st: return the maximum and corresponding value...

[Kit Baum <baum@bc.edu>]

Le Wang said

I have a question regarding Mata. Suppose I have an imaginary matrix
with two columns as follows,
x                      y
2.9479707       .15
2.926226         .16
2.8994321       .17
2.8691981       .18
2.8368784       .19
2.8035694        .2
2.7701303       .21
2.7372142       .22
2.705299         .23
2.6747184        .24

I want Mata to return the maximum values of column 1 (x) AND its
corresonding values in column 2 (y), in this example, 2.9479707 and
0.15. I understand I can use -rowmax- to find the maximum, but I am
not quite sure how to find the corresponding value.



The good news: there is a function undocumented in the manual to  
handle this.
The bad news: it does not appear to work.

This workaround does:

real scalar lewang(real matrix a)
{
real vector i
maxindex(a[.,1],1,i,w)
return(i)
}
a=(4,2\6,3\10,9\1,2\4,5\7,6\9,0)
j=lewang(a)
b=a[j,.]
b

Now all this horsing around should be unnecessary, for you should  
just be able to call

maxindex(a[.,1],1,ii,w)

But that yields

                 <istmt>:  3499  ii not found
r(3499);


minindex() doesn't seem to work either, even on the examples in the  
helpfile:


: v = (3,1,5,7,6)

: minindex(v, 1, i, w)
                 <istmt>:  3499  i not found
r(3499);


Looks like a bug to me.


Kit Baum, Boston College Economics
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html


*
*   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/