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

From cppreference.com
< cpp‎ | algorithm
m (fmt)
(Added LWG issue #1205 DR (part 2/6).)
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===Return value===
 
===Return value===
−
Iterator to the first element in the range {{range|first|last}} that is equal to an element from the range {{range|s_first|s_last}}. If no such element is found, {{c|last}} is returned.
+
Iterator to the first element in the range {{range|first|last}} that is equal to an element from the range {{range|s_first|s_last}}.
 +
 
 +
If {{range|s_first|s_last}} is empty or if no such element is found, {{c|last}} is returned.
  
 
===Complexity===
 
===Complexity===
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int main()
 
int main()
 
{
 
{
−
     const std::vector<int> v {0, 2, 3, 25, 5};
+
     const std::vector<int> v{0, 2, 3, 25, 5};
 
     const auto t1 = {19, 10, 3, 4};
 
     const auto t1 = {19, 10, 3, 4};
 
     const auto t2 = {1, 6, 7, 9};
 
     const auto t2 = {1, 6, 7, 9};
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===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
−
{{dr list item|wg=lwg|dr=576|std=C++98|before={{c|first}} and {{c|last}} needed to be {{named req|ForwardIterator}}s|after=they only need to be {{named req|InputIterator}}s}}
+
{{dr list item|wg=lwg|dr=576|std=C++98|before={{c|first}} and {{c|last}} needed to be {{named req|ForwardIterator}}s|after=they only need to be<br>{{named req|InputIterator}}s}}
 +
{{dr list item|wg=lwg|dr=1205|std=C++98|before=the return value was unclear if {{range|s_first|s_last}} is empty|after=returns {{c|last}} in this case}}
 
{{dr list end}}
 
{{dr list end}}
  

Revision as of 23:46, 27 July 2023

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
(1)
template< class InputIt, class ForwardIt >

InputIt find_first_of( InputIt first, InputIt last,

                       ForwardIt s_first, ForwardIt s_last );
 (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 InputIt, class ForwardIt, class BinaryPredicate >

InputIt find_first_of( InputIt first, InputIt last,
                       ForwardIt s_first, ForwardIt s_last,

                       BinaryPredicate p );
 (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)

Contents

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.
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.
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
ForwardIt1 must meet the requirements of LegacyForwardIterator.
-
ForwardIt2 must meet the requirements of LegacyForwardIterator.
-
BinaryPredicate 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 N as std::distance(first, last) and S as std::distance(s_first, s_last):

1,2) At most N·S comparisons using operator==
3,4) At most N·S applications of the predicate p

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 (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 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 }

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 576 C++98 first and last needed to be LegacyForwardIterators they only need to be
LegacyInputIterators
LWG 1205 C++98 the return value was unclear if [s_first, s_last) is empty returns last in this case

See also

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