merge or cbind using multiple columns as key in R

Question

I am using a toy exammple:

Data set A: 
color  number   valueA
red      18      0.2
blue     21      0.6
green    15      0.9
red      10      1.0
blue     11            


        

Answer 1

merge(data1,data2)


# color number valueA valueB
# 1  blue     11    2.1    2.5
# 2  blue     21    0.6    0.5
# 3 green     13    3.6    3.9
# 4 green     15    0.9    0.1
# 5   red     10      1    1.1
# 6   red     18    0.2    0.3

Please also see documentation of merge function. Specifically by, by.x and by.y argument

By default the data frames are merged on the columns with names they both have, but separate specifications of the columns can be given by by.x and by.y. The rows in the two data frames that match on the specified columns are extracted, and joined together. If there is more than one match, all possible matches contribute one row each. For the precise meaning of ‘match’, see match.