st: RE: Re: mata for-if-else

["Abhimanyu Arora" <abhimanyu.arora@student.kuleuven.be>]

I see. Thanks very much Joseph and Nick for these useful tips and tricks
(just beginning to get used to mata). 
Cheers
Abhimanyu  

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Joseph Coveney
Sent: woensdag 14 april 2010 15:06
To: statalist@hsphsun2.harvard.edu
Subject: st: Re: mata for-if-else

Abhimanyu Arora wrote:

I have simplified my problem to a large extent, but essentially has to do
with using for-if-else commands in mata. I would like to create a matrix 'a'
whose values depend on an existing matrix 'b' (both are 1000X1 vectors to be
precise). I need to tell mata the dimension of a first. But all I get after
executing the commands below is the original vector 'a'. What could be the
possible error? I have tried adding and removing braces but it is not
working... 

a=J(1000,1,.)
for(i=1,i<1001,i++) {
if (b[i]==1){
a[i]=2
}
else {a[i]=3
}
}
a
end

----------------------------------------------------------------------------
----

Not to take away from any of the helpful replies that you've already
received,
but don't forget that Mata has colon operators that help avoid loops when
dealing with vectors and matrices.  For example, your simplified example can
be
handled succinctly without the need for any explicit looping.

: B = (. \ 1 \ 2 \ 3)

: 
: A = J(rows(B), 1, 3) - (B :== 1)

: 
: A
       1
    +-----+
  1 |  3  |
  2 |  2  |
  3 |  3  |
  4 |  3  |
    +-----+

It could be more efficient, too, than an elaborated for-loop with an
imbedded
if-then-else section.  You'll need to consider the trade-offs in code
readability (maintainability) if you get too carried away with one-liners
like
that.

I'm sure that your real problem is more complicated than the simplified
version
you posted for illustration.  And it might not appear, at least at first
glance,
to readily lend itself to a colon operator.  But keep in mind that you've
got
options (operators, matrix functions) available in Mata to help in just
these
kinds of situations. It's a domain-specific programming language that's
designed
to be rich in operations that matter in statistical computation.

Joseph Coveney




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