please help!! stata looping

send everything in Mata and calculate. Afterward send everything back could work......then everything will be in memory and donot need too many variables....

Eg:

mata
x = st_data(.,("x"))
y = st_data(.,("y"))
gen z = J(rows(x),1,0)

for (i=2;i<=rows(x);i++){
   if (y[i]==0&x[i]-x[i-1]<>0{
      z[i] = x[i]-x[i-1]
   }
   else if (y[i]==0&x[i]-x[i-1]==0){
          for(k=i;k>=1;k--){
                if(x[i]-x[k]<>0) {
                   z[i] = x[i]-x[k]
                }
                else z[i] = .
        }  
    }
}   
}
st_matrix("z",z)

end
svmat  z