std::find, std::find_if, std::find_if_not

From cppreference.com

Defined in header `<algorithm>`
	(1)
template< class InputIt, class T > InputIt find( InputIt first, InputIt last, const T& value );			(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 );			(since C++26)
		(2)
template< class ExecutionPolicy, class ForwardIt, class T > ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );				(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 );				(since C++26)
template< class InputIt, class UnaryPred > InputIt find_if( InputIt first, InputIt last, UnaryPred p );			(3)	(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt find_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred p );			(4)	(since C++17)
template< class InputIt, class UnaryPred > InputIt find_if_not( InputIt first, InputIt last, UnaryPred q );			(5)	(since C++11) (constexpr since C++20)
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 range [first, last) that satisfies specific criteria (or last if there is no such iterator).

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

3) find_if searches for an element for which predicate p returns true.

5) find_if_not searches for an element for which predicate q returns false.

2,4,6) Same as (1,3,5), but executed according to policy.

These overloads participate in overload resolution only if all following conditions are satisfied:

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)

[edit] Parameters

first, last	-	the pair of iterators defining the range of elements to examine
value	-	value to compare the elements to
policy	-	the execution policy to use
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).
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `UnaryPredicate` must meet the requirements of Predicate.

[edit] Return value

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

1,2) *it == value is true.

3,4) p(*it) is true.

5,6) q(*it) is false.

[edit] Complexity

Given $\scriptsize N$ $N$ as std::distance(first, last):

1,2) At most

N

comparisons with value using operator==.

3,4) At most

N

applications of the predicate p.

5,6) At most

N

applications of the predicate q.

[edit] 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.

[edit] Possible implementation

find (1)
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 (3)
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 (5)
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; }

[edit] 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 (1,2)

[edit] Example

The following example finds numbers in given sequences.

Run this code

#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

[edit] 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

[edit] See also

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

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