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