generating a conditional variable in mata without loop

[Daniel Abebe]

I wrote a function in mata to generate a variable in such a way that
the variable has a well defined value conditional on the value of
a binary variable. So far, I am able to do this by adding a do operator
inside the function that iterates over each observation. This loop
is, however, taking a very long time. Is there a way to write my
conditional statement in a matrix so that I would not need to loop
over each observation?

Here is an example.

clear all
mata
sampsize=100
function geny(xx1){
y=2*xx1
return(y)
}
u=runiform(sampsize,1,0,1)
Z=u:>0.5
X=rnormal(sampsize,1,0,1)
Y=geny(X)
end

I want to generate Y for Z=1

Thank you in advance

[Niels Henrik Bruun]

Since your Z is zero-one you have at least two possibilities:

clear all
clear all
mata:
    sampsize=100
    function geny(xx1){
    y=2*xx1
    return(y)
    }
    u=runiform(sampsize,1,0,1)
    Z=u:>0.5
    X=rnormal(sampsize,1,0,1)
end
mata: select(geny(X), Z)    //1: select values, disregard positions
mata: geny(X) :/ Z            //2: select values, keep positions

[Daniel Abebe]

Thank you Niels. This is an easy solution.