Check whether values in one data frame column exist in a second data frame

Question

I have two data frames (A and B), both with a column 'C'. I want to check if values in column 'C' in data frame A exists in data frame B.

A = data.frame(C = c(1,2,3,4)) B = data.frame(C = c(1,3,4,7)) 

Answer 1

Use %in% as follows

A$C %in% B$C 

Which will tell you which values of column C of A are in B.

What is returned is a logical vector. In the specific case of your example, you get:

A$C %in% B$C # [1]  TRUE FALSE  TRUE  TRUE 

Which you can use as an index to the rows of A or as an index to A$C to get the actual values:

# as a row index A[A$C %in% B$C,  ]  # note the comma to indicate we are indexing rows  # as an index to A$C A$C[A$C %in% B$C] [1] 1 3 4  # returns all values of A$C that are in B$C 

We can negate it too:

A$C[!A$C %in% B$C] [1] 2   # returns all values of A$C that are NOT in B$C 

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If you want to know if a specific value is in B$C, use the same function:

  2 %in% B$C   # "is the value 2 in B$C ?"     # FALSE    A$C[2] %in% B$C  # "is the 2nd element of A$C in B$C ?"     # FALSE