C ++ max_element每n个元素 [英] c++ max_element every n element
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问题描述
有没有一种方法可以在容器中查找每N个元素进行比较的max元素并返回索引.使用STL,BOOST还是其他lib?
Is there a way to find the max element in a container comparing every N elements and return the index. Using STL, BOOST, or... another lib?
每N个,我的意思是使用std :: max_element,但是将for的增加量从++ first更改为first + = n;
with every N, I mean use the std::max_element, but change the increase in the for, from ++first, to first += n;
// based on std::max_element
#ifndef NEWTON_ALGORITHM_SEARCH_MAX_INDEX_HPP
#define NEWTON_ALGORITHM_SEARCH_MAX_INDEX_HPP
#include <iterator>
#include <newton/functional.hpp>
namespace newton {
// SAME THAT STD::MAX_ELEMENT
template<class ForwardIt, class Compare>
const ForwardIt max_index(ForwardIt first, ForwardIt last, Compare comp)
{
if ( newton::equal(first, last) ) // newton::equal is basically equivalent to std::equal_to
return last;
ForwardIt largest = first;
while ( newton::not_equal(++first, last) )
if (comp(*largest, *first))
largest = first;
return largest;
}
// possible names
// max_index_some
// max_index_every_n
// max_index__n
template<class ForwardIt, class Size, class Compare>
const ForwardIt max_index_every_n(ForwardIt first, ForwardIt last, Size n, Compare comp)
{
if ( newton::equal(first, last) )
return last;
ForwardIt largest = first;
Size blocks = std::distance(first, last) / n; // integer blocks
if ( newton::greater_equal(blocks, 1) ) {
// if there are exacly N elements, can't sum example
// v.size() = 10, first start in 0, so "0 += 10" is "10", but last index is "9"
// but if mod >= 1, then index is at least 10, so can sum
if ( newton::greater_equal( std::distance(first, last) % n, 1) ) {
for (size_t i = 1; newton::less_equal(i, blocks); ++i, first += n) {
if (comp(*largest, *first))
largest = first;
}
}
else {
for (size_t i = 1; newton::less(i, blocks); ++i, first += n) {
if (comp(*largest, *first))
largest = first;
}
}
}
return largest;
}
template<class ForwardIt>
const ForwardIt max_index(ForwardIt first, ForwardIt last)
{
return max_index(first, last, newton::structure::less());
}
} // newton
如果没有,您尝试将其包含在下一个STL版本中的解决方案是什么.记住
if there is not, whats your solution to try to include it in a next STL version. remember
推荐答案
仅使用Boost(可以说是"Range V2"):
Using just Boost ("Range V2" so to speak):
#include <boost/range/adaptors.hpp>
#include <boost/range/algorithm.hpp>
#include <boost/range/algorithm_ext.hpp>
#include <iostream>
#include <vector>
int main() {
std::vector<int> v(100);
boost::iota(v, 0);
int n = 17;
auto it = boost::max_element(v | boost::adaptors::strided(n));
std::cout << "max: " << *it << "\n";
}
打印
max: 85
替代用法
您不必全部使用范围算法.您还可以根据需要选择:
Alternative Usage
You don't have to use range algorithms and all. You can also pick as you require:
#include <boost/range/adaptor/strided.hpp>
#include <iostream>
#include <vector>
int main() {
std::vector<int> v { 1,2,3,4,5,6,7,8,9,10 };
auto v_ = boost::adaptors::stride(v, 7);
auto it = std::max_element(v_.begin(), v_.end());
std::cout << "max: " << *it << "\n";
}
打印
max: 8
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