预增量不能按我预期的那样工作 [英] pre-increment not working as i expect

问题描述

我正在尝试通过在线解决一些问题来学习动态编程.我遇到的一个问题需要处理以下输入

I am trying to learn dynamic programming by solving some questions online. One question that i came across requires the to process the following input

4 10 3 4 4 5 6 7 5 7

4 10 3 4 4 5 6 7 5 7

第一个指向项目数量，第二个指向总容量，其余四个(成对)现在应该指向值和容量.

The first points at number of items, second the total capacity and the rest of the four (in a pair) should now point at the value and capacity.

我遇到的问题是解析它的代码.

The problem i have is with the code which parses it.

#include<iostream>
#include<map>
#include<iterator>
using namespace std;

template<typename T>
void print(typename T::const_iterator start,
           typename T::const_iterator end)
{
    typename T::const_iterator itr=start;
    while(itr!=end)
    {
        std::cout << itr->first << " " << itr->second << std::endl;
        itr++;
    }
}

int main()
{
    int _M_N; // total numbers
    int _M_V; // total values
    std::map<int,int> _M_Values; 

    istream_iterator<int> isi(cin); 
    _M_N = *isi;
    _M_V = *++isi;

    for(int i=0;i<_M_N;i++)
    {
        //int m=*++isi,n=*++isi;
        //_M_Values.insert(make_pair(m,n));
        _M_Values.insert(make_pair(*++isi,*++isi)); 
    }
    print<std::map<int,int> >(_M_Values.begin(),_M_Values.end()); 
}

当我运行此程序时，此输入得到错误的输出

when i run this program i get the wrong output for this input

   akhetarp@desktop> knapsack < test
   4 3 
   5 4 
   7 6

但是，如果我取消注释该行并注释前面的插入行，则会得到预期的结果

if i however uncomment the line and comment the earlier line for insertion, i get the expected

   int m=*++isi,n=*++isi;
   _M_Values.insert(make_pair(m,n));
   // _M_Values.insert(make_pair(*++isi,*++isi));

   3 4
   4 5 
   6 7 
   5 7 

我知道这一定是一些基本错误. 预先感谢，

I know it has to be some basic error. Thanks in advance,

推荐答案

您在此处缺少序列点:

 _M_Values.insert(make_pair(*++isi,*++isi)); 

声明中的逗号为1，函数中的逗号不是:

The comma in a declaration is one, the comma in a function not:

int m=*++isi,n=*++isi;

请查看: http://en.wikipedia.org/wiki/Sequence_point

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