提升 zip_iterator 和 std::sort

问题描述

我有两个长度相同的数组 values 和 keys.我想使用 keys 数组作为键对 values 数组进行排序.有人告诉我，boost 的 zip 迭代器是将两个数组锁定在一起并同时处理它们的正确工具.

I have two arrays values and keys both of the same length. I want to sort-by-key the values array using the keys array as keys. I have been told the boost's zip iterator is just the right tool for locking two arrays together and doing stuff to them at the same time.

这是我尝试使用 boost::zip_iterator 解决无法使用 gcc 编译的排序问题.有人可以帮我修复此代码吗?

Here is my attempt at using the boost::zip_iterator to solve sorting problem which fails to compile with gcc. Can someone help me fix this code?

问题出在线路上

std::sort (boost::make_zip_iterator(keys, values), boost::make_zip_iterator(keys+N, values+N));

#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <vector>
#include <algorithm>
#include <boost/iterator/zip_iterator.hpp>
#include <boost/tuple/tuple.hpp>
#include <boost/tuple/tuple_comparison.hpp>



int main(int argc, char *argv[])
{
  int N=10;
  int    keys[N];
  double values[N];
  int M=100;

  //Create the vectors.
  for (int i = 0; i < N; ++i)
   {
     keys[i]   = rand()%M;
     values[i] = 1.0*rand()/RAND_MAX;
   }


  //Now we use the boost zip iterator to zip the two vectors and sort them "simulatneously"
  //I want to sort-by-key the keys and values arrays
   std::sort ( boost::make_zip_iterator( keys, values  ), 
               boost::make_zip_iterator( keys+N  , values+N    )
             );
    //The values array and the corresponding keys in ascending order. 
   for (int i = 0; i < N; ++i)
    {
      std::cout << keys[i]   <<  "	"  << values[i]    << std::endl;  
     }
  return 0;
}

注意:编译错误信息

g++ -g -Wall boost_test.cpp 
boost_test.cpp: In function ‘int main(int, char**)’:
boost_test.cpp:37:56: error: no matching function for call to ‘make_zip_iterator(int [(((unsigned int)(((int)N) + -0x00000000000000001)) + 1)], double [(((unsigned int)(((int)N) + -0x00000000000000001)) + 1)])’
boost_test.cpp:38:64: error: no matching function for call to ‘make_zip_iterator(int*, double*)’

推荐答案

你不能对一对 zip_iterator 进行排序.

You can't sort a pair of zip_iterators.

首先，make_zip_iterator 将迭代器元组作为输入，因此您可以调用:

Firstly, make_zip_iterator takes a tuple of iterators as input, so you could call:

boost::make_zip_iterator(boost::make_tuple( ... ))

但这也不会编译，因为 keys 和 keys+N 没有相同的类型.我们需要强制 keys 成为一个指针:

but that won't compile either, because keys and keys+N doesn't have the same type. We need to force keys to become a pointer:

std::sort(boost::make_zip_iterator(boost::make_tuple(+keys, +values)),
          boost::make_zip_iterator(boost::make_tuple(keys+N, values+N)));

这会编译，但排序结果仍然是错误的，因为 zip_iterator 只对 可读迭代器，但 std::sort 也需要输入为 Writable 作为 此处描述，因此您无法使用 zip_iterator 进行排序.

this will compile, but the sorted result is still wrong, because a zip_iterator only models a Readable iterator, but std::sort also needs the input to be Writable as described here, so you can't sort using zip_iterator.

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