MATLAB: How to create a semi-diagonal matrix

diagdoublemodonestoeplitzzeros

Hi all,
I have been trying to create a semi-diagonal matrix; Now I know this may not be the right terminology for it, however here it is (also, the matrix doesnt have to be square. I want to have control over its m x n dimensions. The example is thus of a semi-diagonal 3 x 6 matrix):
 M_3x6 = [1 1 0 0 0 0
          0 0 1 1 0 0
          0 0 0 0 1 1] ;
Or for a larger matrix:
 M_4x7 = [1 1 0 0 0 0 0
          0 0 1 1 0 0 0
          0 0 0 0 1 1 0
          0 0 0 0 0 0 1] ;
Thanks for your help in advance,
KMT.

Best Answer

Method one: blkdiag:
>> blkdiag([1,1],[1,1],[1,1])
ans =
   1   1   0   0   0   0
   0   0   1   1   0   0
   0   0   0   0   1   1
>>
You can easily supply the inputs as a comma-separated list:
>> C = repmat({[1,1]},1,4);
>> blkdiag(C{:})
ans =
   1   1   0   0   0   0   0   0
   0   0   1   1   0   0   0   0
   0   0   0   0   1   1   0   0
   0   0   0   0   0   0   1   1
>>
Then use indexing to select a part of the matrix.
Method two: repelem: With newer MATLAB versions you could use eye and repelem, and then use indexing as above.
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