title: Finding rows which match between 2 dataframes, and the index of them in R
body: <p>I have 2 dataframes.  First (and I know how to do this) I want to find which rows (the whole row) match between the 2 dataframes.  So I can create a column in A which tells me if that entire row is in B.  However, the part I don't know how to do is to find which indices in B it would be.  </p>
  
  <p>TL DR;  I need to create a new column in A, which tells me FALSE if the whole row isn't in B, or instead give me the index of where that row is in B.</p>
  
  <pre><code>a = as.data.frame(matrix(data = 1:10,nrow=5))
  b = as.data.frame(matrix(data = c(1:5,10,7,6,9,8), nrow=5))
  
  set.seed(02138)
  b = b[sample(nrow(b)),]
  rownames(b) = 1:nrow(b)
  a_ = do.call("paste", a[,1:2])
  b_ = do.call("paste", b[,1:2])
  
  # Gets me TRUE/FALSE of whether there is a complete row-wise match
  a$InB = a_ %in% b_
  
  # Gets me which rows they are in b
  b[b_ %in% a_,]
  
  # Now is where I need help combining this
  # Expected output
  
  &gt; a$InB = temp
  &gt; a
    V1 V2   InB
  1  1  6 FALSE
  2  2  7     3
  3  3  8 FALSE
  4  4  9     5
  5  5 10 FALSE
  </code></pre>