of each word in a vector of strings:
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <string></span>
<span class="co2">#include <iostream></span>
<span class="co2">#include <vector></span>
<span class="co2">#include <unordered_map></span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../vector.html"><span class="kw1094">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="../../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy1">></span> words <span class="sy1">=</span> <span class="br0">{</span>
<span class="st0">"this"</span>, <span class="st0">"sentence"</span>, <span class="st0">"is"</span>, <span class="st0">"not"</span>, <span class="st0">"a"</span>, <span class="st0">"sentence"</span>,
<span class="st0">"this"</span>, <span class="st0">"sentence"</span>, <span class="st0">"is"</span>, <span class="st0">"a"</span>, <span class="st0">"hoax"</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<a href="../unordered_map.html"><span class="kw1106">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span><a href="../../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a>, size_t<span class="sy1">></span> word_map<span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>w <span class="sy4">:</span> words<span class="br0">)</span> <span class="br0">{</span>
<span class="sy2">++</span>word_map<span class="br0">[</span>w<span class="br0">]</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>pair <span class="sy4">:</span> word_map<span class="br0">)</span> <span class="br0">{</span>
<a href="../../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> pair.<span class="me1">second</span>
<span class="sy1"><<</span> <span class="st0">" occurrences of word '"</span>
<span class="sy1"><<</span> pair.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">"'<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1 occurrences of word 'hoax'
2 occurrences of word 'this'
2 occurrences of word 'a'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/operator_at_assoc&action=edit&section=T-7" title="Template:cpp/container/operator at assoc">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="at.html" title="cpp/container/unordered map/at"> <span class="t-lines"><span>at</span></span></a></div></div>
</td>
<td> access specified element with bounds checking <br /> <span class="t-mark">(public member function)</span> <span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/dsc_at&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="insert_or_assign.html" title="cpp/container/unordered map/insert or assign"> <span class="t-lines"><span>insert_or_assign</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div>
</td>
<td> inserts an element or assigns to the current element if the key already exists <br /> <span class="t-mark">(public member function)</span> <span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/dsc_insert_or_assign&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="try_emplace.html" title="cpp/container/unordered map/try emplace"> <span class="t-lines"><span>try_emplace</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div>
</td>
<td> inserts in-place if the key does not exist, does nothing if the key exists <br /> <span class="t-mark">(public member function)</span> <span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/container/dsc_try_emplace&action=edit">[edit]</a></span>
</td></tr>
</table>
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