clear
input id productid purchase
1     1         1
2     1         1
3     2         1
1     3         1
end

id productid purchase
1     1         1
2     1         1
3     1         0
1     2         0
2     2         0
3     2         1
1     3         1
2     3         0
3     3         0
end

qui sum id, d
local obs = r(N)
expand = `obs'
levelsof productid, local(id)
local j = 1
foreach i of local id {
    replace productid = `i' if `j' == id
    local j = `j' + 1
}