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Difference between revisions of "cpp/algorithm/mismatch"

From cppreference.com
< cpp‎ | algorithm
m (Use parallel overload precondition template)
m (Mathjax update.)
 
(10 intermediate revisions by 4 users not shown)
Line 2: Line 2:
 
{{cpp/algorithm/navbar}}
 
{{cpp/algorithm/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | algorithm}}
+
{{dcl header|algorithm}}
−
{{dcl rev multi | num=1 | since1= | until1=c++20 |dcl1=
+
{{dcla|num=1|notes={{mark constexpr since c++20}}|
 
template< class InputIt1, class InputIt2 >
 
template< class InputIt1, class InputIt2 >
−
std::pair<InputIt1,InputIt2>
+
std::pair<InputIt1, InputIt2>
 
     mismatch( InputIt1 first1, InputIt1 last1,
 
     mismatch( InputIt1 first1, InputIt1 last1,
 
               InputIt2 first2 );
 
               InputIt2 first2 );
−
|dcl2=
 
−
template< class InputIt1, class InputIt2 >
 
−
constexpr std::pair<InputIt1,InputIt2>
 
−
              mismatch( InputIt1 first1, InputIt1 last1,
 
−
                        InputIt2 first2 );
 
 
}}
 
}}
−
{{dcl | since=c++17 | until= | num=2 |1=
+
{{dcl|num=2|since=c++17|
 
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
 
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
−
std::pair<ForwardIt1,ForwardIt2>
+
std::pair<ForwardIt1, ForwardIt2>
−
     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
+
     mismatch( ExecutionPolicy&& policy,
 +
              ForwardIt1 first1, ForwardIt1 last1,
 
               ForwardIt2 first2 );
 
               ForwardIt2 first2 );
 
}}
 
}}
−
{{dcl rev multi | num=3 | since1= | until1=c++20 |dcl1=
+
{{dcla|num=3|notes={{mark constexpr since c++20}}|
−
template< class InputIt1, class InputIt2, class BinaryPredicate >
+
template< class InputIt1, class InputIt2, class BinaryPred >
−
std::pair<InputIt1,InputIt2>
+
std::pair<InputIt1, InputIt2>
 
     mismatch( InputIt1 first1, InputIt1 last1,
 
     mismatch( InputIt1 first1, InputIt1 last1,
−
               InputIt2 first2,
+
               InputIt2 first2, BinaryPred p );
−
              BinaryPredicate p );
+
−
|dcl2=
+
−
template< class InputIt1, class InputIt2, class BinaryPredicate >
+
−
constexpr std::pair<InputIt1,InputIt2>
+
−
              mismatch( InputIt1 first1, InputIt1 last1,
+
−
                        InputIt2 first2,
+
−
                        BinaryPredicate p );
+
 
}}
 
}}
−
{{dcl | since=c++17 | until= | num=4 |1=
+
{{dcl|num=4|since=c++17|
−
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate >
+
template< class ExecutionPolicy,
−
std::pair<ForwardIt1,ForwardIt2>
+
          class ForwardIt1, class ForwardIt2, class BinaryPred >
−
     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
+
std::pair<ForwardIt1, ForwardIt2>
−
               ForwardIt2 first2,
+
     mismatch( ExecutionPolicy&& policy,
−
              BinaryPredicate p );
+
              ForwardIt1 first1, ForwardIt1 last1,
 +
               ForwardIt2 first2, BinaryPred p );
 
}}
 
}}
−
{{dcl rev multi | num=5 | since1=c++14 | until1=c++20 |dcl1=
+
{{dcla|num=5|since=c++14|notes={{mark constexpr since c++20}}|
 
template< class InputIt1, class InputIt2 >
 
template< class InputIt1, class InputIt2 >
−
std::pair<InputIt1,InputIt2>
+
std::pair<InputIt1, InputIt2>
 
     mismatch( InputIt1 first1, InputIt1 last1,
 
     mismatch( InputIt1 first1, InputIt1 last1,
 
               InputIt2 first2, InputIt2 last2 );
 
               InputIt2 first2, InputIt2 last2 );
−
|dcl2=
 
−
template< class InputIt1, class InputIt2 >
 
−
constexpr std::pair<InputIt1,InputIt2>
 
−
              mismatch( InputIt1 first1, InputIt1 last1,
 
−
                        InputIt2 first2, InputIt2 last2 );
 
 
}}
 
}}
−
{{dcl | since=c++17 | until= | num=6 |1=
+
{{dcl|num=6|since=c++17|
 
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
 
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
−
std::pair<ForwardIt1,ForwardIt2>
+
std::pair<ForwardIt1, ForwardIt2>
−
     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
+
     mismatch( ExecutionPolicy&& policy,
 +
              ForwardIt1 first1, ForwardIt1 last1,
 
