Now, I'm aware of the threads existing about this question such as:

How to enter matrices in block matrix format?

How to form a block-diagonal Matrix from a list of matrices?

But I wasn't able to find the solution to my problem there. I need to make a matrix $$ \begin{pmatrix} C_1 &I\\ 0 & C_2 \end{pmatrix} $$

My problem is that $C_1$ and $C_2$ are of different sizes! $C_1$ is of size 30 and $C_2$ is of size 48. So one would expect the following code to work:

MatrixForm[ArrayFlatten[{{c1, IdentityMatrix[30], 0}, {0, c2}}]]

or maybe

MatrixForm[ArrayFlatten[{{c1, IdentityMatrix[30], ConstantArray[0,{18,18}]}, 
{ConstantArray[0,{30,30}], c2}}]]

But neither actually works!


I ended up using J.M's answer:

ArrayFlatten[{{c1, PadRight[IdentityMatrix[30], {Automatic, 48}]}, {0, c2}}]]
  • 2
    $\begingroup$ Why not ArrayFlatten[{{c1, IdentityMatrix[30]}, {0, c2}}]? $\endgroup$ Nov 23, 2015 at 13:59
  • $\begingroup$ this doesn't work either :( $\endgroup$
    – Sertii
    Nov 23, 2015 at 14:03
  • 2
    $\begingroup$ Try ArrayFlatten[{{c1, PadRight[IdentityMatrix[30], {Automatic, 48}]}, {0, c2}}]] then. $\endgroup$ Nov 23, 2015 at 14:09
  • $\begingroup$ Awesome! That worked! $\endgroup$
    – Sertii
    Nov 23, 2015 at 14:10
  • $\begingroup$ Yes they are both square. C1 and C2 are companion matrices and i'm trying to make the Jordan rational normal form. I don't think there is a problem, I might be wrong though $\endgroup$
    – Sertii
    Nov 23, 2015 at 14:15

1 Answer 1

m30 = ConstantArray[3, {30, 30}];
i30 = IdentityMatrix[30];
m48 = ConstantArray[4, {48, 48}];
m12 = ConstantArray[0, {48, 12}];

Transpose@Join[m30, i30]~Join~Join[m12, m48, 2]
  • $\begingroup$ I ended up using an anwer provided above but thank you for taking the time anyway! $\endgroup$
    – Sertii
    Nov 23, 2015 at 14:16
  • 1
    $\begingroup$ They are different answers, J.M.'s follows the idea that C1 and C2 have no columns in common - which can be inferred from your sketch above. Eldo's answer maintains the fact that I is an identity matrix. You can't have both: if I is an identity matrix, then the shape described doesn't make sense $\endgroup$
    – Jason B.
    Nov 23, 2015 at 14:25

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