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<header data-kunai-mdinfo="{"meta": {"header": ["algorithm"], "namespace": ["std"], "id-type": ["function template"]}, "sources": [{"id": "10a349b1a831287fa59b525e2744a2fd779744ee", "source": "#include <iostream>\n#include <algorithm>\n#include <vector>\n#include <string>\n\nstruct X {\n int id;\n std::string name;\n};\n\nint main()\n{\n // \u3053\u306e\u95a2\u6570\u5358\u4f53\u3068\u3057\u3066\u306e\u4f7f\u3044\u65b9\n {\n // lower_bound \u3067 4 \u4ee5\u4e0a\u306e\u8981\u7d20\u306e\u4f4d\u7f6e\u3092\u691c\u7d22\u3059\u308b\u5834\u5408\u3001\n // 4 \u3088\u308a\u5c0f\u3055\u3044\u7269\u3068 4 \u4ee5\u4e0a\u306e\u7269\u304c\u305d\u306e\u9806\u306b\u4e26\u3093\u3067\u3044\u308c\u3070\u3001\n // \u5fc5\u305a\u3057\u3082\u30bd\u30fc\u30c8\u3055\u308c\u3066\u3044\u308b\u5fc5\u8981\u306f\u306a\u3044\u3002\n std::vector<int> v = {3, 1, 4, 6, 5};\n\n // 4\u4ee5\u4e0a\u306e\u8981\u7d20\u3092\u4e8c\u5206\u63a2\u7d22\u3067\u691c\u7d22\n decltype(v)::iterator it = std::lower_bound(v.begin(), v.end(), 4);\n if (it != v.end()) {\n std::size_t pos = std::distance(v.begin(), it);\n std::cout << *it << \" pos=\" << pos << std::endl;\n }\n }\n\n // \u57fa\u672c\u7684\u306a\u7528\u9014\n // \u30bd\u30fc\u30c8\u6e08\u307f\u30a4\u30c6\u30ec\u30fc\u30bf\u7bc4\u56f2\u304b\u3089\u3001\u7279\u5b9a\u306e\u5024\u3092\u4e8c\u5206\u63a2\u7d22\u3067\u898b\u3064\u3051\u308b\n {\n std::vector<int> v = {3, 1, 4, 6, 5};\n std::sort(v.begin(), v.end());\n\n // \u4e8c\u5206\u63a2\u7d22\u3067\u50244\u3092\u691c\u7d22\n decltype(v)::iterator it = std::lower_bound(v.begin(), v.end(), 4);\n if (it != v.end() && *it == 4) { // lower_bound\u3067\u306f4\"\u4ee5\u4e0a\"\u306e\u5024\u304c\u898b\u3064\u304b\u308b\u306e\u3067\u3001\n // \u50244\u3092\u898b\u3064\u3051\u305f\u3044\u306a\u3089\u691c\u7d22\u7d50\u679c\u306e\u5024\u3092\u6bd4\u8f03\u3059\u308b\u5fc5\u8981\u304c\u3042\u308b\n std::size_t pos = std::distance(v.begin(), it);\n std::cout << *it << \" pos=\" << pos << std::endl;\n }\n }\n\n // \u8981\u7d20\u306e\u4e00\u90e8\u306e\u5024\u3092\u6bd4\u8f03\u3057\u3066\u898b\u3064\u3051\u308b\n {\n // \u8981\u7d20\u306f\u8907\u6570\u306e\u30e1\u30f3\u30d0\u5909\u6570\u3092\u3082\u3064\n std::vector<X> v = {\n {1, \"Bob\"},\n {3, \"Alice\"},\n {4, \"Carol\"},\n {2, \"Franklin\"},\n {5, \"Eve\"},\n {6, \"Dave\"}\n };\n\n const std::string key = \"Carol\";\n\n // X::name\u30e1\u30f3\u30d0\u5909\u6570\u3092\u30ad\u30fc\u306b\u3057\u3066\u3001\n // X::name == \"Carol\"\u3068\u306a\u308b\u8981\u7d20\u3092\u4e8c\u5206\u63a2\u7d22\u3067\u898b\u3064\u3051\u308b\n decltype(v)::iterator it = std::lower_bound(\n v.begin(),\n v.end(),\n X{-1, key}, // name\u306e\u307f\u3092\u6bd4\u8f03\u3059\u308b\u306e\u3067\u3001id\u306e\u5024\u306f\u306a\u3093\u3067\u3082\u3088\u3044\n [](const X& a, const X& b) { return a.name < b.name; }\n );\n\n if (it != v.end() && it->name == key) {\n std::size_t pos = std::distance(v.begin(), it);\n std::cout << \"id=\" << it->id\n << \" name=\" << it->name\n << \" pos=\" << pos\n << std::endl;\n }\n }\n}\n"}, {"id": "66465ab9bbf94810abcb7b8d865e5533e76c5607", "source": "#include <algorithm>\n#include <iostream>\n#include <vector>\n#include <iterator>\n\nstruct Point {\n int x;\n int y;\n\n bool operator==(const Point& other) const = default;\n auto operator<=>(const Point& other) const = default;\n};\n\nint main() {\n std::vector<Point> v = {\n {1, 2},\n {1, 2},\n {3, 4},\n {5, 6},\n };\n\n auto it = std::lower_bound(v.begin(), v.end(), {3, 4});\n\n // lower_bound\u3067\u306f\u6307\u5b9a\u3057\u305f\u5024\"\u4ee5\u4e0a\"\u306e\u5024\u304c\u898b\u3064\u304b\u308b\u306e\u3067\u3001\n // \u6307\u5b9a\u3057\u305f\u5024\u3092\u898b\u3064\u3051\u305f\u3044\u306a\u3089\u691c\u7d22\u7d50\u679c\u306e\u5024\u3092\u6bd4\u8f03\u3059\u308b\u5fc5\u8981\u304c\u3042\u308b\n if (it != v.end() && *it == Point{3, 4}) {\n std::size_t pos = std::distance(v.begin(), it);\n std::cout << \"pos=\" << pos << std::endl;\n }\n}\n"}], "page_id": ["reference", "algorithm", "lower_bound"]}">
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最終更新日時(UTC):
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2025年07月08日 15時33分39秒
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<div class="identifier-type">function template</div><div class="header"><algorithm></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">lower_bound</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="k">namespace</span><span class="w"> </span><span class="nn">std</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="w"> </span><span class="n">ForwardIterator</span>