               ForwardIt2 first2, ForwardIt2 last2 );
 
               ForwardIt2 first2, ForwardIt2 last2 );
 
}}
 
}}
−
{{dcl rev multi | num=7 | since1=c++14 | until1=c++20 |dcl1=
+
{{dcla|num=7|since=c++14|notes={{mark constexpr since c++20}}|
−
template< class InputIt1, class InputIt2, class BinaryPredicate >
+
template< class InputIt1, class InputIt2, class BinaryPred >
−
std::pair<InputIt1,InputIt2>
+
std::pair<InputIt1, InputIt2>
 
     mismatch( InputIt1 first1, InputIt1 last1,
 
     mismatch( InputIt1 first1, InputIt1 last1,
−
               InputIt2 first2, InputIt2 last2,
+
               InputIt2 first2, InputIt2 last2, BinaryPred p );
−
              BinaryPredicate p );
+
−
|dcl2=
+
−
template< class InputIt1, class InputIt2, class BinaryPredicate >
+
−
constexpr std::pair<InputIt1,InputIt2>
+
−
              mismatch( InputIt1 first1, InputIt1 last1,
+
−
                        InputIt2 first2, InputIt2 last2,
+
−
                        BinaryPredicate p );
+
 
}}
 
}}
−
{{dcl | since=c++17 | until= | num=8 |1=
+
{{dcl|num=8|since=c++17|
−
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate >
+
template< class ExecutionPolicy,
−
std::pair<ForwardIt1,ForwardIt2>
+
          class ForwardIt1, class ForwardIt2, class BinaryPred >
−
     mismatch( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1,
+
std::pair<ForwardIt1, ForwardIt2>
−
               ForwardIt2 first2, ForwardIt2 last2,
+
     mismatch( ExecutionPolicy&& policy,
−
              BinaryPredicate p );
+
              ForwardIt1 first1, ForwardIt1 last1,
 +
               ForwardIt2 first2, ForwardIt2 last2, BinaryPred p );
 
}}
 
}}
−
 
 
{{dcl end}}
 
{{dcl end}}
  
−
Returns the first mismatching pair of elements from two ranges: one defined by {{tt|[first1, last1)}} and another defined by {{tt|[first2,last2)}}. If {{tt|last2}} is not provided (overloads {{v|1-4}}), it denotes {{tt|first2 + (last1 - first1)}}.
+
Returns a pair of iterators to the first mismatching of elements from {{range|first1|last1}} and a range starting from {{c|first2}}:
 +
* For overloads {{v|1-4}}, the second range has {{c|std::distance(first1, last1)}} elements.
 +
* For overloads {{v|5-8}}, the second range is {{range|first2|last2}}.
 +
:* If {{c|std::distance(first1, last1)}} and {{c|std::distance(first2, last2)}} are different, the comparison stops when {{c|last1}} or {{c|last2}} is reached.
  
−
@1,5@ Elements are compared using {{tt|operator{{==}}}}.
+
@1,5@ Elements are compared using {{c/core|1=operator==}}.
−
@3,7@ Elements are compared using the given binary predicate {{tt|p}}.
+
@3,7@ Elements are compared using the given binary predicate {{c|p}}.
  
−
@2,4,6,8@ Same as {{v|1,3,5,7}}, but executed according to {{tt|policy}}. {{cpp/algorithm/parallel overload precondition|plural=true}}
+
@2,4,6,8@ Same as {{v|1,3,5,7}}, but executed according to {{c|policy}}.
 +
@@ {{cpp/algorithm/parallel overload precondition|plural=yes}}
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | first1, last1 | the first range of the elements}}
+
{{par|first1, last1|the first range of the elements}}
−
{{par | first2, last2 | the second range of the elements}}
+
{{par|first2, last2|the second range of the elements}}
 
{{par exec pol}}
 
{{par exec pol}}
−
{{par pred2 eq | p | p1=InputIt1 | p2=InputIt2}}
+
{{par pred2 eq|p|p1=InputIt1|p2=InputIt2}}
 
{{par hreq}}
 
{{par hreq}}
−
{{par req named | InputIt1 | InputIterator}}
+
{{par req named|InputIt1|InputIterator}}
−
{{par req named | InputIt2 | InputIterator}}
+
{{par req named|InputIt2|InputIterator}}
−
{{par req named | ForwardIt1 | ForwardIterator}}
+
{{par req named|ForwardIt1|ForwardIterator}}
−
{{par req named | ForwardIt2 | ForwardIterator}}
+
{{par req named|ForwardIt2|ForwardIterator}}
−
{{par req named | BinaryPredicate | BinaryPredicate}}
+
{{par req named|BinaryPred|BinaryPredicate}}
 
{{par end}}
 
{{par end}}
  
Line 104: Line 87:
 
{{lc|std::pair}} with iterators to the first two non-equal elements.
 
