Defined in header <algorithm> | ||
---|---|---|
template< class ForwardIt, class T > std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value ); | (1) | |
template< class ForwardIt, class T, class Compare > std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp ); | (2) |
Returns a range containing all elements equivalent to value
in the range [first, last)
.
The range [first, last)
must be partitioned with respect to comparison with value
, i.e. it must satisfy all of the following requirements:
- partitioned with respect to
element < value
orcomp(element, value)
- partitioned with respect to
!(value < element)
or!comp(value, element)
- for all elements, if
element < value
orcomp(element, value)
istrue
then!(value < element)
or!comp(value, element)
is alsotrue
A fully-sorted range meets these criteria, as does a range resulting from a call to std::partition
.
The returned range is defined by two iterators, one pointing to the first element that is not less than value
and another pointing to the first element greater than value
. The first iterator may be alternatively obtained with std::lower_bound()
, the second - with std::upper_bound()
.
The first version uses operator<
to compare the elements, the second version uses the given comparison function comp
.
Parameters
first, last | - | the range of elements to examine | |||
value | - | value to compare the elements to | |||
comp | - | comparison function which returns true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following:
The signature does not need to have | |||
Type requirements | |||||
- ForwardIt must meet the requirements of ForwardIterator . |
Return value
std::pair
containing a pair of iterators defining the wanted range, the first pointing to the first element that is not less than value
and the second pointing to the first element greater than value
.
If there are no elements not less than value
, last
is returned as the first element. Similarly if there are no elements greater than value
, last
is returned as the second element.
Complexity
The number of comparisons performed is logarithmic in the distance between first
and last
(At most 2 * log
2(last - first) + O(1) comparisons). However, for non-RandomAccessIterator
s, the number of iterator increments is linear.
Possible implementation
First version |
---|
template<class ForwardIt, class T> std::pair<ForwardIt,ForwardIt> equal_range(ForwardIt first, ForwardIt last, const T& value) { return std::make_pair(std::lower_bound(first, last, value), std::upper_bound(first, last, value)); } |
Second version |
template<class ForwardIt, class T, class Compare> std::pair<ForwardIt,ForwardIt> equal_range(ForwardIt first, ForwardIt last, const T& value, Compare comp); { return std::make_pair(std::lower_bound(first, last, value, comp), std::upper_bound(first, last, value, comp)); } |
Example
#include <algorithm> #include <vector> #include <iostream> struct S { int number; char name; S ( int number, char name ) : number ( number ), name ( name ) {} // only the number is relevant with this comparison bool operator< ( const S& s ) const { return number < s.number; } }; int main() { // note: not ordered, only partitioned w.r.t. S defined below std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4,'G'}, {3,'F'} }; S value ( 2, '?' ); auto p = std::equal_range(vec.begin(),vec.end(),value); for ( auto i = p.first; i != p.second; ++i ) std::cout << i->name << ' '; }
Output:
B C D
Example With Comparator
#include <algorithm> #include <vector> #include <iostream> struct S { int number; char name; S ( int number, char name ) : number ( number ), name ( name ) {} // only the number is relevant with this comparison bool operator< ( const S& s ) const { return number < s.number; } }; struct Comp { bool operator() ( const S& s, int i ) { return s.number < i; } bool operator() ( int i, const S& s ) { return i < s.number; } }; int main() { // note: not ordered, only partitioned w.r.t. S defined below std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4,'G'}, {3,'F'} }; auto p = std::equal_range(vec.begin(),vec.end(),2,Comp()); for ( auto i = p.first; i != p.second; ++i ) std::cout << i->name << ' '; }
Output:
B C D
See also
returns an iterator to the first element not less than the given value (function template) | |
returns an iterator to the first element greater than a certain value (function template) | |
determines if an element exists in a certain range (function template) | |
divides a range of elements into two groups (function template) |
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