How do i...calculate the dot product of an array of structure

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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.

C++

<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;
}

What I have tried:

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

Posted 19-Sep-17 16:30pm

Mwater07

Updated 19-Sep-17 22:20pm

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Comments

Nelek 20-Sep-17 1:01am

I am not understanding what you try to do. What I think you are saying doesn't match what I see in your code.

It would be nice if you edit the question and give a bit more explanation. And a "what it is supposed to be" example of the matrix / vectors in each step.

CPallini 20-Sep-17 4:12am

Sorry, what's the logic of discarding zero values in favour of garbage?

2 solutions

Solution 2

You are trying to complicate a simple matter. Try

C++

#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;
}

Posted 19-Sep-17 22:20pm

CPallini

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Jochen Arndt · Accepted Answer · 2017-09-19T21:43:00

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];
    }
}

The next is here:

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