To Get Diagonals of a Matrix in Matlab?

Question

I want to get diagonals from a matrix in Matlab. For example, given the following matrix

M = [1  1   4   5   4
     2  5   1   2   2
     4  1   2   1   3
     1  3   1   1   1
     1  2   3   3   1]

I want to get a list of vectors which make the upper diagonals: {[1 1 1 1], [4 2 3], [5 2] [4]}.

One attempt

I want to replace if-clauses by a while-loop in Matlab

     if (i==j)
        A(t+1,i) = M(i,j)
     end
     if (i==j+1) 
        A(t+1,i) = M(i,j)
     end
     ...
     if (i==j+N) 
        A(t+1,i) = M(i,j)
     end

To store all diagonals of the matrix

t = 0;
while (t < 5)   % if-clauses replaced here by this
   for i=1:5
      for j=i:5
          if (i == j+t)
             trend(t+1,i) = M(i,j); 
          end
      end
   end
t = t + 1;
end

The idea is to store elements into the matrix trend with the condition i == j+t that is a condition I find holds for the diagonals of the upper triangle. The if-clauses are replaced by false pseudocode, which tries to go through the upper triangle of the matrix, but false because no "1" vector found in trend

---

How can you get diagonals from a matrix in Matlab?

Answer 1

If I understood correctly, you want to use the spdiags function to extract the upper off-diagonals. Example:

N = 5;
M = magic(N)
D = spdiags(M, 1:N-1)
DD = arrayfun(@(i) D(i+1:end,i).', 1:N-1, 'Uniform',false)
celldisp(DD)

The result:

M =
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9

D =
     0     0     0     0
    24     0     0     0
     7     1     0     0
    20    14     8     0
     3    22    16    15

DD = 
    [1x4 double]    [1x3 double]    [1x2 double]    [15]
DD{1} =
    24     7    20     3
DD{2} =
     1    14    22
DD{3} =
     8    16
DD{4} =
    15