<span class="w"> </span><span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">ForwardIterator</span>
<span class="w"> </span><span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc"><a href="../iterator/iterator_traits.html">iterator_traits</a></span><span class="o"><</span><span class="n">ForwardIterator</span><span class="o">>::</span><span class="n">value_type</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">ForwardIterator</span>
<span class="w"> </span><span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++26</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Compare</span><span class="o">></span>
<span class="w"> </span><span class="n">ForwardIterator</span>
<span class="w"> </span><span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">,</span>
<span class="w"> </span><span class="n">Compare</span><span class="w"> </span><span class="n">comp</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Compare</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">ForwardIterator</span>
<span class="w"> </span><span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">,</span>
<span class="w"> </span><span class="n">Compare</span><span class="w"> </span><span class="n">comp</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc"><a href="../iterator/iterator_traits.html">iterator_traits</a></span><span class="o"><</span><span class="n">ForwardIterator</span><span class="o">>::</span><span class="n">value_type</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Compare</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">ForwardIterator</span>
<span class="w"> </span><span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">,</span>
<span class="w"> </span><span class="n">Compare</span><span class="w"> </span><span class="n">comp</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++26</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a><code>[first, last)</code>のうち、指定された要素以上の値が現れる最初の位置のイテレータを取得する。</p>
<p>この関数の用途としては、ソート済み<a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a>に対して、任意の値を二分探索で見つけるために使用できる。<code><a href="../set/multiset.html">std::multiset</a></code>のように同じキーを持つ要素が複数あり、その全てを列挙したい場合にはこの関数の代わりに<code><a href="equal_range.html">std::equal_range()</a></code>関数を使用できる。</p>
<h2>要件</h2>
<ul>
<li>C++03 まで<ul>
<li><code>first</code>、<code>last</code> は前方向イテレータの要件を満たすこと。</li>
<li><code>comp</code> は 2 引数の関数オブジェクトで、結果の型は <code>bool</code> 型に変換可能であること。また、引数に非 <code>const</code> の関数を適用しないこと。</li>
<li><code>T</code> は <code>LessThanComparable</code> であること。</li>
<li><code>operator<</code> または <code>comp</code> は「<a href="../algorithm.html#strict-weak-ordering">狭義の弱順序</a>」であること。</li>
<li><a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a> <code>[first, last)</code> は <code>operator<</code> または <code>comp</code> を基準として昇順に並んでいること。</li>
</ul>
</li>
<li>C++11 から <ul>
<li><code>first</code>、<code>last</code> は前方向イテレータの要件を満たすこと。</li>
<li><code>comp</code> は 2 引数の関数オブジェクトで、結果の型は <code>bool</code> 型に変換可能であること。また、引数に非 <code>const</code> の関数を適用しないこと。</li>
<li><a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a><code>[first,last)</code> の要素 <code>e</code> は <code>e < value</code> または <code>comp(e, value)</code> によって<a href="../algorithm.html#sequence-is-partitioned">区分化</a>されていること。
つまり、<code>e < value</code> または <code>comp(e, value)</code> が <code>true</code> となる全ての要素 <code>e</code> は、<code>false</code> となる全ての要素よりも左側（<code>first</code> に近い方）になければならない。</li>
</ul>
</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a></h2>
<p><code>[first, last]</code> 内のイテレータ <code>i</code> のうち、以下の条件を満たす、最も右側（<code>first</code> から遠い方）のもの</p>
<ul>
<li><code>[first, i)</code> 内の全てのイテレータ <code>j</code> が <code>*j < value</code> または <code>comp(*j, value) != false</code> である</li>
</ul>
<p>（つまり、<code>value</code> 以上の要素のうち最初のものを指すイテレータ。<code>value</code> 以上の要素が無ければ <code>last</code>）</p>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Complexity。関数が使うアルゴリズムの空間計算量・時間計算量">計算量</a></h2>
<p>最大で log2(<code>last - first</code>) + O(1) 回の比較を行う</p>
<h2>備考</h2>
<ul>
<li><code><a href="../set/set.html">std::set</a></code>や<code><a href="../set/multiset.html">std::multiset</a></code>に対しては専用の<code><a href="../set/set/lower_bound.html">lower_bound</a></code>メンバ関数が定義されているため、そちらを使用すること</li>
<li>本関数は、本質的に C++11 で追加された <code><a href="partition_point.html">partition_point</a></code> と等価である。<br />
具体的には、<code><a href="partition_point.html">partition_point</a>(first, last, [value](const T& e) { return e < value; })</code>、あるいは、<code><a href="partition_point.html">partition_point</a>(first, last, [value, comp](const T& e) { return comp(e, value); })</code> とすることで等価の結果が得られる。</li>
