切矢量

Question

我有一个std :: vector.我想创建表示该向量切片的迭代器.我该怎么做？在伪C++中:

class InterestingType;

void doSomething(slice& s) {
    for (slice::iterator i = s.begin(); i != s.end(); ++i) {
       std::cout << *i << endl;
    }
}
int main() {
   std::vector v();
   for (int i= 0; i < 10; ++i) { v.push_back(i); }
   slice slice1 = slice(v, 1, 5);
   slice slice2 = slice(v, 2, 4);
   doSomething(slice1);
   doSomething(slice2);
   return 0;
}

我宁愿不必将元素复制到新的数据结构.

Answer 1

你只需要使用一对迭代器:

typedef std::vector<int>::iterator vec_iter;

void doSomething(vec_iter first, vec_iter last) {
    for (vec_iter cur = first; cur != last; ++cur) {
       std::cout << *cur << endl;
    }
}

int main() {
   std::vector v();
   for (int i= 0; i < 10; ++i) { v.push_back(i); }

   doSomething(v.begin() + 1, v.begin() + 5);
   doSomething(v.begin() + 2, v.begin() + 4);
   return 0;
}

或者,Boost.Range库应该允许您将迭代器对表示为单个对象,但以上是执行它的规范方法.

Answer 2

在学习C++之前,我学习了Python.我想知道C++是否提供了像Python列表中的切片一样的切片.花了几分钟来编写这个函数,它允许你切割一个类似于它在Python中完成的向量.

vector<int> slice(const vector<int>& v, int start=0, int end=-1) {
    int oldlen = v.size();
    int newlen;

    if (end == -1 or end >= oldlen){
        newlen = oldlen-start;
    } else {
        newlen = end-start;
    }

    vector<int> nv(newlen);

    for (int i=0; i<newlen; i++) {
        nv[i] = v[start+i];
    }
    return nv;
}

用法:

vector<int> newvector = slice(vector_variable, start_index, end_index);

start_index元素将包含在切片中,而end_index将不包括在内.

例:

对于像{1,3,5,7,9}这样的向量v1

切片(v1,2,4)返回{5,7}

Answer 3

取自这里:

std::vector<myvector::value_type>(myvector.begin()+start, myvector.begin()+end).swap(myvector);

用法示例:

#include <iostream>
#include <vector>

int main ()
{
    std::vector<int> indexes{3, 6, 9};

    for( auto index : indexes )
    {
        int slice = 3;
        std::vector<int> bar{1, 2, 3, 4, 5, 6, 7, 8, 9};
        std::vector<int>( bar.begin() + index - slice, bar.begin() + index ).swap(bar);

        std::cout << "bar index " << index << " contains:";
        for (unsigned i=0; i<bar.size(); i++)
            std::cout << ' ' << bar[i];
        std::cout << '\n';
    }

    return 0;
}

输出:

bar index 3 contains: 1 2 3
bar index 6 contains: 4 5 6
bar index 9 contains: 7 8 9