How can I do Data distribution ?

I have a data like this

a = 
2 3 4
5 4 1
7 8 4

And I will apply some function to this data(give label) like below

a=
2 3 4 A
5 4 1 B
7 8 4 C

Then I want to distribute that label at some data like this

b =          b=
7 8 4        7 8 4 C
7 8 4        7 8 4 C
2 3 4   ->   2 3 4 A
2 3 4        2 3 4 A
7 8 4        7 8 4 C
5 4 1        5 4 1 B

Actually this question is easy if I use 'for loop' but, I work with large data speed is very important for me. Is there any way to solve this problem ??