I have two data frames:
x <- structure(list(Cluster1 = c(53L, NA, NA), Cluster10 = c(48L, 
46L, NA), Cluster11 = c(2L, NA, NA)), row.names = c("Cluster1", 
"Cluster10", "Cluster11"), class = "data.frame")
y <- tructure(list(Cluster1 = c(53L, NA), Cluster10 = c(46L, NA), 
    Cluster11 = c(2L, NA)), row.names = c("Cluster1", "Cluster11"
), class = "data.frame")
It looks like this:
> x
          Cluster1 Cluster10 Cluster11
Cluster1        53        48         2
Cluster10       NA        46        NA
Cluster11       NA        NA        NA
> y
          Cluster1 Cluster10 Cluster11
Cluster1        53        46         2
Cluster11       NA        NA        NA
Now, y miss a row in x that is Cluster10. How can I insert that missing row and fill it with value NAs. 
The final desired output for y is:
          Cluster1 Cluster10 Cluster11
Cluster1        53        48         2
Cluster10       NA        NA        NA
Cluster11       NA        NA        NA
 
    