# st: Re: Re: replacing a scalar from mat

 From "R.E. De Hoyos" <[email protected]> To <[email protected]> Subject st: Re: Re: replacing a scalar from mat Date Wed, 13 Jul 2005 02:29:42 +0300

Michael,

Your suggestion works fine. However I still face the row and column name problem when I try to replace a the scalar (or 1x1 matrix) into another vector. I should've explained myself better. The problem is the following. Consider:

reg z1 x1 x2
mat A=e(b)

reg z2 x1 x2
mat B=e(b)

I want to know how much of the difference between z1 and z2 is explained by the difference in parameter x2 (a similar exercise like the Oaxaca decomposition). Therefore I get the parameter of x2 from mat A:

mat X2 = A["y1","x2"]

The second step is to replace this scalar (or 1x1 matrix) into mat B. To do so, I have to let Stata know the name of the variable--column--in mat B that has to be replaced with mat X2. My first though would be:

mat B["y1","x2"] = X2["y1","x2"]

However this syntax is not valid. The second thing I tried--following your advice--was to create a scalar from X2:

scalar x2 = X2[1,1]

Again I tried to replace mat B with scalar x2:
mat B["y1","x2"] = x2

And it didn't work either. To replace an element of a matrix you have to use numbers of rows and columns a not the names of them (mat B[1,2] = x2 works fine). Perhaps there is a strong reason for that but I cannot see it. In any case the problem was solved by assigning a number to each element of the independent variables:

local indv x1 x2
local i=1
foreach k in `indv' {
local `k' = `i'
local i=`i'+1
}
mat X`x1' = A[1,`x1']
mat B[1,`x1'] = X`x1'

I still wonder why Stata does not accept rows and columns names?

Best regards,

Rafa

----- Original Message ----- From: "Michael Blasnik" <[email protected]>
To: <[email protected]>
Sent: Tuesday, July 12, 2005 9:33 PM
Subject: st: Re: replacing a scalar from mat

If you know that you are only extract one row and column, why not
mat B = e(b)
mat B1 = B["y1","eq1:x1"]
scalar s=B1[1,1]

Michael Blasnik
[email protected]

----- Original Message ----- From: "R.E. De Hoyos" <[email protected]>
To: <[email protected]>
Sent: Tuesday, July 12, 2005 12:48 PM
Subject: st: replacing a scalar from mat

I am trying to get a scalar from a matrix (vector). Within a multiple equations framework (-mlogit-) I want to get one of the parameters estimated. After running -mlogit- I tried the following:

mat B = e(b)
scalar s = B["y1","eq1:x1"]

The following error appears:

matrix operators that return matrices not allowed in this context
r(509);

However if I try one of the following, there is no error reported:

mat A = B["y1","eq1:x1"]
scalar s = B[1,1]

Using the rows and columns names of the matrix is important since these scalars will be replaced into a second vector of estimated parameters.

Thanks,

Rafa
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