What STL algorithm can determine if exactly one item in a container satisfies a predicate?
I need an STL algorithm that takes a predicate and a collection and returns true
if one and only one member of the collection satisfies the predicate, otherwise returns false
.
How would I do this using STL algorithms?
E.g., to replace the following with STL algorithm code to express the same return value.
int count = 0;
for( auto itr = c.begin(); itr != c.end(); ++itr ) {
if ( predicate( *itr ) ) {
if ( ++count > 1 ) {
break;
}
}
}
return 1 == count;
Solution 1:
Two things come to my mind:
std::count_if
and then compare the result to 1
.
To avoid traversing the whole container in case eg the first two elements already match the predicate I would use two calls looking for matching elements. Something along the line of
auto it = std::find_if(begin,end,predicate);
if (it == end) return false;
++it;
return std::none_of(it,end,predicate);
Or if you prefer it more compact:
auto it = std::find_if(begin,end,predicate);
return (it != end) && std::none_of(std::next(it),end,predicate);
Credits goes to Remy Lebeau for compacting, Deduplicator for debracketing and Blastfurnance for realizing that we can also use none_of
the std algorithms.
Solution 2:
You can use std::count_if
† to count and return if it is one.
For example:
#include <iostream>
#include <algorithm> // std::count_if
#include <vector> // std::vector
#include <ios> // std::boolalpha
template<class Iterator, class UnaryPredicate>
constexpr bool is_count_one(Iterator begin, const Iterator end, UnaryPredicate pred)
{
return std::count_if(begin, end, pred) == 1;
}
int main()
{
std::vector<int> vec{ 2, 4, 3 };
// true: if only one Odd element present in the container
std::cout << std::boolalpha
<< is_count_one(vec.cbegin(), vec.cend(),
[](const int ele) constexpr noexcept -> bool { return ele & 1; });
return 0;
}
†Update: However, std::count_if
counts entire element in the container, which is not good as the algorithm given in the question. The best approach using the standard algorithm collections has been mentioned in @formerlyknownas_463035818 's answer.
That being said, OP's approach is also good as the above mentioned best standard approach, where a short-circuiting happens when count
reaches 2
. If someone is interested in a non-standard algorithm template function for OP's approach, here is it.
#include <iostream>
#include <vector> // std::vector
#include <ios> // std::boolalpha
#include <iterator> // std::iterator_traits
template<class Iterator, class UnaryPredicate>
bool is_count_one(Iterator begin, const Iterator end, UnaryPredicate pred)
{
typename std::iterator_traits<Iterator>::difference_type count{ 0 };
for (; begin != end; ++begin) {
if (pred(*begin) && ++count > 1) return false;
}
return count == 1;
}
int main()
{
std::vector<int> vec{ 2, 3, 4, 2 };
// true: if only one Odd element present in the container
std::cout << std::boolalpha
<< is_count_one(vec.cbegin(), vec.cend(),
[](const int ele) constexpr noexcept -> bool { return ele & 1; });
return 0;
}
Now that can be generalized, by providing one more parameter, the number of N
element(s) has/ have to be found in the container.
template<typename Iterator>
using diff_type = typename std::iterator_traits<Iterator>::difference_type;
template<class Iterator, class UnaryPredicate>
bool has_exactly_n(Iterator begin, const Iterator end, UnaryPredicate pred, diff_type<Iterator> N = 1)
{
diff_type<Iterator> count{ 0 };
for (; begin != end; ++begin) {
if (pred(*begin) && ++count > N) return false;
}
return count == N;
}