Merging data frames in a list [duplicate]

Answer 1

You can try Reduce with merge

 Reduce(function(...) merge(..., by='custID', all=TRUE), res)
 #     custID top_pct.x top_pct.y
 #1      1        10        10
 #2      2        20        20
 #3      6        NA        10
 #4      7        NA        20

Or as @Colonel Beauvel suggested, a more readable approach would be wrapping it with Curry from library(functional)

 library(functional)
 Reduce(Curry(merge, by='custID', all=T), res)