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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/api/basic_json_view/count/" rel="canonical"><link href=../contains/ rel=prev><link href=../dump/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>count - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/api/basic_json_view/count/" rel="canonical"><link href=../contains/ rel=prev><link href=../dump/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>count - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> count </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" 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<span class=n>size_type</span><span class=w> </span><span class=nf>count</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=w> </span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=k>const</span><span class=p>;</span>
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<span class=n>size_type</span><span class=w> </span><span class=nf>count</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>string_t</span><span class=o>&</span><span class=w> </span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=k>const</span><span class=p>;</span>
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</code></pre></div> <p>Returns the number of members with key <code>key</code> if the value is an object, <code class=highlight><span class=mi>0</span></code> otherwise. Members with a repeated key are all counted.</p> <h2 id=parameters>Parameters<a class=headerlink href=#parameters title="Permanent link">¶</a></h2> <dl> <dt><code>key</code> (in)</dt> <dd>key value of the element to count</dd> </dl> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">¶</a></h2> <p>the number of members of the object with key <code>key</code> (duplicates included), or <code class=highlight><span class=mi>0</span></code> if the value is not an object or has no such member.</p> <h2 id=exception-safety>Exception safety<a class=headerlink href=#exception-safety title="Permanent link">¶</a></h2> <p>No-throw guarantee: this function never throws exceptions.</p> <h2 id=complexity>Complexity<a class=headerlink href=#complexity title="Permanent link">¶</a></h2> <p>Linear in the number of members: unlike the lookup functions, <code>count()</code> does not stop at the first match, because it has to count every member with the key. Members are compared one after another, in document order. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. The hash index of the keys of objects with 128 or more members does not help here, since every member is visited.</p> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <p>This method always returns <code class=highlight><span class=mi>0</span></code> when the value is not an object -- including a <a href=../is_discarded/ >discarded</a> view.</p> <p>If the source text repeats a key, <code>count()</code> returns more than <code class=highlight><span class=mi>1</span></code> for it, even though the <a href=../materialize/ >materialized</a> value (like the result of <code>parse()</code>) would have only one member with that key. Hence <code class=highlight><span class=n>count</span><span class=p>(</span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=o>></span><span class=w> </span><span class=mi>1</span></code> detects duplicate keys. This is unlike the lookup functions (<a href=../operator%5B%5D/ ><code>operator[]</code></a>, <a href=../at/ ><code>at</code></a>, <a href=../find/ ><code>find</code></a>, <a href=../contains/ ><code>contains</code></a>), which resolve to the <em>first</em> member; see the <a href=../operator%5B%5D/#notes>Notes on duplicate keys</a> of <code>operator[]</code>.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <details class=example> <summary>Example</summary> <p>The example below validates that every transaction of a batch carries exactly one <code>amount</code> field, using <code>count()</code> before deciding whether to materialize a transaction at all. It reports a transaction with no <code>amount</code> field and one with a duplicated <code>amount</code> field.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
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