
Category: algorithms  Component type: function 
template <class InputIterator1, class InputIterator2> int lexicographical_compare_3way(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2);
As with lexicographical_compare, lexicographical comparison means "dictionary" (elementbyelement) ordering. That is, lexicographical_compare_3way returns a negative number if *first1 is less than *first2, and a positive number if *first1 is greater than *first2. If the two first elements are equivalent [2] then lexicographical_compare_3way compares the two second elements, and so on. Lexicographical_compare_3way returns 0 only if the two ranges [first1, last1) and [first2, last2) have the same length and if every element in the first range is equivalent to its corresponding element in the second.
int main() { int A1[] = {3, 1, 4, 2, 8, 5, 7}; int A2[] = {3, 1, 4, 1, 5, 9, 3}; int A3[] = {1, 2, 3, 4}; int A4[] = {1, 2, 3, 4, 5}; const int N1 = sizeof(A1) / sizeof(int); const int N2 = sizeof(A2) / sizeof(int); const int N3 = sizeof(A3) / sizeof(int); const int N4 = sizeof(A4) / sizeof(int); int C12 = lexicographical_compare_3way(A1, A1 + N1, A2, A2 + N2); int C34 = lexicographical_compare_3way(A3, A3 + N3, A4, A4 + N4); cout << "A1[] and A2[]: " << C12 << endl; cout << "A3[] and A4[]: " << C34 << endl; }
[1] Lexicographical_compare_3way is almost, but not quite, redundant: the call lexicographical_compare_3way(f1,l1, f2,l2) could be written as lexicographical_compare(f1,l1, f2,l2) ? 1 : (lexicographical_compare(f2,l2, f1,l1) ? 1 : 0). The single call to lexicographical_compare_3way, however, is much faster than the two calls to lexicographical_compare.
[2] "Equivalent", not "equal", because two equivalent elements (that is, two elements with the property that neither one is less than the other) are not necessarily equal. Operator< is required to induce a strict weak ordering, not necessarily a total ordering. See the LessThan Comparable requirements for a discussion.
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