Question

I have a vector containing something like

{1,  2, 1,  4, 5, 1,  7, 8 ,9, 1 }

So the vector represent the columns of an upper right matrix

  1 2 4 7
  0 1 5 8
  0 0 1 9
  0 0 0 1 

How could I use the vector and get from

{1,  2, 1,  4, 5, 1,  7, 8 ,9, 1 }

to

  1 2 4 7
  2 1 5 8
  4 5 1 9
  7 8 9 1 

as a vector

{ 1,2,4,7,
  2,1,5,8, 
  4,5,1,9,
  7,8,9,1 }

In fact After reviewing the code, the solution is:

for (int i = 0; i < cols; ++i)
    for (int j = 0; j <= i; ++j)
        v[cols * i + j] = v[cols * j + i] = w[k++];

Answer 1

int k=0;
for(int i = 0; i<4; ++i)
    for(int j = 0; j<=i; ++j)
        v[i][j]=v[j][i]=w[k++];

Assuming source vector is w, dest is v.

If it should be 1D, not 2D, then [i][j] should be changed to [4*i+j].

EDIT
This can be done ‘in-place’, but it is a little tricky. In order not to overwrite values you haven’t used yet, you have to loop backwards and to make two passes:

int k=10;
for(int i = 3; i >=0; --i)
    for(int j = i; j >= 0; --j)
        v[4*i+j]=v[--k];
for(int i = 0; i < 4; ++i)
    for(int j = 0; j < i; ++j)
        v[4*j+i]=v[4*i+j];