<li>本関数の要件は、上記の通り C++03 までの方が C++11 よりも厳しい。
しかし、本アルゴリズムの特性上、処理系が C++03 までにしか準拠していない場合でも、昇順に並んでいなくても正常に動作する可能性は高いものと思われる。</li>
<li>(1), (2) :<ul>
<li>C++26 : 引数として波カッコ初期化<code>{}</code>を受け付ける
<br />
<div class="codehilite"><pre><span></span><code><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="p">;</span>
<span class="k">auto</span><span class="w"> </span><span class="n">it</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">lower_bound</span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="p">{</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">});</span>
</code></pre></div>
</li>
</ul>
</li>
</ul>
<h2>例</h2>
<h3>基本的な使い方</h3>
<p><div class="yata" id="10a349b1a831287fa59b525e2744a2fd779744ee"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../algorithm.html"><algorithm></a></span>
<span class="cp">#include <a href="../vector.html"><vector></a></span>
<span class="cp">#include <a href="../string.html"><string></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">id</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">name</span><span class="p">;</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// この関数単体としての使い方</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// lower_bound で 4 以上の要素の位置を検索する場合、</span>
<span class="w"> </span><span class="c1">// 4 より小さい物と 4 以上の物がその順に並んでいれば、</span>
<span class="w"> </span><span class="c1">// 必ずしもソートされている必要はない。</span>
<span class="w"> </span><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// 4以上の要素を二分探索で検索</span>
<span class="w"> </span><span class="k">decltype</span><span class="p">(</span><span class="n">v</span><span class="p">)</span><span class="o">::</span><span class="n">iterator</span><span class="w"> </span><span class="n">it</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::lower_bound</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="mi">4</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">it</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">pos</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../iterator/distance.html">std::distance</a></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n">it</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="o">*</span><span class="n">it</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" pos="</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">pos</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="c1">// 基本的な用途</span>
<span class="w"> </span><span class="c1">// ソート済みイテレータ範囲から、特定の値を二分探索で見つける</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">};</span>
<span class="w"> </span><span class="n"><a href="sort.html">std::sort</a></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">);</span>
<span class="w"> </span><span class="c1">// 二分探索で値4を検索</span>
<span class="w"> </span><span class="k">decltype</span><span class="p">(</span><span class="n">v</span><span class="p">)</span><span class="o">::</span><span class="n">iterator</span><span class="w"> </span><span class="n">it</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::lower_bound</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="mi">4</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">it</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="o">*</span><span class="n">it</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">4</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// lower_boundでは4"以上"の値が見つかるので、</span>
<span class="w"> </span><span class="c1">// 値4を見つけたいなら検索結果の値を比較する必要がある</span>
<span class="w"> </span><span class="n"><a href="../cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">pos</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../iterator/distance.html">std::distance</a></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n">it</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="o">*</span><span class="n">it</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" pos="</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">pos</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="c1">// 要素の一部の値を比較して見つける</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// 要素は複数のメンバ変数をもつ</span>
<span class="w"> </span><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n">X</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s">"Bob"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="s">"Alice"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="s">"Carol"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="s">"Franklin"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">5</span><span class="p">,</span><span class="w"> </span><span class="s">"Eve"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">6</span><span class="p">,</span><span class="w"> </span><span class="s">"Dave"</span><span class="p">}</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">key</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"Carol"</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// X::nameメンバ変数をキーにして、</span>