{{lc|std::pair}} with iterators to the first two non-equal elements.
  
−
{{rev begin}}
+
If {{c|last1}} is reached, the second iterator in the pair is the {{c|std::distance(first1, last1)}}{{su|b=th}} iterator after {{c|first2}}.
−
{{rev | until=c++14 | If no mismatches are found when the comparison reaches {{tt|last1}}, the pair holds {{tt|last1}} and the corresponding iterator from the second range. The behavior is undefined if the second range is shorter than the first range.
+
 
−
}}
+
For overloads {{v|5-8}}, if {{c|last2}} is reached, the first iterator in the pair is the {{c|std::distance(first2, last2)}}{{su|b=th}} iterator after {{c|first1}}.
−
{{rev | since=c++14 | If no mismatches are found when the comparison reaches {{tt|last1}} or {{tt|last2}}, whichever happens first, the pair holds the end iterator and the corresponding iterator from the other range.
+
−
}}
+
−
{{rev end}}
+
  
 
===Complexity===
 
===Complexity===
−
@1-4@At most {{tt|last1}} - {{tt|first1}} applications of {{tt|operator{{==}}}} or the predicate {{tt|p}}
+
Given {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} as {{c|std::distance(first1, last1)}} and {{mathjax-or|\(\scriptsize N_2\)|N{{su|b=2}}}} as {{c|std::distance(first2, last2)}}:
−
@5-8@At most min({{tt|last1}} - {{tt|first1}}, {{tt|last2}} - {{tt|first2}}) applications of {{tt|operator{{==}}}} or the predicate {{tt|p}}.
+
@1,2@ At most {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} comparisons using {{c/core|1=operator==}}.
 +
@3,4@ At most {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} applications of the predicate {{c|p}}.
 +
@5,6@ At most {{mathjax-or|\(\scriptsize \min(N_1,N_2)\)|min(N{{su|b=1}},N{{su|b=2}})}} comparisons using {{c/core|1=operator==}}.
 +
@7,8@ At most {{mathjax-or|\(\scriptsize \min(N_1,N_2)\)|min(N{{su|b=1}},N{{su|b=2}})}} applications of the predicate {{c|p}}.
  
 
===Exceptions===
 
===Exceptions===
−
{{cpp/algorithm/parallel_exceptions_reporting_behavior|singular=no}}
+
{{cpp/algorithm/parallel exceptions reporting behavior|singular=no}}
  
 
===Possible implementation===
 
===Possible implementation===
−
{{eq fun
+
{{eq impl
−
| 1=
+
|title1=mismatch (1)|ver1=1|1=
 
template<class InputIt1, class InputIt2>
 
template<class InputIt1, class InputIt2>
 
std::pair<InputIt1, InputIt2>
 
std::pair<InputIt1, InputIt2>
 
     mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2)
 
     mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2)
 
{
 
{
−
     while (first1 != last1 && *first1 == *first2) {
+
     while (first1 != last1 && *first1 == *first2)
 
         ++first1, ++first2;
 
         ++first1, ++first2;
−
     }
+
      
 
     return std::make_pair(first1, first2);
 
     return std::make_pair(first1, first2);
 
}
 
}
−
| 2=
+
|title2=mismatch (3)|ver2=3|2=
−
template<class InputIt1, class InputIt2, class BinaryPredicate>
+
template<class InputIt1, class InputIt2, class BinaryPred>
 
std::pair<InputIt1, InputIt2>
 
std::pair<InputIt1, InputIt2>
−
     mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPredicate p)
+
     mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPred p)
 
{
 
{
−
     while (first1 != last1 && p(*first1, *first2)) {
+
     while (first1 != last1 && p(*first1, *first2))
 
         ++first1, ++first2;
 
         ++first1, ++first2;
−
     }
+
      
 
     return std::make_pair(first1, first2);
 
     return std::make_pair(first1, first2);
 
}
 
}
−
| 3=
+
|title3=mismatch (5)|ver3=5|3=
 
template<class InputIt1, class InputIt2>
 
template<class InputIt1, class InputIt2>
 
std::pair<InputIt1, InputIt2>
 
std::pair<InputIt1, InputIt2>
 
     mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2)
 
     mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2)
 
