Loop or tool to replace missing components of an aggregate

[Harald Leber]

I have an aggregate X that consists of components A + B + C + ...
Suppose now that C is missing. If all others are available, I can use X - A - B - ... to obtain C.
I can do the same thing now with every (missing) component (as long as there is only one). However, is there a faster way in Stata than using:
replace A = X - B - C - ... if A == .
replace B = X - A - C - ... if B == .
and so on?

[Maarten Buis]

local vars a b c
egen xprime = rowtotal(`vars')
foreach var of local vars {
    local othervars : list vars - var
    local othervars : subinstr local othervars " " ", ", all
    replace `var' = x - xprime if missing(var) & !missing(`othervars')
}

[Harald Leber]

Many thanks!