home | help 
std::find_first_of(3)	       C++ Standard Libary	   std::find_first_of(3)

NAME
     std::find_first_of - std::find_first_of

Synopsis
	Defined in header <algorithm>
	template< class InputIt, class ForwardIt >

	InputIt  find_first_of( InputIt first, InputIt last,	  (1) (constexpr
     since C++20)

			       ForwardIt s_first, ForwardIt
	s_last );
	template< class ExecutionPolicy, class ForwardIt1, class
	ForwardIt2 >

	ForwardIt1 find_first_of( ExecutionPolicy&& policy,
				  ForwardIt1  first,  ForwardIt1    (2)   (since
     C++17)
	last,

				  ForwardIt2 s_first, ForwardIt2
	s_last );
	template< class InputIt, class ForwardIt, class
	BinaryPred >

	InputIt  find_first_of( InputIt first, InputIt last,	  (3) (constexpr
     since C++20)
			       ForwardIt s_first, ForwardIt
	s_last,

			       BinaryPred p );
	template< class ExecutionPolicy,

		  class ForwardIt1, class ForwardIt2, class
	BinaryPred >
	ForwardIt1 find_first_of( ExecutionPolicy&& policy,
				  ForwardIt1  first,  ForwardIt     (4)   (since
     C++17)
	last,
				  ForwardIt2 s_first, ForwardIt2
	s_last,

				  BinaryPred p );

	Searches the range [first, last) for any of the elements in the range
	[s_first, s_last).

	1) Elements are compared using operator==.
	3) Elements are compared using the given binary predicate p.
	2,4) Same as (1,3), but executed according to policy.
	These overloads participate in overload resolution only if

	std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>    is    true.
     (until
										  C++20)
	std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>      is
     true. (since
										  C++20)

Parameters
	first, last	- the range of elements to examine
	s_first, s_last - the range of elements to search for
	policy		 - the execution policy to use. See execution policy for
     details.
			  binary predicate which returns true  if  the	elements
     should be
			  treated as equal.

			  The  signature  of  the  predicate  function should be
     equivalent to
			  the following:

			   bool pred(const Type1 &a, const Type2 &b);

	p		- While the signature does not need to have const &, the
     function
			  must not modify the objects passed to it and	must  be
     able to
			  accept  all  values of type (possibly const) Type1 and
     Type2
			  regardless of value category (thus, Type1 & is not al-
     lowed
			  , nor is Type1 unless for Type1 a move  is  equivalent
     to a copy
			  (since C++11)).
			  The types Type1 and Type2 must be such that objects of
     types
			  ForwardIt1 and ForwardIt2 can be dereferenced and then
     implicitly
			  converted to Type1 and Type2 respectively.

Type requirements
	-
	InputIt must meet the requirements of LegacyInputIterator.
	-
	ForwardIt must meet the requirements of LegacyForwardIterator.
	-
	ForwardIt1 must meet the requirements of LegacyForwardIterator.
	-
	ForwardIt2 must meet the requirements of LegacyForwardIterator.
	-
	BinaryPred must meet the requirements of BinaryPredicate.

Return value
	Iterator  to  the first element in the range [first, last) that is equal
     to an element
	from the range [s_first, s_last).

	If [s_first, s_last) is empty or if no such element is	found,	last  is
     returned.

Complexity
	Given \(\scriptsize N\)N as std::distance(first, last) and \(\scriptsize
     S\)S as
	std::distance(s_first, s_last):

	1,2) At most \(\scriptsize N\cdot S\)NA.S comparisons using operator==.
	3,4)  At most \(\scriptsize N\cdot S\)NA.S applications of the predicate
     p.

Exceptions
	The overloads with a template parameter named ExecutionPolicy report er-
     rors as
	follows:

	  * If execution of a function invoked as part of the  algorithm  throws
     an exception
	    and  ExecutionPolicy is one of the standard policies, std::terminate
     is called.
	    For any other ExecutionPolicy, the	behavior  is  implementation-de-
     fined.
	  * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
			    find_first_of (1)
	template<class InputIt, class ForwardIt>
	InputIt find_first_of(InputIt first, InputIt last,
			      ForwardIt s_first, ForwardIt s_last)
	{
	    for (; first != last; ++first)
		for (ForwardIt it = s_first; it != s_last; ++it)
		    if (*first == *it)
			return first;
	    return last;
	}
			    find_first_of (3)
	template<class InputIt, class ForwardIt, class BinaryPred>
	InputIt find_first_of(InputIt first, InputIt last,
			      ForwardIt s_first, ForwardIt s_last,
			      BinaryPred p)
	{
	    for (; first != last; ++first)
		for (ForwardIt it = s_first; it != s_last; ++it)
		    if (p(*first, *it))
			return first;
	    return last;
	}

Example
	The following code searches for any of specified integers in a vector of
     integers:

     // Run this code

      #include <algorithm>
      #include <iostream>
      #include <vector>

      auto print_sequence = [](const auto id, const auto& seq, int pos = -1)
      {
	  std::cout << id << "{ ";
	  for (int i{}; auto const& e : seq)
	  {
	      const bool mark{i == pos};
	      std::cout << (i++ ? ", " : "");
	      std::cout << (mark ? "[ " : "") << e << (mark ? " ]" : "");
	  }
	  std::cout << " }\n";
      };

      int main()
      {
	  const std::vector<int> v{0, 2, 3, 25, 5};
	  const auto t1 = {19, 10, 3, 4};
	  const auto t2 = {1, 6, 7, 9};

	  auto find_any_of = [](const auto& v, const auto& t)
	  {
	      const auto result = std::find_first_of(v.begin(), v.end(),
						     t.begin(), t.end());
	      if (result == v.end())
	      {
		  std::cout << "No elements of v are equal to any element of ";
		  print_sequence("t = ", t);
		  print_sequence("v = ", v);
	      }
	      else
	      {
		  const auto pos = std::distance(v.begin(), result);
		  std::cout  << "Found a match (" << *result << ") at position "
     << pos;
		  print_sequence(", where t = ", t);
		  print_sequence("v = ", v, pos);
	      }
	  };

	  find_any_of(v, t1);
	  find_any_of(v, t2);
      }

Output:
      Found a match (3) at position 2, where t = { 19, 10, 3, 4 }
      v = { 0, 2, [ 3 ], 25, 5 }
      No elements of v are equal to any element of t = { 1, 6, 7, 9 }
      v = { 0, 2, 3, 25, 5 }

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	  Applied  to	      Behavior as published		 Correct
     behavior
	LWG 576  C++98	    first and last needed  to  be	      they  only
     need to be
			    LegacyForwardIterators		       LegacyIn-
     putIterators
	LWG 1205 C++98	    the return value was unclear if	   returns  last
     in this case
			    [s_first, s_last) is empty

See also
	find
	find_if 	      finds the first element satisfying specific crite-
     ria
	find_if_not	      (function template)
	(C++11)
	ranges::find_first_of searches for any one of a set of elements
	(C++20) 	      (niebloid)

http://cppreference.com 	   2024.06.10		   std::find_first_of(3)
home | help