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mismatch

The header file 'mismatch.hpp' contains two variants of a the stl algorithm mismatch. The algorithm finds the first point in two sequences where they do not match.

Before (the proposed) C++14 the algorithm std::mismatch took three iterators and an optional comparison predicate. The first two iterators [first1, last1) defined a sequence, and the second one first2 defined the start of the second sequence. The second sequence was assumed to be the same length as the first.

In C++14, two new variants were introduced, taking four iterators and an optional comparison predicate. The four iterators define two sequences [first1, last1) and [first2, last2) explicitly, rather than defining the second one implicitly. This leads to correct answers in more cases (and avoid undefined behavior in others).

Consider the two sequences:

auto seq1 = { 0, 1, 2 };
auto seq2 = { 0, 1, 2, 3, 4 };

std::mismatch ( seq1.begin (), seq1.end (), seq2.begin ()); // <3, 3>
std::mismatch ( seq2.begin (), seq2.end (), seq1.begin ()); // Undefined behavior
std::mismatch ( seq1.begin (), seq1.end (), seq2.begin (), seq2.end ()); // <3, 3>

The first N entries in seq2 are the same as the entries in seq1 - but that's not all that's in seq2. In the second case, the algorithm will read past the end of seq1, resulting in undefined behavior (large earthquake, incorrect results, pregnant cat, etc).

However, if the two sequences are specified completely, it's clear that where the mismatch occurs.

interface

The function mismatch returns a pair of iterators which denote the first mismatching elements in each sequence. If the sequences match completely, mismatch returns their end iterators. One version uses std::equal_to to do the comparison; the other lets the caller pass predicate to do the comparisons.

template <class InputIterator1, class InputIterator2>
std::pair<InputIterator1, InputIterator2>
mismatch ( InputIterator1 first1, InputIterator1 last1,
           InputIterator2 first2, InputIterator2 last2 );

template <class InputIterator1, class InputIterator2, class BinaryPredicate>
std::pair<InputIterator1, InputIterator2>
mismatch ( InputIterator1 first1, InputIterator1 last1,
           InputIterator2 first2, InputIterator2 last2, BinaryPredicate pred );

Examples

Given the container c1 containing { 0, 1, 2, 3, 14, 15 }, and c2 containing { 1, 2, 3 }, then

mismatch ( c1.begin(),     c1.end(),       c2.begin(), c2.end()) --> <c1.begin(), c2.begin()> // first elements do not match
mismatch ( c1.begin() + 1, c1.begin() + 4, c2.begin(), c2.end()) --> <c1.begin() + 4, c2.end ()> // all elements of `c2` match
mismatch ( c1.end(),       c1.end(),       c2.end(),   c2.end()) --> <c1.end(), c2.end()> // empty sequences don't match at the end.

Iterator Requirements

mismatch works on all iterators except output iterators.

Complexity

Both of the variants of mismatch run in O(N) (linear) time; that is, they compare against each element in the list once. If the sequence is found to be equal at any point, the routine will terminate immediately, without examining the rest of the elements.

Exception Safety

Both of the variants of mismatch take their parameters by value and do not depend upon any global state. Therefore, all the routines in this file provide the strong exception guarantee.

Notes

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