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api/basic_json/count/index.md

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+# nlohmann::basic_json::count
+
+```
+// (1)
+size_type count(const typename object_t::key_type& key) const;
+
+// (2)
+template<typename KeyType>
+size_type count(KeyType&& key) const;
+```
+
+1. Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will always be `0` (`key` was not found) or `1` (`key` was found).
+1. See 1. This overload is only available if `KeyType` is comparable with `typename object_t::key_type` and `typename object_comparator_t::is_transparent` denotes a type.
+
+## Template parameters
+
+`KeyType` : A type for an object key other than [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md) that is comparable with [`string_t`](https://json.nlohmann.me/api/basic_json/string_t/index.md) using [`object_comparator_t`](https://json.nlohmann.me/api/basic_json/object_comparator_t/index.md). This can also be a string view (C++17).
+
+## Parameters
+
+`key` (in) : key value of the element to count.
+
+## Return value
+
+Number of elements with key `key`. If the JSON value is not an object, the return value will be `0`.
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Complexity
+
+Logarithmic in the size of the JSON object.
+
+## Notes
+
+This method always returns `0` when executed on a JSON type that is not an object.
+
+## Examples
+
+Example: (1) count number of elements
+
+The example shows how `count()` is used.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON object
+    json j_object = {{"one", 1}, {"two", 2}};
+
+    // call count()
+    auto count_two = j_object.count("two");
+    auto count_three = j_object.count("three");
+
+    // print values
+    std::cout << "number of elements with key \"two\": " << count_two << '\n';
+    std::cout << "number of elements with key \"three\": " << count_three << '\n';
+}
+```
+
+Output:
+
+```
+number of elements with key "two": 1
+number of elements with key "three": 0
+```
+
+Example: (2) count number of elements using string_view
+
+The example shows how `count()` is used.
+
+```
+#include <iostream>
+#include <string_view>
+#include <nlohmann/json.hpp>
+
+using namespace std::string_view_literals;
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON object
+    json j_object = {{"one", 1}, {"two", 2}};
+
+    // call count()
+    auto count_two = j_object.count("two"sv);
+    auto count_three = j_object.count("three"sv);
+
+    // print values
+    std::cout << "number of elements with key \"two\": " << count_two << '\n';
+    std::cout << "number of elements with key \"three\": " << count_three << '\n';
+}
+```
+
+Output:
+
+```
+number of elements with key "two": 1
+number of elements with key "three": 0
+```
+
+## See also
+
+- [find](https://json.nlohmann.me/api/basic_json/find/index.md) find a value in an object
+- [contains](https://json.nlohmann.me/api/basic_json/contains/index.md) checks whether a key exists
+
+## Version history
+
+1. Added in version 3.11.0.
+1. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0.