{
 
{
−
     while (first1 != last1 && first2 != last2 && *first1 == *first2) {
+
     while (first1 != last1 && first2 != last2 && *first1 == *first2)
 
         ++first1, ++first2;
 
         ++first1, ++first2;
−
     }
+
      
 
     return std::make_pair(first1, first2);
 
     return std::make_pair(first1, first2);
 
}
 
}
−
| 4=
+
|title4=mismatch (7)|ver4=7|4=
−
template<class InputIt1, class InputIt2, class BinaryPredicate>
+
template<class InputIt1, class InputIt2, class BinaryPred>
 
std::pair<InputIt1, InputIt2>
 
std::pair<InputIt1, InputIt2>
−
     mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, BinaryPredicate p)
+
     mismatch(InputIt1 first1, InputIt1 last1,
 +
            InputIt2 first2, InputIt2 last2, BinaryPred p)
 
{
 
{
−
     while (first1 != last1 && first2 != last2 && p(*first1, *first2)) {
+
     while (first1 != last1 && first2 != last2 && p(*first1, *first2))
 
         ++first1, ++first2;
 
         ++first1, ++first2;
−
     }
+
      
 
     return std::make_pair(first1, first2);
 
     return std::make_pair(first1, first2);
 
}
 
}
Line 163: Line 147:
  
 
===Example===
 
===Example===
−
 
 
{{example
 
{{example
−
| This program determines the longest substring that is simultaneously found at the very beginning of the given string and at the very end of it, in reverse order (possibly overlapping)
+
|This program determines the longest substring that is simultaneously found at the very beginning of the given string and at the very end of it, in reverse order (possibly overlapping).
−
| code=
+
|code=
 +
#include <algorithm>
 
#include <iostream>
 
#include <iostream>
 
#include <string>
 
#include <string>
−
#include <algorithm>
 
  
 
std::string mirror_ends(const std::string& in)
 
std::string mirror_ends(const std::string& in)
Line 183: Line 166:
 
               << mirror_ends("aba") << '\n';
 
               << mirror_ends("aba") << '\n';
 
}
 
}
−
| output=
+
|output=
 
ab
 
ab
 
a
 
a
Line 191: Line 174:
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/algorithm/dsc equal}}
+
{{dsc inc|cpp/algorithm/dsc equal}}
−
{{dsc inc | cpp/algorithm/dsc find}}
+
{{dsc inc|cpp/algorithm/dsc find}}
−
{{dsc inc | cpp/algorithm/dsc lexicographical_compare}}
+
{{dsc inc|cpp/algorithm/dsc lexicographical_compare}}
−
{{dsc inc | cpp/algorithm/dsc search}}
+
{{dsc inc|cpp/algorithm/dsc search}}
−
 
+
{{dsc inc|cpp/algorithm/ranges/dsc mismatch}}
 
{{dsc end}}
 
{{dsc end}}
  
−
[[de:cpp/algorithm/mismatch]]
+
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
−
[[es:cpp/algorithm/mismatch]]
+
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[[fr:cpp/algorithm/mismatch]]
+
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[[it:cpp/algorithm/mismatch]]
+
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[[ja:cpp/algorithm/mismatch]]
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[[pt:cpp/algorithm/mismatch]]
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[[ru:cpp/algorithm/mismatch]]
+
−
[[zh:cpp/algorithm/mismatch]]
+

Latest revision as of 01:37, 3 April 2024

 
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)
mismatch

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>
template< class InputIt1, class InputIt2 >

std::pair<InputIt1, InputIt2>
    mismatch( InputIt1 first1, InputIt1 last1,

              InputIt2 first2 );
 (1) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >

std::pair<ForwardIt1, ForwardIt2>
    mismatch( ExecutionPolicy&& policy,
              ForwardIt1 first1, ForwardIt1 last1,

              ForwardIt2 first2 );
 (2) (since C++17)
template< class InputIt1, class InputIt2, class BinaryPred >

std::pair<InputIt1, InputIt2>
    mismatch( InputIt1 first1, InputIt1 last1,

              InputIt2 first2, BinaryPred p );
 (3) (constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class BinaryPred >
std::pair<ForwardIt1, ForwardIt2>
    mismatch( ExecutionPolicy&& policy,
              ForwardIt1 first1, ForwardIt1 last1,

