std::find, std::find_if, std::find_if_not - cppreference.com

Defined in header `<algorithm>`
template< class InputIt, class T > InputIt find( InputIt first, InputIt last, const T& value );	(1)	(constexpr since C++20) (until C++26)
template< class InputIt, class T = typename std::iterator_traits <InputIt>::value_type > constexpr InputIt find( InputIt first, InputIt last, const T& value );	(1)	(since C++26)
template< class InputIt, class UnaryPred > InputIt find_if( InputIt first, InputIt last, UnaryPred p );	(2)	(constexpr since C++20)
template< class InputIt, class UnaryPred > InputIt find_if_not( InputIt first, InputIt last, UnaryPred q );	(3)	(since C++11) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class T > ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );	(4)	(since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );	(4)	(since C++26)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt find_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred p );	(5)	(since C++17)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt find_if_not( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred q );	(6)	(since C++17)

Returns an iterator to the first element in the source range [first, last) that satisfies specific criteria (or last if there is no such iterator).

1) find searches for the first element equal to value (using operator==).

2) find_if searches for the first element for which predicate p returns true.

3) find_if_not searches for the first element for which predicate q returns false.

4-6) Same as (1-3), but executed according to policy.

These overloads participate in overload resolution only if the value of the following expression is true:

`std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>`	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>`	(since C++20)

Parameters

first, last	-	the pair of iterators defining the source range
value	-	value to compare the elements to
p	-	unary predicate which returns `true` for the required element. The expression `p(v)` must be convertible to `bool` for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of `VT&`is not allowed, nor is `VT` unless for `VT` a move is equivalent to a copy(since C++11).
q	-	unary predicate which returns `false` for the required element. The expression `q(v)` must be convertible to `bool` for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of `VT&`is not allowed, nor is `VT` unless for `VT` a move is equivalent to a copy(since C++11).
policy	-	the execution policy to use
Type requirements
-`InputIt` must meet the requirements of LegacyInputIterator.
-`ForwardIt` must meet the requirements of LegacyForwardIterator.
-`UnaryPredicate` must meet the requirements of Predicate.

Return value

The first iterator it in the source range satisfying the following condition or last if there is no such iterator:

1,4) *it == value is true.

2,5) p(*it) is true.

3,6) q(*it) is false.

Complexity

Given N as std::distance(first, last):

1) At most N comparisons with value using operator==.

2) At most N applications of p.

3) At most N applications of q.

4) 𝓞(N) comparisons with value using operator==.

5) 𝓞(N) applications of p.

6) 𝓞(N) applications of q.

Exceptions

4-6) During the execution process:

If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Possible implementation

find
template<class InputIt, class T = typename std::iterator_traits<InputIt>::value_type> constexpr InputIt find(InputIt first, InputIt last, const T& value) { for (; first != last; ++first) if (*first == value) return first; return last; }
find_if
template<class InputIt, class UnaryPred> constexpr InputIt find_if(InputIt first, InputIt last, UnaryPred p) { for (; first != last; ++first) if (p(*first)) return first; return last; }
find_if_not
template<class InputIt, class UnaryPred> constexpr InputIt find_if_not(InputIt first, InputIt last, UnaryPred q) { for (; first != last; ++first) if (!q(*first)) return first; return last; }

Notes

If C++11 is not available, an equivalent to std::find_if_not is to use std::find_if with the negated predicate.

template<class InputIt, class UnaryPred>
InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
{
    return std::find_if(first, last, std::not1(q));
}

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	`202403`	(C++26)	List-initialization for algorithms (1,4)

Example

The following example finds numbers in given sequences.

#include <algorithm>
#include <array>
#include <cassert>
#include <complex>
#include <initializer_list>
#include <iostream>
#include <vector>

bool is_even(int i)
{
    return i % 2 == 0;
}

void example_contains()
{
    const auto haystack = {1, 2, 3, 4};
    
    for (const int needle : {3, 5})
        if (std::find(haystack.begin(), haystack.end(), needle) == haystack.end())
            std::cout << "haystack does not contain " << needle << '\n';
        else
            std::cout << "haystack contains " << needle << '\n';
}

void example_predicate()
{
    for (const auto& haystack : {std::array{3, 1, 4}, {1, 3, 5}})
    {
        const auto it = std::find_if(haystack.begin(), haystack.end(), is_even);
        if (it != haystack.end())
            std::cout << "haystack contains an even number " << *it << '\n';
        else
            std::cout << "haystack does not contain even numbers\n";
    }
}

void example_list_init()
{
    std::vector<std::complex<double>> haystack{{4.0, 2.0}};
#ifdef __cpp_lib_algorithm_default_value_type
    // T gets deduced making list-initialization possible
    const auto it = std::find(haystack.begin(), haystack.end(), {4.0, 2.0});
#else
    const auto it = std::find(haystack.begin(), haystack.end(), std::complex{4.0, 2.0});
#endif
    assert(it == haystack.begin());  
}

int main()
{
    example_contains();
    example_predicate();
    example_list_init();
}

Output:

haystack contains 3
haystack does not contain 5
haystack contains an even number 4
haystack does not contain even numbers

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	`T` was required to be EqualityComparable, but the value type of `InputIt` might not be `T`	removed the requirement

ranges::findranges::find_ifranges::find_if_not (C++20)(C++20)(C++20)	finds the first element satisfying specific criteria (algorithm function object)[edit]
adjacent_find	finds the first two adjacent items that are equal (or satisfy a given predicate) (function template & algorithm function object)[edit]
ranges::adjacent_find (C++20)
find_end	finds the last sequence of elements in a certain range (function template & algorithm function object)[edit]
ranges::find_end (C++20)
find_first_of	searches for any one of a set of elements (function template & algorithm function object)[edit]
ranges::find_first_of (C++20)
mismatch	finds the first position where two ranges differ (function template & algorithm function object)[edit]
ranges::mismatch (C++20)
search	searches for the first occurrence of a range of elements (function template & algorithm function object)[edit]
ranges::search (C++20)