[英]Using a struct member in STL algorithms
#include <iostream>
#include <vector>
#include <iterator>
using namespace std;
struct Point
{
int x;
int y;
Point(int x, int y) :
x(x),
y(y)
{}
};
int main()
{
vector<Point> points;
points.push_back(Point(1, 2));
points.push_back(Point(4, 6));
vector<int> xs;
for(vector<Point>::iterator it = points.begin();
it != points.end();
++it)
{
xs.push_back(it->x);
}
copy(xs.begin(), xs.end(), ostream_iterator<int>(cout, " "));
cout << endl;
return 0;
}
I'm wondering how I would achieve the same result as the for loop above using an STL algorithm? 我想知道如何使用STL算法获得与上述for循环相同的结果? I've tried a few things using for_each, but wasn't able to get it to work.
我已经尝试过使用for_each进行一些操作,但是无法使其正常工作。
You wouldn't use std::for_each
, but rather std::transform
(you're transforming a point into a single number.) 您不会使用
std::for_each
,而是使用std::for_each
std::transform
(将点转换为单个数字。)
For example: 例如:
#include <algorithm> // transform resides here
#include <iostream>
#include <iterator>
#include <vector>
struct Point
{
int x;
int y;
Point(int x, int y) :
x(x),
y(y)
{
}
};
int point_to_int(const Point& p)
{
return p.x;
}
int main()
{
std::vector<Point> points;
points.push_back(Point(1, 2));
points.push_back(Point(4, 6));
std::vector<int> xs;
std::transform(points.begin(), points.end(),
std::back_inserter(xs), point_to_int);
std::copy(xs.begin(), xs.end(),
std::ostream_iterator<int>(std::cout, " "));
std::cout << std::endl;
return 0;
}
Because you know the size of the container you'll be transforming, you might get a slight performance improvement from the following. 因为您知道将要转换的容器的大小,所以您可能会从以下内容获得稍微的性能改进。 I also find it more readable:
我还发现它更具可读性:
std::vector<int> xs;
xs.reserve(points.size());
std::transform(points.begin(), points.end(),
std::back_inserter(xs), point_to_int);
And with boost::lambda
along with boost::bind
: 并带有
boost::lambda
和boost::bind
:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
#include <boost/bind.hpp>
#include <boost/lambda/lambda.hpp>
struct Point
{
int x;
int y;
Point(int x, int y) :
x(x),
y(y)
{
}
};
int main()
{
using namespace boost;
std::vector<Point> points;
points.push_back(Point(1, 2));
points.push_back(Point(4, 6));
std::vector<int> xs;
xs.reserve(points.size());
std::transform(points.begin(), points.end(),
std::back_inserter(xs), bind(&Point::x, lambda::_1));
std::copy(xs.begin(), xs.end(),
std::ostream_iterator<int>(std::cout, " "));
std::cout << std::endl;
return 0;
}
Removes the need to specify a function elsewhere. 无需在其他地方指定函数。 This keeps the code close to the calling site, and generally improves readability.
这样可以使代码保持靠近调用站点的位置,并通常提高了可读性。
In C++0x, it will simply be: 在C ++ 0x中,它将简单地是:
std::transform(points.begin(), points.end(),
std::back_inserter(xs), [](const Point& p){ return p.x; } );
(To the best of my knowledge, anyway) (据我所知)
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