selecting elements with varying indices

[charlie wong]

hello

Sorry if this is too elementary - for each row, I want to select a column specified in another column. Wondering if there is a better Mata approach than the follows:

clear * 
set obs 10

g a1=cond(_n<5,_n,.)
g a2=runiform(0,1)
g b1=cond(_n>5,_n,.)
g b2=runiform(0,1)
g c1=_n


g c2= cond( c1==a1 , a2, cond(c1==b1,b2,.)) //the Stata way of doing it


putmata ab =(a* b*) c1
mata
index=J(10,1,.)
c2=J(10,1,.)
for (i=1; i<=10; i++) {
if (length(selectindex(ab[i,]:==c1[i]))>0) {
    index[i]=(selectindex(ab[i,]:==c1[i]))
    c2[i,1]=ab[i,index[i,1]+1]
    }
} 
end

Thanks a lot!

[charlie wong]

Similarly, is there a better way to obtain matrix b below (for each row, select the last non-missing col)?

mata a=(1\2\3),(4\5\.),(7\.\.)
mata rnm=rownonmissing(a)
mata b=a[1,rnm[1]]\a[2,rnm[2]]\a[3,rnm[3]]

Thanks.