Comparisons across multiple rows in Stata (household dataset) - loops

Question

I'm working on a household dataset and my data looks like this:
input id id_family mother_id male
1 2 12 0
2 2 13 1
3 3 15 1
4 3 17 0
5 3 4 0
end
What I want to do is identify the mother in each family. A mother is a member of the family whose id is equal to one of the mother_id's of another family member. In the example above, for the family with id_family=3, individual 5 has mother_id=4, which makes individual 4 her mother.
I create a family size variable that tells me how many members there are per family. I also create a rank variable for each member within a family. For families of three, I then have the following piece of code that works:
bysort id_family: gen family_size=_N
bysort id_family: gen rank=_n
gen mother=.
bysort id_family: replace mother=1 if male==0 & rank==1 & family_size==3 & (id[_n]==id[_n+1] | id[_n]==id[_n+2])
bysort id_family: replace mother=1 if male==0 & rank==2 & family_size==3 & (id[_n]==id[_n-1] | id[_n]==id[_n+1])
bysort id_family: replace mother=1 if male==0 & rank==3 & family_size==3 & (id[_n]==id[_n-1] | id[_n]==id[_n-2])
What I get is:
id id_family mother_id male family_size rank mother
1 2 12 0 2 1 .
2 2 13 1 2 2 .
3 3 15 1 3 1 .
4 3 17 0 3 2 1
5 3 4 0 3 3 .
However, in my real data set, I have to get the mother for families of size 4 and higher (up to 9), which makes this procedure very inefficient (in the sense that there are too many row elements to compare "manually").
How would you obtain this in a cleaner way? Would you make use of permutations to index the rows? Or would you use a for-loop?

Answer 1

Here's an approach using merge.
// create sample data
clear
input id id_family mother_id male
1 2 12 0
2 2 13 1
3 3 15 1
4 3 17 0
5 3 4 0
end
save families, replace
clear
// do the job
use families
drop id male
rename mother_id id
sort id_family id
duplicates drop
list, clean abbreviate(10)
save mothers, replace
use families, clear
merge 1:1 id_family id using mothers, keep(master match)
generate byte is_mother = _merge==3
list, clean abbreviate(10)
The second list yields
id id_family mother_id male _merge is_mother
1. 1 2 12 0 master only (1) 0
2. 2 2 13 1 master only (1) 0
3. 3 3 15 1 master only (1) 0
4. 4 3 17 0 matched (3) 1
5. 5 3 4 0 master only (1) 0
where I retained _merge only for expositional purposes.