RE: st: matrix question

 From "Nick Cox" <[email protected]> To <[email protected]> Subject RE: st: matrix question Date Fri, 25 Mar 2005 15:38:25 -0000

```This is a bit easier in some other software
than it is in Stata. But is also a bit
easier than this. I agree with the main
idea, of looping over rows to select those
you want, but Paul's example can be condensed
and then extended a bit roughly along these
lines:

forval i=1/`=rowsof(mat1)' {
if mat1[`i',1]==999999 {
mat mat2 = nullmat(mat2) \ mat1[`i',2]
local rownames "`rownames' `i'"
}
}
matrix rownames mat2 = `rownames'

The key details are

1. use -rowsof()- to work out the number of rows
on the fly.

2. use -nullmat()- either to initialise or to
enlarge a matrix

3. use the loop to select the rownames as well,
if of interest.

That said, some canned routines in -matvsort-
on SSC are other possibilities in nearby terrain.

Nick
[email protected]

Paul Millar

> As far as I know, there is no easy way to do this in Stata.
> You will probably need at least 1 loop to do this:
>
> mat mat2=J(260,1,0)
> local rows=0
> forvalues i=1/260 {
>   if mat1[`i',1]==999999 {
>     local nrows=`nrows'+1
>     mat mat2[`nrows',1]=mat1[`i',2]
>     }
>   }
> di "`nrows' rows updated in mat2"

Indranil Majumdar

> > I have a matrix (mat1) with the following data (extract).
> >
> > . matrix list mat1
> >
> > mat1[260,2]
> >        ratsect     cutoff
> >  r1     999999  3.435e-06
> >  r2     999999  4.572e-06
> >  r3     999999  6.596e-06
> >  r4     999999  9.606e-06
> >  r5     999999  .00001548
> > r22     999998  5.470e-06
> > r23     999998   .0000119
> > r24     999998   .0000177
> > r25     999998   .0000295
> > r26     999998   .0000439
> >
> > I want to create another matrix (say mat2) with only those rows
> > that have a
> > particular 'ratsect' e.g. 999999
> >
> > I guess the command will look something like the following.
> > matrix mat2 = mat1["some expression ???",2]
> > I am not sure how to filter rows based on a criterion.
> > Any help will be appreciated.

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