Dear William,
Thank you very much for the complete solutions.I am just starting
using mata since yesterday and no manual with me.
--
Greetings,
Xiaoheng Zhang (Kevin)
On 25/10/2007, William Gould, StataCorp LP <[email protected]> wrote:
> Xiaoheng Zhang <[email protected]> is having trouble understanding and using
> Mata syntax, and offers the following:
>
> . mata
> -------------------------- mata (type end to exit)
> : for (k=1; k<=2; k++) {
> > r`k' = rowmin(D)
> > C`k' = J(3 , 3 , 2)
> > for (i=1; i<=3; i++) {
> > for (j=1; j<=3; j++) {
> > if (D[ i , j ] == r`k'[ i , 1 ])
> > C`k'[ i , j ] = 1
> > D[ i , j ] = D[ i , j ] + 10000
> > else C`k'[ i , j ] = 0
> 'else' found where almost anything else expected
> r(3000);
>
> : }
> expression invalid
> r(3000);
>
> : }
> expression invalid
> r(3000);
>
> : }
> expression invalid
> r(3000);
>
> : end
> -----------------------------------------------------------
>
> Xiaoheng has one typogrpahical error and one conceptual error in the above.
> The typographical error is in
>
> if (D[ i , j ] == r`k'[ i , 1 ])
> C`k'[ i , j ] = 1
> D[ i , j ] = D[ i , j ] + 10000
> else C`k'[ i , j ] = 0
>
> I suspect he means to code
>
> if (D[ i , j ] == r`k'[ i , 1 ]) {
> C`k'[ i , j ] = 1
> D[ i , j ] = D[ i , j ] + 10000
> }
> else C`k'[ i , j ] = 0
>
> Note that I added braces.
>
> The conceptual error, however, is more serious. Throughout his code, Xiaohang
> refers to r`k' and C`k'. The Stata macro-substitution syntax makes no sense
> to Mata and the entire approach is simply not going to work.
>
> Xiaoheng wants to start with a Distance matrix D
>
> Distance between firms
> | 1 2 3
> --+---------------------------
> 1 | 0 5 7
> 2 | 5 0 2
> 3 | 7 2 0
> ------------------------------
>
> (Three firms shown for illustration)
>
> and create two indicator matrices, C1 showing the nearest firm:
>
> | 1 2 3 meaning
> --+---------------------------
> 1 | 0 1 0 nearest to 1 is 2
> 2 | 0 0 1 nearest to 2 is 3
> 3 | 0 1 0 nearest to 3 is 2
> ------------------------------
>
> and C2, showing the second nearest firm:
>
> | 1 2 3 meaning
> --+---------------------------
> 1 | 0 0 1 2nd nearest to 1 is 3
> 2 | 1 0 0 2nd nearest to 2 is 1
> 3 | 1 0 0 2nd nearest to 3 is 1
> ------------------------------
>
> So that's the problem: Given D, produce C1 and C2.
>
> The first thing I'm goping to do is change the problem a little. I'm going
> to produce vectors c1 and c2, c1 containing
>
> | meaning
> --+---
> 1 | 2 nearest to 1 is 2
> 2 | 3 nearest to 2 is 3
> 3 | 2 nearest to 3 is 2
> ------
>
> and c2, a vector defined similarly. It will be easy enough to produce C1
> from c1 and produce C2 from c2, as I will show.
>
> First, here's a function that, given D, produces c1:
>
> real colvector closest(D)
> {
> c = J(rows(D), 1, 0)
> for (i=1; i<=rows(D); i++) {
> j_min = d_min = .
> for (j=1; j<=cols(D); j++) {
> if (i!=j) {
> if (D[i,j]<d_min) {
> j_min = j
> d_min = D[i,j]
> }
> }
> }
> c[i] = j_min
> }
> return(c)
> }
>
> and here's a function that will produce c2 given D and c1:
>
>
> real colvector closest2(D, c1)
> {
> D2 = D
> for (i=1; i<=rows(D); i++) D2[i,c1[i]] = .
> return(closest(D2))
> }
>
> and finally, here's a function that will produce a C matrix from a c vector:
>
>
> real matrix C_of_c(c)
> {
> C = J(rows(c), rows(c), 0)
> for (i=1; i<=rows(C); i++) C[i,c[i]] = 1
> return(C)
> }
>
> Below I try them out:
>
> : D = (0,5,7 \ 5,0,2 \ 7,2,0)
>
> : D
> [symmetric]
> 1 2 3
> +-------------+
> 1 | 0 |
> 2 | 5 0 |
> 3 | 7 2 0 |
> +-------------+
>
> : c1 = closest(D)
>
> : c1
> 1
> +-----+
> 1 | 2 |
> 2 | 3 |
> 3 | 2 |
> +-----+
>
> : c2 = closest2(D, c1)
>
> : c2
> 1
> +-----+
> 1 | 3 |
> 2 | 1 |
> 3 | 1 |
> +-----+
>
> : C_of_c(c1)
> 1 2 3
> +-------------+
> 1 | 0 1 0 |
> 2 | 0 0 1 |
> 3 | 0 1 0 |
> +-------------+
>
> : C_of_c(c2)
> 1 2 3
> +-------------+
> 1 | 0 0 1 |
> 2 | 1 0 0 |
> 3 | 1 0 0 |
> +-------------+
>
> -- Bill
> [email protected]
> *
> * For searches and help try:
> * http://www.stata.com/support/faqs/res/findit.html
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
>
>
--
Greetings,
Xiaoheng Zhang (Kevin)
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
