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# std::mismatch

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt1, class InputIt2 > std::pair<InputIt1,InputIt2> mismatch( InputIt1 first1, InputIt1 last1, InputIt2 first2 ); | (1) |
template< class InputIt1, class InputIt2, class BinaryPredicate > std::pair<InputIt1,InputIt2> mismatch( InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPredicate p ); | (2) |
template< class InputIt1, class InputIt2 > std::pair<InputIt1,InputIt2> mismatch( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 ); | (3) | (since C++14)
template< class InputIt1, class InputIt2, class BinaryPredicate > std::pair<InputIt1,InputIt2> mismatch( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, BinaryPredicate p ); | (4) | (since C++14)

Returns the first mismatching pair of elements from two ranges: one defined by `[first1, last1)` and another defined by `[first2,last2)`. If `last2` is not provided (overloads (1) and (2)), it denotes `first2 + (last1 - first1)`.

Overloads (1) and (3) use `operator==` to compare the elements, the overloads (2) and (4) use the given binary predicate `p`.

### [edit] Parameters

first1, last1 | - | the first range of the elements
first2, last2 | - | the second range of the elements
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 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 `InputIterator`.
- `InputIt2` must meet the requirements of `InputIterator`.
- `BinaryPredicate` must meet the requirements of `BinaryPredicate`.

### [edit] Return value

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

If no mismatches are found when the comparison reaches `last1`, the pair holds `last1` and the corresponding iterator from the second range. The behavior is undefined if the second range is shorter than the first range. | (until C++14)
If no mismatches are found when the comparison reaches `last1` or `last2`, whichever happens first, the pair holds the end iterator and the corresponding iterator from the other range. | (since C++14)

### [edit] Complexity

At most `last1` - `first1` applications of the predicate

### [edit] Possible implementation

First version
    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);
    }
Second version
    template<class InputIt1, class InputIt2, class BinaryPredicate>
    std::pair<InputIt1, InputIt2>
        mismatch(InputIt1 first1, InputIt1 last1, InputIt2 first2, BinaryPredicate p)
    {
        while (first1 != last1 && 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 <iostream>
    #include <string>
    #include <algorithm>

    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

equal | determines if two sets of elements are the same (function template) [edit]
find find_if find_if_not (C++11) | finds the first element satisfying specific criteria (function template) [edit]
lexicographical_compare | returns true if one range is lexicographically less than another (function template) [edit]
search | searches for a range of elements (function template) [edit]

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