Difference between revisions of "cpp/algorithm/find first of"

Revision as of 13:55, 29 December 2022

Defined in header `<algorithm>`
	(1)
template< class ForwardIt1, class ForwardIt2 > ForwardIt1 find_first_of( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last );			(until C++11)
template< class InputIt, class ForwardIt > InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last );			(since C++11) (until C++20)
template< class InputIt, class ForwardIt > constexpr InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last );			(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt1 find_first_of( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last );		(2)	(since C++17)
		(3)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate > ForwardIt1 find_first_of( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p );				(until C++11)
template< class InputIt, class ForwardIt, class BinaryPredicate > InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last, BinaryPredicate p );				(since C++11) (until C++20)
template< class InputIt, class ForwardIt, class BinaryPredicate > constexpr InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last, BinaryPredicate p );				(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > ForwardIt1 find_first_of( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p );			(4)	(since C++17)

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)

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.
p	-	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); 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 allowed, 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.
- `ForwardIt1` must meet the requirements of LegacyForwardIterator.
- `ForwardIt2` must meet the requirements of LegacyForwardIterator.

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 no such element is found, last is returned.

Complexity

Does at most (S*N) comparisons where S = std::distance(s_first, s_last) and N = std::distance(first, last).

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors 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-defined.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

find_first_of

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

template<class InputIt, class ForwardIt, class BinaryPredicate>
InputIt find_first_of(InputIt first, InputIt last,
                      ForwardIt s_first, ForwardIt s_last,
                      BinaryPredicate 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 = [](auto const id, auto const& 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 }

Compiler support
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Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

findfind_iffind_if_not (C++11)	finds the first element satisfying specific criteria (function template) [edit]
ranges::find_first_of (C++20)	searches for any one of a set of elements (niebloid)[edit]

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Difference between revisions of "cpp/algorithm/find first of"

Revision as of 13:55, 29 December 2022

Contents

Parameters

Return value

Complexity

Exceptions

Possible implementation

Example

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@@ Line 3: / Line 3: @@
 {{dcl begin}}
 {{dcl header|algorithm}}
-{{dcl rev begin|num=1}}
+{{dcl rev multi|num=1|anchor=1|until1=c++11|dcl1=
-{{dcla|anchor=1|since=|until=c++11|num=|1=
 template< class ForwardIt1, class ForwardIt2 >
 ForwardIt1 find_first_of( ForwardIt1 first, ForwardIt1 last,
                            ForwardIt2 s_first, ForwardIt2 s_last );
+|until2=c++20|dcl2=
-}}
-{{dcl|since=c++11|until=c++20|1=
 template< class InputIt, class ForwardIt >
 InputIt find_first_of( InputIt first, InputIt last,
                         ForwardIt s_first, ForwardIt s_last );
-}}
+|dcl3=
-{{dcl|since=c++20|1=
 template< class InputIt, class ForwardIt >
 constexpr InputIt find_first_of( InputIt first, InputIt last,
                                   ForwardIt s_first, ForwardIt s_last );
 }}
-{{dcl rev end}}
 {{dcl|since=c++17|until=|num=2|1=
 template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
@@ Line 27: / Line 22: @@
                            ForwardIt2 s_first, ForwardIt2 s_last );
 }}
-{{dcl rev begin|num=3}}
+{{dcl rev multi|num=3|anchor=3|until1=c++11|dcl1=
-{{dcla|anchor=3|since=|until=c++11|num=|1=
 template< class ForwardIt1, class ForwardIt2, class BinaryPredicate >
 ForwardIt1 find_first_of( ForwardIt1 first, ForwardIt1 last,
                            ForwardIt2 s_first, ForwardIt2 s_last,
                            BinaryPredicate p );
+|until2=c++20|dcl2=
-}}
-{{dcl|since=c++11|until=c++20|1=
 template< class InputIt, class ForwardIt, class BinaryPredicate >
 InputIt find_first_of( InputIt first, InputIt last,
                         ForwardIt s_first, ForwardIt s_last,
                         BinaryPredicate p );
-}}
+|dcl3=
-{{dcl|since=c++20|1=
 template< class InputIt, class ForwardIt, class BinaryPredicate >
 constexpr InputIt find_first_of( InputIt first, InputIt last,
@@ Line 48: / Line 38: @@
                                   BinaryPredicate p );
 }}
-{{dcl rev end}}
 {{dcl|since=c++17|until=|num=4|1=
 template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,