# std::experimental::ranges::lexicographical_compare

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< cpp‎ | experimental‎ | ranges

Defined in header `<experimental/ranges/algorithm>` |  |
template< InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I2> S2, class Proj1 = ranges::identity, class Proj2 = ranges::identity, class Comp = ranges::less<> > requires IndirectStrictWeakOrder<Comp, projected<I1, Proj1>, projected<I2, Proj2>> bool lexicographical_compare(I1 first1, S1 last1, I2 first2, S2 last2, Comp comp = Comp{}, Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{}); | (1) | (ranges TS)
template< InputRange R1, InputRange R2, class Proj1 = ranges::identity, class Proj2 = ranges::identity, class Comp = ranges::less<> > requires IndirectStrictWeakOrder<Comp, projected<ranges::iterator_t<R1>, Proj1>, projected<ranges::iterator_t<R2>, Proj2>> bool lexicographical_compare(R1&& r1, R2&& r2, Comp comp = Comp{}, Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{}); | (2) | (ranges TS)

1) Checks if the first range [first1, last1) is lexicographically less than the second range [first2, last2). Elements are compared using the given binary comparison function `comp`, after being projected with `proj1` and `proj2` respectively.
2) Same as (1), but uses `r1` as the first source range and `r2` as the second source range, as if using ranges::begin(r1) as `first1`, ranges::end(r1) as `last1`, ranges::begin(r2) as `first2`, and ranges::end(r2) as `last2`.

Lexicographical comparison is a operation with the following properties:

- Two ranges are compared element by element.
- The first mismatching element defines which range is lexicographically less or greater than the other.
- If one range is a prefix of another, the shorter range is lexicographically less than the other.
- If two ranges have equivalent elements and are of the same length, then the ranges are lexicographically equal.
- An empty range is lexicographically less than any non-empty range.
- Two empty ranges are lexicographically equal.

### Parameters

first1, last1 | - | the first range of elements to examine
r1 | - | the first range of elements to examine
first2, last2 | - | the second range of elements to examine
r2 | - | the second range of elements to examine
comp | - | comparison function to apply to the projected elements
proj1 | - | projection to apply to the elements in the first range
proj2 | - | projection to apply to the elements in the second range

### Return value

true if the first range is lexicographically less than the second.

### Complexity

At most 2·min(N1, N2) applications of the comparison operation, where N1 = last1 - first1 and N2 = last2 - first2.

### Possible implementation

    template< InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I2> S2,
              class Proj1 = ranges::identity, class Proj2 = ranges::identity,
              class Comp = ranges::less<> >
      requires IndirectStrictWeakOrder<Comp, projected<I1, Proj1>, projected<I2, Proj2>>
    bool lexicographical_compare(I1 first1, S1 last1, I2 first2, S2 last2,
                                 Comp comp = Comp{},
                                 Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{})
    {
        for ( ; (first1 != last1) && (first2 != last2); (void) ++first1, (void) ++first2 ) {
            if (ranges::invoke(comp, ranges::invoke(proj1, *first1),
                                     ranges::invoke(proj2, *first2))) return true;
            if (ranges::invoke(comp, ranges::invoke(proj2, *first2),
                                     ranges::invoke(proj1, *first1))) return false;
        }
        return (first1 == last1) && (first2 != last2);
    }

### Example

| This section is incomplete Reason: no example

### See also

lexicographical_compare | returns true if one range is lexicographically less than another (function template)
equal | determines if two sets of elements are the same (function template)

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