If I have the following matrix for example: enter image description here

The values in the tables are an index of the elements.

for (int count =0; index<9; count  ) {
     //row = function of index
     //column = function of index

}

In other words, how can I get the row and column from the index of an upper triangle of a matrix if it was ordered as in the photo.

I've been thinking about this for a while but I can't seem to figure it out!

CodePudding user response：

int i, j, v[10][10], k=0, n;
cin>>n;
for(i=0; i<n; i  )
{
     for(j=0; j<n; j  )
     if(i<j)
     {
           v[i][j]=k;
           k  ;
      }
}
for(i=0; i<n; i  )
{
     for(j=0; j<n; j  )
          cout<<v[i][j]<<" ";
     cout<<endl;
}

I think this is what you want. The result should be as in your image. Tell me if you want something else. The n you write from the keyboard and it's the number of rows and columns from your matrix.

CodePudding user response：

int matrix[n][n];
vector<pair<int, int>> indexMap;
for(int i=0;i<n;i  )
  for(int j=i;j<n;j  )
    indexMap.push_back(make_pair(i, j));
for(int i=0;i<indexMap.size();i  )
    cout << matrix[indexMap[i].first][indexMap[i].second] << " ";

I hope this is what you are expected. You can access each element by its indexes of indexMap vector ( If n=4, indexes are from 0-9 as you mentioned in your example figure ). indexMap vector size depends on the n. Here n is the size of the matrix.

And also you can get the row and column from the index of an upper triangle of the matrix.

CodePudding user response：

Thank you guys for the answers!

Here's a link algorithm for index numbers of triangular matrix coefficients which gives the answer algebraically. credit to https://stackoverflow.com/users/4958/shreevatsar

Along with the comment from https://stackoverflow.com/users/2019794/michael-bauer