<span class="w"> </span><span class="c1">// X::name == "Carol"となる要素を二分探索で見つける</span>
<span class="w"> </span><span class="k">decltype</span><span class="p">(</span><span class="n">v</span><span class="p">)</span><span class="o">::</span><span class="n">iterator</span><span class="w"> </span><span class="n">it</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::lower_bound</span></span><span class="p">(</span>
<span class="w"> </span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span>
<span class="w"> </span><span class="n">X</span><span class="p">{</span><span class="mi">-1</span><span class="p">,</span><span class="w"> </span><span class="n">key</span><span class="p">},</span><span class="w"> </span><span class="c1">// nameのみを比較するので、idの値はなんでもよい</span>
<span class="w"> </span><span class="p">[](</span><span class="k">const</span><span class="w"> </span><span class="n">X</span><span class="o">&</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">X</span><span class="o">&</span><span class="w"> </span><span class="n">b</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">a</span><span class="p">.</span><span class="n">name</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">b</span><span class="p">.</span><span class="n">name</span><span class="p">;</span><span class="w"> </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">it</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="n">it</span><span class="o">-></span><span class="n">name</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">key</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">pos</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../iterator/distance.html">std::distance</a></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n">it</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"id="</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">it</span><span class="o">-></span><span class="n">id</span>
<span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" name="</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">it</span><span class="o">-></span><span class="n">name</span>
<span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" pos="</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">pos</span>
<span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></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>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>4 pos=2
4 pos=2
id=4 name=Carol pos=2
</code></pre></p>
<h3>波カッコ初期化を入力として使用する (C++26)</h3>
<p><div class="yata" id="66465ab9bbf94810abcb7b8d865e5533e76c5607"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../algorithm.html"><algorithm></a></span>
<span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../vector.html"><vector></a></span>
<span class="cp">#include <a href="../iterator.html"><iterator></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Point</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">y</span><span class="p">;</span>
<span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="k">operator</span><span class="o">==</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Point</span><span class="o">&</span><span class="w"> </span><span class="n">other</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">default</span><span class="p">;</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="k">operator</span><span class="o"><=></span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Point</span><span class="o">&</span><span class="w"> </span><span class="n">other</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">default</span><span class="p">;</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n">Point</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">5</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="p">},</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">it</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::lower_bound</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">});</span>
<span class="w"> </span><span class="c1">// lower_boundでは指定した値"以上"の値が見つかるので、</span>
