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Showing content from https://cplusplus.com/reference/algorithm/mismatch/ below:

function template

std::mismatch equality (1)

template <class InputIterator1, class InputIterator2>  pair<InputIterator1, InputIterator2>    mismatch (InputIterator1 first1, InputIterator1 last1,              InputIterator2 first2);

predicate (2)

template <class InputIterator1, class InputIterator2, class BinaryPredicate>  pair<InputIterator1, InputIterator2>    mismatch (InputIterator1 first1, InputIterator1 last1,              InputIterator2 first2, BinaryPredicate pred);

Return first position where two ranges differ

Compares the elements in the range [first1,last1) with those in the range beginning at first2, and returns the first element of both sequences that does not match.

The elements are compared using operator== (or pred, in version (2)).

The function returns a pair of iterators to the first element in each range that does not match.

The behavior of this function template is equivalent to:

template <class InputIterator1, class InputIterator2>
  pair<InputIterator1, InputIterator2>
    mismatch (InputIterator1 first1, InputIterator1 last1, InputIterator2 first2 )
{
  while ( (first1!=last1) && (*first1==*first2) )  // or: pred(*first1,*first2), for version 2
  { ++first1; ++first2; }
  return std::make_pair(first1,first2);
}

Parameters

first1, last1: Input iterators to the initial and final positions of the first sequence. The range used is [first1,last1), which contains all the elements between first1 and last1, including the element pointed by first1 but not the element pointed by last1.
first2: Input iterator to the initial position of the second sequence. Up to as many elements as in the range [first1,last1) can be accessed by the function.
pred: Binary function that accepts two elements as argument (one of each of the two sequences, in the same order), and returns a value convertible to bool. The value returned indicates whether the elements are considered to match in the context of this function.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

Return value A pair, where its members first and second point to the first element in both sequences that did not compare equal to each other.
If the elements compared in both sequences have all matched, the function returns a pair with first set to last1 and second set to the element in that same relative position in the second sequence.
If none matched, it returns make_pair(first1,first2).

Example

// mismatch algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::mismatch
#include <vector>       // std::vector
#include <utility>      // std::pair

bool mypredicate (int i, int j) {
  return (i==j);
}

int main () {
  std::vector<int> myvector;
  for (int i=1; i<6; i++) myvector.push_back (i*10); // myvector: 10 20 30 40 50

  int myints[] = {10,20,80,320,1024};                //   myints: 10 20 80 320 1024

  std::pair<std::vector<int>::iterator,int*> mypair;

  // using default comparison:
  mypair = std::mismatch (myvector.begin(), myvector.end(), myints);
  std::cout << "First mismatching elements: " << *mypair.first;
  std::cout << " and " << *mypair.second << '\n';

  ++mypair.first; ++mypair.second;

  // using predicate comparison:
  mypair = std::mismatch (mypair.first, myvector.end(), mypair.second, mypredicate);
  std::cout << "Second mismatching elements: " << *mypair.first;
  std::cout << " and " << *mypair.second << '\n';

  return 0;
}

Output:

First mismatching elements: 30 and 80
Second mismatching elements: 40 and 320

Complexity Up to linear in the distance between first1 and last1: Compares elements until a mismatch is found.

Data races Some (or all) of the objects in both ranges are accessed (once at most).

Exceptions Throws if any element comparison (or pred) throws or if any of the operations on iterators throws.
Note that invalid parameters cause undefined behavior.

See also

find_first_of: Find element from set in range (function template)

find_end: Find last subsequence in range (function template)

search: Search range for subsequence (function template)

equal: Test whether the elements in two ranges are equal (function template)

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