I want to create a Toeplitz matrix which I need to invert. Do anyone know a good way to create one? The way I have take way too many steps.

Int_t n = 3; // n x n matrix
Double_t a[] = {1,2,3};
TVectorD aa;
aa.Use(n,a);
TMatrixD A(n,n);
TMatrixDRow(A,0) = aa;
//here is a for to reallocate the elements and fill the matrix
for(Int_t i = 1; i<n; i++){ // this for to fill the matrix
Double_t temp = aa[0];
aa[0] = aa[n-1];
for(Int_t j = 1; j<n;j++){
Double_t temp2 = aa[j];
aa[j] = temp;
temp = temp2;
}
TMatrixDRow(A,i) = aa;
}
for(Int_t i = 0; i<n; i++){
for(Int_t j = 0; j<n; j++){
cout << A[i][j] << " ";
}
cout << "\n";
}

Sorry, I didn’t follow you. I am using the definition of the matrix there. It is just that root does not accept negative values, so I though it would be easy this way.

Not sure what you expect because what you wrote is already quite compact.
The swapping could be done in one line :

void toeplitz()
{
Int_t n = 3; // n x n matrix
Double_t a[] = {1,-2,3};
TVectorD aa;
aa.Use(n,a);
TMatrixD A(n,n);
TMatrixDRow(A,0) = aa;
//here is a for to reallocate the elements and fill the matrix
for(Int_t i = 1; i<n; i++){ // this for to fill the matrix
Double_t temp = aa[0];
aa[0] = aa[n-1];
for(Int_t j = 1; j<n;j++)
std::swap(aa[j],temp);
TMatrixDRow(A,i) = aa;
}
A.Print();
}

or are you looking for something more “elegant” ? It could be done with TMatrixDDiag and TMatrixDSub but it will not be shorter .

Thanks a lot for the reply. I liked the std::swap method there.
I was wondering if there was any function member of TMatrixD that could do it
I’ve stepped into a solution for M x M matrices:

Int_t M = 3; // M x M matrix
Double_t a[] = {1,-2,3};
TMatrixD A(M,M);
for (int i=0; i<M; i++)
{
for (int j=0; j<M; j++)
{
A[i][j] = aa[abs(i-j)];
}
}

Just for the record, your code examples do create Toeplitz matrices but very specific ones, where the last one is symmetric and the first one permutates the off-diagonal values.

Any Toeplitz can be written by the following code (Notice the upper and lower boundary of the TVectorD:

Int_t M = 3; // M x M matrix
TMatrixD A(M,M);
Double_t a[] = {1,2,3,4,5};
TVectorD aa(-M+1,M-1);
aa.Use(-M+1,M-1,a);
for (int i=0; i<M; i++)
{
for (int j=0; j<M; j++)
{
A[i][j] = aa[j-i];
}
}