<span class="w"> </span><span class="c1">// 指定した値を見つけたいなら検索結果の値を比較する必要がある</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">it</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="o">*</span><span class="n">it</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">Point</span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">})</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">pos</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../iterator/distance.html">std::distance</a></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n">it</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"pos="</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">pos</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>pos=2
</code></pre></p>
<h2>実装例</h2>
<p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Compare</span><span class="o">></span>
<span class="n">ForwardIterator</span>
<span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">,</span><span class="w"> </span><span class="n">Compare</span><span class="w"> </span><span class="n">comp</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">diff</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc"><a href="../iterator/iterator_traits.html">std::iterator_traits</a></span><span class="o"><</span><span class="n">ForwardIterator</span><span class="o">>::</span><span class="n">difference_type</span><span class="p">;</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">diff</span><span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../iterator/distance.html">std::distance</a></span><span class="p">(</span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">last</span><span class="p">);</span><span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">diff</span><span class="w"> </span><span class="n">half</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="mi">2</span><span class="p">;</span>
<span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">mid</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../iterator/advance.html">std::advance</a></span><span class="p">(</span><span class="n">mid</span><span class="p">,</span><span class="w"> </span><span class="n">half</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">comp</span><span class="p">(</span><span class="o">*</span><span class="n">mid</span><span class="p">,</span><span class="w"> </span><span class="n">value</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">-=</span><span class="w"> </span><span class="n">half</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span>
<span class="w"> </span><span class="n">first</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">++</span><span class="n">mid</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">half</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="p">}</span>
<span class="c1">// operator< 用の関数オブジェクト（std::less は特殊化されているかもしれないので）</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">less_inner</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">U</span><span class="o">></span>
<span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="k">operator</span><span class="p">()(</span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">U</span><span class="o">&</span><span class="w"> </span><span class="n">rhs</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="kt">bool</span><span class="p">(</span><span class="n">lhs</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">ForwardIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="kr">inline</span><span class="w"> </span><span class="n">ForwardIterator</span>
<span class="n">lower_bound</span><span class="p">(</span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">ForwardIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">value</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">lower_bound</span><span class="p">(</span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">value</span><span class="p">,</span><span class="w"> </span><span class="n">less_inner</span><span class="p">());</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#384" target="_blank">LWG Issue 384. <code>equal_range</code> has unimplementable runtime complexity</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2150" target="_blank">LWG Issue 2150. Unclear specification of <code>find_end</code></a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0202r3.html" target="_blank">P0202R3 Add Constexpr Modifiers to Functions in <code><algorithm></code> and <code><utility></code> Headers</a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2248r8.html" target="_blank">P2248R8 Enabling list-initialization for algorithms</a><ul>
<li>C++26で波カッコ初期化 (リスト初期化) に対応した</li>
</ul>
</li>
</ul></div>
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