std::ranges::mismatch, std::ranges::mismatch_result

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< cpp‎ | algorithm‎ | ranges
 
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Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
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C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
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Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
Constrained numeric operations
(C++23)
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Return types
 
Defined in header <algorithm>
Call signature
template< std::input_iterator I1, std::sentinel_for<I1> S1,

          std::input_iterator I2, std::sentinel_for<I2> S2,
          class Pred = ranges::equal_to,
          class Proj1 = std::identity, class Proj2 = std::identity >
requires  std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr mismatch_result<I1, I2>
          mismatch( I1 first1, S1 last1, I2 first2, S2 last2,

                    Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
 (1) (since C++20)
template< ranges::input_range R1, ranges::input_range R2,

          class Pred = ranges::equal_to,
          class Proj1 = std::identity, class Proj2 = std::identity >
requires  std::indirectly_comparable<
               ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2>
constexpr mismatch_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>>

          mismatch( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
 (2) (since C++20)
Helper types
template<class I1, class I2>
using mismatch_result = ranges::in_in_result<I1, I2>;
 (3) (since C++20)

Returns the first mismatching pair of projected elements from two ranges: one defined by [first1, last1) or r1 and another defined by [first2,last2) or r2.

1) Elements are compared using the given binary predicate p.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first1, last1 - an iterator-sentinel pair denoting the first range of the elements to compare
r1 - the first range of the elements to compare
first2, last2 - an iterator-sentinel pair denoting the second range of the elements to compare
r2 - the second range of the elements to compare
pred - predicate to apply to the projected elements
proj1 - projection to apply to the first range of elements
proj2 - projection to apply to the second range of elements

Return value

ranges::mismatch_result with iterators to the first two non-equal elements.

If no mismatches are found when the comparison reaches last1 or last2, whichever happens first, the object holds the end iterator and the corresponding iterator from the other range.

Complexity

At most min(last1 - first1, last2 - first2) applications of the predicate and corresponding projections.

Possible implementation

struct mismatch_fn {
  template<std::input_iterator I1, std::sentinel_for<I1> S1,
           std::input_iterator I2, std::sentinel_for<I2> S2,
           class Pred = ranges::equal_to,
           class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
  constexpr std::mismatch_result<I1, I2>
  operator()(I1 first1, S1 last1, I2 first2, S2 last2,
             Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
  {
      for (; first1 != last1 && first2 != last2; ++first1, (void)++first2) {
          if (!std::invoke(pred, std::invoke(proj1, *first1), std::invoke(proj2, *first2))) {
            break;
          }
      }
      return {first1, first2};
  }
 
  template<ranges::input_range R1, ranges::input_range R2,
           class Pred = ranges::equal_to,
           class Proj1 = std::identity, class Proj2 = std::identity >
  requires std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                                      Pred, Proj1, Proj2>
  constexpr
    ranges::mismatch_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>>
  operator()(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
  {
     return (*this)(ranges::begin(r1), ranges::end(r1),
                    ranges::begin(r2), ranges::end(r2),
                    std::ref(pred), std::ref(proj1), std::ref(proj2));
  }
};
 
inline constexpr mismatch_fn mismatch;

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 <cstddef>
#include <iostream>
#include <ranges>
#include <string_view>
 
constexpr std::string_view mirror_ends(const std::string_view in)
{
    const auto end = std::ranges::mismatch(in, in | std::views::reverse).in1;
    const std::size_t length = std::ranges::distance(in.begin(), end);
    return { in.cbegin(), length };
}
 
int main()
{
    std::cout << mirror_ends("abXYZba") << '\n'
              << mirror_ends("abca") << '\n'
              << mirror_ends("ABBA") << '\n'
              << mirror_ends("level") << '\n';
 
    using namespace std::literals::string_view_literals;
 
    static_assert("123"sv == mirror_ends("123!@#321"));
    static_assert("radar"sv == mirror_ends("radar"));
}

Output: 

ab
a
ABBA
level

See also

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