              ForwardIt2 first2, BinaryPred p );
 (4) (since C++17)
template< class InputIt1, class InputIt2 >

std::pair<InputIt1, InputIt2>
    mismatch( InputIt1 first1, InputIt1 last1,

              InputIt2 first2, InputIt2 last2 );
 (5) (since C++14)
(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >

std::pair<ForwardIt1, ForwardIt2>
    mismatch( ExecutionPolicy&& policy,
              ForwardIt1 first1, ForwardIt1 last1,

              ForwardIt2 first2, ForwardIt2 last2 );
 (6) (since C++17)
template< class InputIt1, class InputIt2, class BinaryPred >

std::pair<InputIt1, InputIt2>
    mismatch( InputIt1 first1, InputIt1 last1,

              InputIt2 first2, InputIt2 last2, BinaryPred p );
 (7) (since C++14)
(constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class BinaryPred >
std::pair<ForwardIt1, ForwardIt2>
    mismatch( ExecutionPolicy&& policy,
              ForwardIt1 first1, ForwardIt1 last1,

              ForwardIt2 first2, ForwardIt2 last2, BinaryPred p );
 (8) (since C++17)

Returns a pair of iterators to the first mismatching of elements from [first1, last1) and a range starting from first2:

  • For overloads (1-4), the second range has std::distance(first1, last1) elements.
  • For overloads (5-8), the second range is [first2, last2).
  • If std::distance(first1, last1) and std::distance(first2, last2) are different, the comparison stops when last1 or last2 is reached.
1,5) Elements are compared using operator==.
3,7) Elements are compared using the given binary predicate p.
2,4,6,8) Same as (1,3,5,7), 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

[edit] Parameters

first1, last1 - the first range of the elements
first2, last2 - the second range of the elements
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 InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​

Type requirements
-
InputIt1 must meet the requirements of LegacyInputIterator.
-
InputIt2 must meet the requirements of LegacyInputIterator.
-
ForwardIt1 must meet the requirements of LegacyForwardIterator.
-
ForwardIt2 must meet the requirements of LegacyForwardIterator.
-
BinaryPred must meet the requirements of BinaryPredicate.

[edit] Return value

std::pair with iterators to the first two non-equal elements.

If last1 is reached, the second iterator in the pair is the std::distance(first1, last1)th iterator after first2.

For overloads (5-8), if last2 is reached, the first iterator in the pair is the std::distance(first2, last2)th iterator after first1.

[edit] Complexity

Given N1 as std::distance(first1, last1) and N2 as std::distance(first2, last2):

1,2) At most N1 comparisons using operator==.
3,4) At most N1 applications of the predicate p.
5,6) At most min(N1,N2) comparisons using operator==.
7,8) At most min(N1,N2) applications of the predicate p.

[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

mismatch (1) 
template<class InputIt1, class InputIt2>
std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2)
{
    while (first1 != last1 && *first1 == *first2)
        ++first1, ++first2;
 
    return std::make_pair(first1, first2);
}
mismatch (3) 
template<class InputIt1, class InputIt2, class BinaryPred>
std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPred p)
{
    while (first1 != last1 && p(*first1, *first2))
        ++first1, ++first2;
 
    return std::make_pair(first1, first2);
}
mismatch (5) 
template<class InputIt1, class InputIt2>
std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2)
{
    while (first1 != last1 && first2 != last2 && *first1 == *first2)
        ++first1, ++first2;
 
    return std::make_pair(first1, first2);
}
mismatch (7) 
template<class InputIt1, class InputIt2, class BinaryPred>
std::pair<InputIt1, InputIt2>
    mismatch(InputIt1 first1, InputIt1 last1,
             InputIt2 first2, InputIt2 last2, BinaryPred p)
{
    while (first1 != last1 && first2 != last2 && p(*first1, *first2))
        ++first1, ++first2;
 
    return std::make_pair(first1, first2);
}

[edit] Example

This program determines the longest substring that is simultaneously found at the very beginning of the given string and at the very end of it, in reverse order (possibly overlapping).

Run this code
#include <algorithm>
#include <iostream>
#include <string>
 
std::string mirror_ends(const std::string& in)
{
    return std::string(in.begin(),
                       std::mismatch(in.begin(), in.end(), in.rbegin()).first);
}
 
int main()
{
    std::cout << mirror_ends("abXYZba") << '\n'
              << mirror_ends("abca") << '\n'
              << mirror_ends("aba") << '\n';
}

Output: 

ab
a
aba

[edit] See also

 determines if two sets of elements are the same
(function template) [edit]
(C++11)
 finds the first element satisfying specific criteria
(function template) [edit]
returns true if one range is lexicographically less than another
(function template) [edit]
searches for the first occurrence of a range of elements
(function template) [edit]
(C++20)
 finds the first position where two ranges differ
(niebloid)[edit]
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