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## Create a block diagonal matrix from a to-be-partitioned matrix

I have a matrix A={1 1,1 2,2 1,2 2,2 3,3 1}. I would like to create a block diagonal matrix (B) such that B={1 1 0 0 0 0, 1 2 0 0 0 0, 0 0 2 1 0 0, 0 0 2 2 0 0, 0 0 2 3 0 0, 0 0 0 0 3 1}. I know of the block function but it does not work here (in its basic form at least). How can I do this in IML? Is it possible without looping because the number of rows in A can get very large (millions or more)?

George

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## Re: Create a block diagonal matrix from a to-be-partitioned matrix

You don't need to loop over rows of A; you can loop over the unique blocks defined by A.  Use the UNIQUE function on the first column of A to find out how many blocks there are. Then for each block, use the BLOCK function to iteratively build up the matrix.  For you example, the loop would have three iterations and be equivalent to the following statements:

B = A[1:2,];  /* initialize outside loop */

/* do i = 2 to 3; */

B = block(B, A[3:5,]);

B = block(B, A[6,]);

/* end do */

print B;

2 REPLIES 2
SAS Super FREQ

## Re: Create a block diagonal matrix from a to-be-partitioned matrix

You don't need to loop over rows of A; you can loop over the unique blocks defined by A.  Use the UNIQUE function on the first column of A to find out how many blocks there are. Then for each block, use the BLOCK function to iteratively build up the matrix.  For you example, the loop would have three iterations and be equivalent to the following statements:

B = A[1:2,];  /* initialize outside loop */

/* do i = 2 to 3; */

B = block(B, A[3:5,]);

B = block(B, A[6,]);

/* end do */

print B;

Calcite | Level 5

Thanks Rick

From The DO Loop