您可以编写自己的迭代器,例如:
#include <iterator>
template<typename Container>
class column_iterator : public std::iterator<std::random_access_iterator_tag,
typename std::decay<decltype(std::declval<Container>()[0][0])>::type>
{
typedef typename Container::iterator iterator;
typedef typename std::decay<decltype(std::declval<Container>()[0][0])>::type type;
public:
column_iterator(iterator it, int n) : it(it), n(n) {}
column_iterator& operator++() {++it; return *this;}
column_iterator& operator++(int) { auto res(*this); ++*this; return res;}
column_iterator& operator +=(std::ptrdiff_t offset) { it += offset; return *this;}
column_iterator operator +(std::ptrdiff_t offset) const { auto res(*this); res += offset; return res;}
column_iterator& operator--() {--it; return *this;}
column_iterator& operator--(int) { auto res(*this); --*this; return res;}
column_iterator& operator -=(std::ptrdiff_t offset) { it -= offset; return *this;}
column_iterator operator -(std::ptrdiff_t offset) const { auto res(*this); res -= offset; return res;}
type& operator*() { return (*it)[n];}
type* operator->() { return &(it)[n];}
bool operator == (const column_iterator& rhs) const { return it == rhs.it && n == rhs.n; }
bool operator != (const column_iterator& rhs) const { return !(*this == rhs); }
bool operator < (const column_iterator& rhs) const { return it < rhs.it; }
std::ptrdiff_t operator -(const column_iterator& rhs) const { return it - rhs.it; }
private:
iterator it;
int n;
};
template<typename Container>
column_iterator<Container> begin(Container& cont, int n)
{
return column_iterator<Container>(cont.begin(), n);
}
template<typename Container>
column_iterator<Container> end(Container& cont, int n)
{
return column_iterator<Container>(cont.end(), n);
}
现在,我们来测试一下:
#include <algorithm>
#include <array>
#include <iostream>
#include <vector>
#include <cassert>
void display(const std::vector<std::array<int, 5>>& v)
{
for (auto rows : v) {
for (auto elem : rows) {
std::cout << elem << " ";
}
std::cout << std::endl;
}
}
int main() {
std::vector<std::array<int, 5>> input {
{{ 13, 27, 4 , 1 , 11 }},
{{ 11, 19, 2 , 37, 1 }},
{{ 32, 64, 11, 22, 41 }},
{{ 71, 13, 27, -8, -2 }},
{{ 0 , -9, 11, 99, 13 }} };
for (int i = 0; i != 5; ++i) {
std::sort(begin(input, i), end(input, i));
}
display(input);
const std::vector<std::array<int, 5>> output {
{{ 0 , -9, 2 , -8, -2 }},
{{ 11, 13, 4 , 1 , 1 }},
{{ 13, 19, 11, 22, 11 }},
{{ 32, 27, 11, 37, 13 }},
{{ 71, 64, 27, 99, 41 }} };
assert(input == output);
return 0;
}