Difference between revisions of "cpp/algorithm/find"

Latest revision as of 21:13, 20 May 2024

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

[edit] Parameters

[edit] Return value

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

[edit] Complexity

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

[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

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;
}

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;
}

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));
}

[edit] Example

#include <algorithm>
#include <array>
#include <cassert>
#include <complex>
#include <iostream>
#include <vector>
 
int main()
{
    const auto v = {1, 2, 3, 4};
 
    for (const int n : {3, 5})
        (std::find(v.begin(), v.end(), n) == std::end(v))
            ? std::cout << "v does not contain " << n << '\n'
            : std::cout << "v contains " << n << '\n';
 
    auto is_even = [](int i) { return i % 2 == 0; };
 
    for (const auto& w : {std::array{3, 1, 4}, {1, 3, 5}})
        if (auto it = std::find_if(begin(w), end(w), is_even); it != std::end(w))
            std::cout << "w contains an even number " << *it << '\n';
        else
            std::cout << "w does not contain even numbers\n";
 
    std::vector<std::complex<double>> nums{{4, 2}};
    #ifdef __cpp_lib_algorithm_default_value_type
        // T gets deduced making list-initialization possible
        const auto it = std::find(nums.begin(), nums.end(), {4, 2});
    #else
        const auto it = std::find(nums.begin(), nums.end(), std::complex<double>{4, 2});
    #endif
    assert(it == nums.begin());   
}

v contains 3
v does not contain 5
w contains an even number 4
w does not contain even numbers

[edit] Defect reports

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

[edit] See also