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lexicographical_compare
function template
<algorithm>
template <class InputIterator1, class InputIterator2>
bool lexicographical_compare ( InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2 );
template <class InputIterator1, class InputIterator2, class Compare>
bool lexicographical_compare ( InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
Compare comp );
Lexicographical less-than comparison
Returns true if range [first1,last1) compares lexicographically less than the range [first2,last2).
Lexicographical comparison is used for example to sort words alphabetically in dictionaries (or in the list of algorithms right here on the left); It involves comparing sequentially the elements that have the same position in both ranges against each other until one element is not equivalent to the other. The result of comparing these first non-matching elements is the result of the lexicographical comparison.
The behavior of this function template is equivalent to:
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template <class InputIterator1, class InputIterator2>
bool lexicographical_compare ( InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2 )
{
while (first1!=last1)
{
if (first2==last2 || *first2<*first1) return false;
else if (*first1<*first2) return true;
first1++; first2++;
}
return (first2!=last2);
}
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Lexicographical comparisons of equivalence with a strict weak ordering criterion between two ranges can be achieved by negating two reflexive less-than comparisons: (a==b) implies (!a<b && !a>b).
Parameters
- first1, last1
- Input iterators to the initial and final positions of the first sequence. The range used is [first1,last1), which contains all the elements between first1 and last1, including the element pointed by first1 but not the element pointed by last1.
- first2, last2
- Input iterators to the initial and final positions of the second sequence. The range used is [first2,last2).
- comp
- Comparison function object that, taking two values of the same type than those contained in the range, returns true if the first argument is to be considered less than the second argument.
Return value
true if the first range compares lexicographically less than than the second.
false if either both ranges are entirely equivalent or if is the second that compares less than the first.
Example
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// lexicographical_compare example
#include <iostream>
#include <algorithm>
#include <cctype>
using namespace std;
// a case-insensitive comparison function:
bool mycomp (char c1, char c2)
{ return tolower(c1)<tolower(c2); }
int main () {
char first[]="Apple"; // 5 letters
char second[]="apartment"; // 9 letters
cout << "Using default comparison (operator<): ";
if (lexicographical_compare(first,first+5,second,second+9))
cout << first << " is less than " << second << endl;
else
if (lexicographical_compare(second,second+9,first,first+5))
cout << first << " is greater than " << second << endl;
else
cout << first << " and " << second << " are equivalent\n";
cout << "Using mycomp as comparison object: ";
if (lexicographical_compare(first,first+5,second,second+9,mycomp))
cout << first << " is less than " << second << endl;
else
if (lexicographical_compare(second,second+9,first,first+5,mycomp))
cout << first << " is greater than " << second << endl;
else
cout << first << " and " << second << " are equivalent\n";
return 0;
}
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The default comparison compares plain ASCII character codes, where 'A' (65) compares less than 'a' (97).
Our mycomp function transforms the letters to lowercase before comparing them, so here the first letter not matching is the third ('a' vs 'p').
Output:
Using default comparison (operator<): Apple is less than apartment
Using mycomp as comparison object: Apple is greater than apartment
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Complexity
At most, performs 2*min(count1,count2) comparisons or applications of comp (where countX is the distance between firstX and lastX).
See also
| mismatch | Return first position where two ranges differ (function template) |
| equal | Test whether the elements in two ranges are equal (function template) |
| search | Find subsequence in range (function template) |
| find_end | Find last subsequence in range (function template) |
| includes | Test whether sorted range includes another sorted range (function template) |
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