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142. lexicographical_compare complexity wrong

Section: 28.7.10 [alg.lex.comparison] Status: TC1 Submitter: Howard Hinnant Opened: 1999-06-20 Last modified: 2017-02-03

Priority: Not Prioritized

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The lexicographical_compare complexity is specified as:

     "At most min((last1 - first1), (last2 - first2)) applications of the corresponding comparison."

The best I can do is twice that expensive.

Nicolai Josuttis comments in lib-6862: You mean, to check for equality you have to check both < and >? Yes, IMO you are right! (and Matt states this complexity in his book)

Proposed resolution:

Change 28.7.10 [alg.lex.comparison] complexity to:

At most 2*min((last1 - first1), (last2 - first2)) applications of the corresponding comparison.

Change the example at the end of paragraph 3 to read:


    for ( ; first1 != last1 && first2 != last2 ; ++first1, ++first2) {
      if (*first1 < *first2) return true;
      if (*first2 < *first1) return false;
    return first1 == last1 && first2 != last2;
--end example]