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## Class template category

boost::histogram::axis::category — Maps at a set of unique values to bin indices.

## Synopsis

    // In header: <boost/histogram/axis/category.hpp>

    template<typename Value, typename MetaData, typename Options,
             typename Allocator>
    class category : public boost::histogram::axis::iterator_mixin< category< Value, MetaData, Options, Allocator > >,
                     public metadata_base_t< MetaData >
    {
    public:
      // construct/copy/destruct
      category() = default;
      explicit category(allocator_type);
      template<typename It>
        category(It, It, metadata_type = {}, options_type = {},
                 allocator_type = {});
      template<typename It, typename A> category(It, It, metadata_type, A);
      template<typename C>
        category(const C &, metadata_type = {}, options_type = {},
                 allocator_type = {});
      template<typename C, typename A> category(const C &, metadata_type, A);
      template<typename U>
        category(std::initializer_list< U >, metadata_type = {},
                 options_type = {}, allocator_type = {});
      template<typename U, typename A>
        category(std::initializer_list< U >, metadata_type, A);
      category(const category &, index_type, index_type, unsigned);

      // public member functions
      index_type index(const value_type &) const noexcept;
      std::pair< index_type, index_type > update(const value_type &);
      auto value(index_type) const;
      decltype(auto) bin(index_type) const;
      index_type size() const noexcept;
      template<typename V, typename M, typename O, typename A>
        bool operator==(const category< V, M, O, A > &) const noexcept;
      template<typename V, typename M, typename O, typename A>
        bool operator!=(const category< V, M, O, A > &) const noexcept;
      allocator_type get_allocator() const;
      template<typename Archive> void serialize(Archive &, unsigned);

      // public static functions
      static constexpr unsigned options() noexcept;
      static constexpr bool inclusive() noexcept;
      static constexpr bool ordered() noexcept;
    };

## Description

The axis maps a set of values to bins, following the order of arguments in the constructor. The optional overflow bin for this axis counts input values that are not part of the set. Binning has O(N) complexity, but with a very small factor. For small N (the typical use case) it beats other kinds of lookup.

The options `underflow` and`circular` are not allowed. The options`growth` and`overflow` are mutually exclusive.

### Template Parameters

1.     typename Value

   input value type, must be equal-comparable.

2.     typename MetaData

   type to store meta data.

3.     typename Options

   see boost::histogram::axis::option.

4.     typename Allocator

   allocator to use for dynamic memory management.

### `category` public construct/copy/destruct

1.     category() = default;

2.     explicit category(allocator_type alloc);

3.     template<typename It>
         category(It begin, It end, metadata_type meta = {},
                  options_type options = {}, allocator_type alloc = {});

   Construct from forward iterator range of unique values.

   The constructor throws `std::invalid_argument` if iterator range is invalid. If the range contains duplicated values, the behavior of the axis is undefined.

   The arguments meta and alloc are passed by value. If you move either of them into the axis and the constructor throws, their values are lost. Do not move if you cannot guarantee that the bin description is not valid.

   Parameters:

   `alloc` | allocator instance to use (optional).
   `begin` | begin of category range of unique values.
   `end` | end of category range of unique values.
   `meta` | description of the axis (optional).
   `options` | see boost::histogram::axis::option (optional).

4.     template<typename It, typename A>
         category(It begin, It end, metadata_type meta, A alloc);

5.     template<typename C>
         category(const C & iterable, metadata_type meta = {},
                  options_type options = {}, allocator_type alloc = {});

   Construct axis from iterable sequence of unique values.

   Parameters:

   `alloc` | allocator instance to use.
   `iterable` | sequence of unique values.
   `meta` | description of the axis.
   `options` | see boost::histogram::axis::option (optional).

6.     template<typename C, typename A>
         category(const C & iterable, metadata_type meta, A alloc);

7.     template<typename U>
         category(std::initializer_list< U > list, metadata_type meta = {},
                  options_type options = {}, allocator_type alloc = {});

   Construct axis from an initializer list of unique values.

   Parameters:

   `alloc` | allocator instance to use.
   `list` | `std::initializer_list` of unique values.
   `meta` | description of the axis.
   `options` | see boost::histogram::axis::option (optional).

8.     template<typename U, typename A>
         category(std::initializer_list< U > list, metadata_type meta, A alloc);

9.     category(const category & src, index_type begin, index_type end,
                unsigned merge);

   Constructor used by algorithm::reduce to shrink and rebin (not for users).

### `category` public member functions

1.     index_type index(const value_type & x) const noexcept;

   Return index for value argument.

2.     std::pair< index_type, index_type > update(const value_type & x);

   Returns index and shift (if axis has grown) for the passed argument.

3.     auto value(index_type idx) const;

   Return value for index argument.

   Throws `std::out_of_range` if the index is out of bounds.

4.     decltype(auto) bin(index_type idx) const;

   Return value for index argument; alias for value(...).

5.     index_type size() const noexcept;

   Returns the number of bins, without over- or underflow.

6.     template<typename V, typename M, typename O, typename A>
         bool operator==(const category< V, M, O, A > & o) const noexcept;

7.     template<typename V, typename M, typename O, typename A>
         bool operator!=(const category< V, M, O, A > & o) const noexcept;

8.     allocator_type get_allocator() const;

9.     template<typename Archive> void serialize(Archive & ar, unsigned);

### `category` public static functions

1.     static constexpr unsigned options() noexcept;

   Returns the options.

2.     static constexpr bool inclusive() noexcept;

   Whether the axis is inclusive (see `axis::traits::is_inclusive`).

3.     static constexpr bool ordered() noexcept;

   Indicate that the axis is not ordered.