Referring to matrix columns and rows

[Sumedha Gupta]

Dear All,
It should be fairly simply, but I am very confused as to how I should refer to a location in a matrix. I want to store certain column's of the regression output in a new matrix. To do so I start by creating an empty matrix and try to save first column (all rows) of matrix m1 in first column of matrix m:

mat m=J(15,27, .)

svy: regress y x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12 x13 x14
mat m1=r(table)'
mat m[1..,1]=m1[1..,1]

But I get the following error:

 mat m[1..,1]=m1[1..,1]
1.. invalid name
r(198);

1 is supposed to be a location (r x c) and not a name. How does one specify a "location" within a matrix in Stata? Seems like such a basic thing but I have tried many versions and it seems I am missing something fundamental about row-column address spec in Stata matrices.

I will be grateful for your help and also reference to a guide to using matrices in Stata please.
Many thanks.
Sincerely,
Sumedha.

[Nick Cox]

Stata's matrix language is much more restricted than Mata. I am open to correction but I think all you can do here is specify matrix matname[r, c] on the left-hand side where r and c are just constants that define the row and column indexes of a particular element. That still allows submatrices to be replaced.

See

help matrix substitution

[Wouter Wakker]

From the Stata User's Guide 14.9:

7. When defining an element of a matrix, use
matrix matname[i,j] = expression
where i and j are scalar expressions. The matrix matname must already exist.

Example:
matrix A = J(2,2,0)
matrix A[1,2] = sqrt(2)

8. To replace a submatrix within a matrix, use the same syntax. If the expression on the right evaluates
to a scalar or 11 matrix, the element is replaced. If it evaluates to a matrix, the submatrix with
top-left element at (i; j) is replaced. The matrix matname must already exist.

Example:
matrix A = J(4,4,0)
matrix A[2,2] = C'*C

[Sumedha Gupta]

Prof. Cox,
Thank you for the suggestion. I thought I was doing something wrong... Following your suggestion I wrote a loop (a bit ridiculous probably) to make it work...

mat m1=r(table)'
local rows=rowsof(m1)
forvalues t=1(1)`rows'{
mat m[`t',1]=m1[`t',1]
 mat m[`t',2]=m1[`t',5]
 mat m[`t',3]=m1[`t',6]
}

So not elegant...but it does what I need it to do..

Thank you again.
Sincerely,
Sumedha.