a question about matrix subscripting in mata

[Frank Erickson]

I'm going to construct a vector "myvec". Each element of "myvec" will come from either "vecA" or "vecB" according to a the selection vector, "sel":

vecA  = ("a","b","c")
vecZ  = ("x","y","z")
p     = (.5,.5)

rseed(1)
sel = rdiscrete(1,n_pl,(.5,.5)) // 2,1,2

// desired answer: x,b,z

My current solution to this is very clumsy:

len   = length(vecA)
myvec = (vecA,vecZ)[(1..len):+len:*(sel:-1)]

So I wanted to ask if there is some more natural way to do this in Mata. As is, I think I may end up tossing my clumsy code into a function...though it will get messier if/when I want to select from more than two vectors. Any suggestions would be appreciated. Thanks!

(I'm used to such a thing in R, which has a vectorized version of Mata's cond ? ifyes : ifno as well as a matrix subsetting syntax (vecA \ vecZ)[...] that would do the trick here.)

[Aljar Meesters]

You can use:

(vecA \ vecZ)[(1..length(vecA) \ sel)]

which is a bit less clumsy and would work with more than two vectors in a straight forward way. Although I agree with you that it would be nice if the ternary operator (? : ) would work on matrices as well.
Best,

Aljar

[Niels Henrik Bruun]

I find your solutions quite elegant allthough I also miss a vectorized version of if or at least a tool like map in Python to make if or other functions vectorized if needed

[Frank Erickson]

Thanks for the feedback!

@Aljar: I can't get your code to work with the inputs I have:

("a","b","c" \ "x","y","z")[(1..3 \ (2,1,2))]

gives

<istmt>:  3201  vector required
r(3201);

I'd be very interested in an approach that extends to more vectors, though, if you could explain.

[Aljar Meesters]

Hi Frank,

I am pretty sure that I have tried it before I have posted it, but apparently I have mixed things up because I get the same error as you.
The only thing that I can come up with to solve it is:

mata
vecA  = ("a","b","c")
vecZ  = ("x","y","z")
p     = (.5,.5)
n_pl = 3
rseed(1)
sel = rdiscrete(1,n_pl,(.5,.5)) // 2,1,2
vecA :* (sel :== 1) + vecZ :* (sel :== 2)
end

With a for loop it is easy to extend it to more vectors. I hope this helps.
Best,

Aljar

[Frank Erickson]

Ok, thanks again Aljar!