我如何...计算结构数组的点积 [英] How do i...calculate the dot product of an array of structure

问题描述

我正在尝试编写计算两个给定向量的点积的C ++程序。在向量a和b中，只有非零元素将存储到结构数组（行和列）中。每次我得到不相关的结果。正确的结果应该是50.请建议。

提前谢谢你。

I am trying write C++ programs that compute the dot product of two given vectors. In vectors a and b only nonzero elements will be stored into array of structures(row and col). Each time I am getting irrelevant results. The correct result should be 50. Please advice.
Thank you in advance.

<pre>#include <iostream>
#include <vector>
using namespace std;

const int n=10; /* vector size */
struct element {
int index; /* original index of non-zero array element */
int value ; /* integer non-zero value at index x */
};

element row[n];
element col[n];

vector<int> a={0,0,7,0,5,0,0,8,0,4,-1};
vector<int> b={0,0,0,5,6,0,0,0,0,5,-1};

void generate_row_and_col()
{
    for (int i=0; i<=n; i++)
    {
        if(a[i]!=0)
        {
            row[i].index=i;
            row[i].value=a[i];
        }
    }
    for (int j=0; j<=n; j++)
    {
        if(b[j]!=0)
        {
           col[j].index=j;
           col[j].value=b[j];
        }
    }
}
int dotproduct()
{
/* calculate the dot product of row and col output the result*/
int i=0;
int j=0;
int product=0;
while(row[i].index!=-1 && col[j].index!=-1)
{
    if(row[i].index == col[j].index)
    {
        product=product+row[i].value*col[j].value;
        i++;
        j++;
    }
    else if(row[i].index<col[j].index)
    {
        i++;
    }
    else
    {
        j++;
    }
}
return product;
}

int main()
{
    generate_row_and_col() ;
    int r;
    r=dotproduct();
    cout<<"result="<<r<<endl;
    return 0;
}

我的尝试：



我想我无法正确地将向量读入结构数组。

What I have tried:

I think i am not able to read the vectors properly into array of structures.

推荐答案

代码中有三个错误。



前两个：对于 row 和 col 但是正在访问11个元素（越界访问）并且没有初始化所有元素：

There are three errors in your code.

The first two ones: You have a fixed array size of 10 for row and col but are accessing 11 elements (out of bound access) and did not initialise all elements:

for (int i=0; i<=n; i++)
{
    // Not all row elements are initialised
    if(a[i]!=0)
    {
        // Out of bound access here when i == 10
        row[i].index=i;
        row[i].value=a[i];
    }
}
// And similar for col
for (int j=0; j<=n; j++)
{
    if(b[j]!=0)
    {
        col[j].index=j;
        col[j].value=b[j];
    }
}

下一个是：

The next is here:

while(row[i].index!=-1 && col[j].index!=-1)

在这里检查 index 成员为-1。但情况永远不会是这样，因为索引只是从0运行到n（假设所有元素都已初始化）。你可能想在这里检查 -1 的值成员。



所以你必须：

• 将 n 设为11
  
  
• 将向量中的所有元素复制到 row 和 col （可选地检查数组大小并在-1之后中断已复制）
  
  
• 检查值成员是否为-1而不是索引 member（或者当vector元素为-1时，将上面步骤中的两个成员设置为-1）
  
  

where you check the index member for being -1. But that will never be the case because the index is just running from 0 to n (assuming all elements has been initialised). You might want to check the value member for -1 here.

So you would have to:




• Set n to 11
  
• Copy all elements from the vectors to row and col (optionally check for the array size and break after -1 has been copied)
  
• Check the value member for being -1 instead of the index member (or set both members in the above step to -1 when the vector element is -1)
  




你试图让一个简单的问题复杂化。尝试



You are trying to complicate a simple matter. Try


#include <iostream>
#include <vector>
using namespace std;


// Please note: error handling (e.g. a.size()!=b.size()) left as exercise
int dot(const vector<int> & a, const vector <int> &b)
{
  int dp = 0;
  for (size_t n = 0; n< a.size(); ++n)
  {
    dp += a[n]*b[n];
  }
  return dp;
}


int main()
{
  vector<int> a={0,0,7,0,5,0,0,8,0,4};
  vector<int> b={0,0,0,5,6,0,0,0,0,5};
  int dp = dot(a,b);

  cout << "result = " << dp << endl;
}




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