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From |
"Xiaoheng Zhang" <zhangx@tcd.ie> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Q about programming in mata re. syntax |

Date |
Thu, 25 Oct 2007 19:03:19 +0100 |

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 <wgould@stata.com> wrote: > Xiaoheng Zhang <zhangx@tcd.ie> 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 > wgould@stata.com > * > * 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/

**References**:**Re: st: Q about programming in mata re. syntax***From:*wgould@stata.com (William Gould, StataCorp LP)

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