From cppreference.com
Defined in header
template<classForwardIt>ForwardItadjacent_find(ForwardItfirst,ForwardItlast); (1)(constexpr since C++20)template<classForwardIt,classBinaryPred>ForwardItadjacent_find(ForwardItfirst,ForwardItlast,BinaryPredp); (2)(constexpr since C++20)template<classExecutionPolicy,classForwardIt>ForwardItadjacent_find(ExecutionPolicy&&policy,ForwardItfirst,ForwardItlast); (3) (since C++17)template<classExecutionPolicy,classForwardIt,classBinaryPred>ForwardItadjacent_find(ExecutionPolicy&&policy,ForwardItfirst,ForwardItlast,BinaryPredp); (4) (since C++17)Searches the source range [first, last) for the first pair of adjacent elements satisfying the specified condition.
1) Searches for the first pair of adjacent equal elements. Equality is determined by operator==.
2) Searches for the first pair of adjacent elements satisfying the given binary predicate p.
3,4) Same as (1,2), 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
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:
boolpred(constType1&a,constType2&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
(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 an object of type ForwardIt can be dereferenced and then implicitly converted to both of them.
policy - the
to use Type requirements -ForwardIt must meet the requirements of
. -BinaryPred must meet the requirements of
. Return value
The first iterator iter in the source range such that the following expression evaluates to true:
1,3)bool(*iter==*std::next(iter))
2,4)bool(p(*iter,std::next(iter))
If no such iterator is found, last is returned.
Complexity
Given result as the return value of adjacent_find, M as std::distance(first,result), and N as std::distance(first,last):
1) Exactly min(M+1,N-1) comparisons using operator==.
2) Exactly min(M+1,N-1) applications of p.
3)𝓞(N) comparisons using operator==.
4)𝓞(N) applications of p.
Exceptions
3,4) During the execution process:
If the temporary memory resources required for parallelization are not available,
is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for
,
is invoked).
Possible implementation
template<classForwardIt>ForwardItadjacent_find(ForwardItfirst,ForwardItlast){if(first==last)returnlast;ForwardItnext=first;++next;for(;next!=last;++next,++first)if(*first==*next)returnfirst;returnlast;}
template<classForwardIt,classBinaryPred>ForwardItadjacent_find(ForwardItfirst,ForwardItlast,BinaryPredp){if(first==last)returnlast;ForwardItnext=first;++next;for(;next!=last;++next,++first)if(p(*first,*next))returnfirst;returnlast;}Example
Run this code
#include<algorithm>#include<functional>#include<iostream>#include<vector>intmain(){std::vector<int>v1{0,1,2,3,40,40,41,41,5};autoi1=std::adjacent_find(v1.begin(),v1.end());if(i1==v1.end())std::cout<<"No matching adjacent elements\n";elsestd::cout<<"The first adjacent pair of equal elements is at "<<std::distance(v1.begin(),i1)<<", *i1 = "<<*i1<<'\n';autoi2=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";elsestd::cout<<"The last element in the non-decreasing subsequence is at "<<std::distance(v1.begin(),i2)<<", *i2 = "<<*i2<<'\n';}Output:
The first adjacent pair of equal elements is at 4, *i1 = 40 The last element in the non-decreasing subsequence is at 7, *i2 = 41 Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 the complexity requirement was unclear made clear See also
(C++20)
finds the first two adjacent items that are equal (or satisfy a given predicate)
(algorithm function object)
removes consecutive duplicate elements in a range
(function template & algorithm function object)
(C++20)