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std::lexicographical_compare

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Defined in header <algorithm>
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template< class InputIt1, class InputIt2 >

bool lexicographical_compare( InputIt1 first1, InputIt1 last1,

                              InputIt2 first2, InputIt2 last2 );
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template< class InputIt1, class InputIt2, class Compare >

bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
                              InputIt2 first2, InputIt2 last2,

                              Compare comp );
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Checks if the first range [first1, last1) is lexicographically less than the second range [first2, last2). The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

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.

Contents

Parameters

first1, last1 - the first range of elements to examine
first2, last2 - the second range of elements to examine
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than the second.

The signature of the comparison function should be equivalent to the following:

 bool cmp(const Type1 &a, const Type2 &b);

The signature does not need to have const &, but the function object 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, InputIt2 must meet the requirements of InputIterator.

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 = std::distance(first1, last1) and N2 = std::distance(first2, last2).

Possible implementation

First version
template<class InputIt1, class InputIt2>
bool lexicographical_compare(InputIt1 first1, InputIt1 last1,
                             InputIt2 first2, InputIt2 last2)
{
    for ( ; (first1 != last1) && (first2 != last2); first1++, first2++ ) {
        if (*first1 < *first2) return true;
        if (*first2 < *first1) return false;
    }
    return (first1 == last1) && (first2 != last2);
}
Second version
template<class InputIt1, class InputIt2, class Compare>
bool lexicographical_compare(InputIt1 first1, InputIt1 last1,
                             InputIt2 first2, InputIt2 last2,
                             Compare comp)
{
    for ( ; (first1 != last1) && (first2 != last2); first1++, first2++ ) {
        if (comp(*first1, *first2)) return true;
        if (comp(*first2, *first1)) return false;
    }
    return (first1 == last1) && (first2 != last2);
}

Example

#include <algorithm>
#include <iostream>
#include <vector>
#include <cstdlib>
#include <ctime>
 
int main()
{
    std::vector<char> v1 {'a', 'b', 'c', 'd'};
    std::vector<char> v2 {'a', 'b', 'c', 'd'};
 
    std::srand(std::time(0));
    while (!std::lexicographical_compare(v1.begin(), v1.end(),
                                         v2.begin(), v2.end())) {
        for (auto c : v1) std::cout << c << ' ';
        std::cout << ">= ";
        for (auto c : v2) std::cout << c << ' ';
        std::cout << '\n';
 
        std::random_shuffle(v1.begin(), v1.end());
        std::random_shuffle(v2.begin(), v2.end());
    }
 
    for (auto c : v1) std::cout << c << ' ';
    std::cout << "< ";
    for (auto c : v2) std::cout << c << ' ';
    std::cout << '\n';
}

Possible output:

a b c d >= a b c d 
d a b c >= c b d a 
b d a c >= a d c b 
a c d b < c d a b