Question

I have a single line/multi column matrix, multiplied by a square matrix. SAS gives me a single line result. So far, from what I remember from college math, and asking people, okay.

But the line from SAS that does it is this one:

fieldA = matrix1`*matrix2[,1:fieldB]

while I understand “fieldA = matrix1`*matrix2”, I have no idea what [,1:fieldB] does, and I can’t seem to find any document that explains it.

Answer 1

matrix2[,1:fieldB] is subsetting matrix2. It includes all rows of matrix2, and only those columns that are included in the list 1:fieldB. Presumably fieldB identifies the number of columns in matrix1 (which become rows when transposed) so the * works [matrix1' rows must equal matrix2 columns for the operation to be legal].

For example, imagine matrix1 is the following matrix:

[1,3,5]

and matrix2 is the following matrix:

[1,2,3,4,
 4,5,6,7,
 8,9,10,11,
 13,14,15,16]

Now,

matrix1`*matrix2

is illegal – matrix1` has 3 rows (transposed) and matrix2 has 4 columns. So:

matrix1`*matrix2[,1:3]

will now be legal, yielding the matrix product of matrix1` by the first 3 columns of matrix2.