std::adjacent_find

Defined in header `<algorithm>`
template< class ForwardIt > ForwardIt adjacent_find( ForwardIt first, ForwardIt last );	(1)
template< class ForwardIt, class BinaryPredicate> ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );	(2)

Searches the range [first, last) for two consecutive identical elements. The first version uses operator== to compare the elements, the second version uses the given binary predicate p.

first, last	-	the range of elements to examine
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); The signature does not need to have const &, but the function must not modify the objects passed to it. The types Type1 and Type2 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to both of them.
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.

[edit] Return value

an iterator to the first of the first pair of identical elements, that is, the first iterator it such that *it == *(it+1) for the first version or p(*it, *(it + 1)) != false for the second version.

If no such elements are found, last is returned

[edit] Complexity

Exactly the smaller of std::distance(first, result) + 1 and std::distance(first, last) - 1 applications of the predicate where result is the return value.

[edit] Possible implementation

First version

template<class ForwardIt>
ForwardIt adjacent_find(ForwardIt first, ForwardIt last)
{
    if (first == last) {
        return last;
    }
    ForwardIt next = first;
    ++next;
    for (; next != last; ++next, ++first) {
        if (*first == *next) {
            return first;
        }
    }
    return last;
}

Second version

template<class ForwardIt, class BinaryPredicate>
ForwardIt adjacent_find(ForwardIt first, ForwardIt last, 
                        BinaryPredicate p)
{
    if (first == last) {
        return last;
    }
    ForwardIt next = first;
    ++next;
    for (; next != last; ++next, ++first) {
        if (p(*first, *next)) {
            return first;
        }
    }
    return last;
}

[edit] Example

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v1{0, 1, 2, 3, 40, 40, 41, 41, 5};
 
    auto i1 = std::adjacent_find(v1.begin(), v1.end());
 
    if (i1 == v1.end()) {
        std::cout << "no matching adjacent elements\n";
    } else {
        std::cout << "the first adjacent pair of equal elements at: "
                  << std::distance(v1.begin(), i1) << '\n';
    }
 
    auto i2 = std::adjacent_find(v1.begin(), v1.end(), std::greater<int>());
    if (i2 == v1.end()) {
        std::cout << "The entire vector is sorted in ascending order\n";
    } else {
        std::cout << "The last element in the non-decreasing subsequence is at: "
                  << std::distance(v1.begin(), i2) << '\n';
    }
}

Output:

The first adjacent pair of equal elements at: 4
The last element in the non-decreasing subsequence is at: 7

[edit] See also

unique	removes consecutive duplicate elements in a range (function template)
std::experimental::parallel::ajacent_find (parallelism TS)	parallelized version of `std::ajacent_find` (function template)

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