I would like help with a simple operation in a data frame. The data frame contains several variables corresponding to countries that are represented with a three letter coding. I would like to add or subtract values from these variables of specific countries.

There is a similar question asked here. In that case, however, the values are strings.

As an example, say I have the following data frame:

set.seed(10)

a <- data.frame(country = rep(c("aaa" , "bbb" , "ccc") , times = 3) ,
                year = rep(c(1990 , 1991 , 1992), each = 3) ,
                c = sample(9))

giving:

  country year c
1     aaa 1990 9
2     bbb 1990 2
3     ccc 1990 4
4     aaa 1991 6
5     bbb 1991 3
6     ccc 1991 7
7     aaa 1992 5
8     bbb 1992 1
9     ccc 1992 8

For example, I would like to add 5 to every country corresponding to "aaa", giving:

   country year  c
1     aaa 1990 14
2     bbb 1990  2
3     ccc 1990  4
4     aaa 1991 11
5     bbb 1991  3
6     ccc 1991  7
7     aaa 1992 10
8     bbb 1992  1
9     ccc 1992  8

Is there an efficient to code to do this? I appreciate any help.