How to create a semi-diagonal matrix

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.

Answer 1

[Stephen23]

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.

[Konstantinos Tsitsilonis]

Thanks for your answer, works well!