title: Matching values from multiple columns in 1 data frame to key in second data frame and creating columns
body: <p>I have 2 data frames. One (df1) looks like this:</p>
  
  <pre><code> var.1 var.2 var.3 var.4
  1     7     9     1     2
  2     4     6     9     7
  3     2     NA    NA    NA
  </code></pre>
  
  <p>And the other (df2) looks like this:</p>
  
  <pre><code> var.a var.b var.c var.d
  1     1     b     c     d
  2     2     f     g     h
  3     4     j     k     l
  3     7     j     k     z
  ...
  </code></pre>
  
  <p>with all of the values listed out in var.1-var.4 in df1 in var.a of df2. </p>
  
  <p>I want to match var.a from df2 across all of the columns listed in df1 and then add these columns to df1 with new/combined column names. So for instance it'll look like this:</p>
  
  <pre><code> var.1 var1.b var1.c var1.d ... var.4 var4.b var4.c var4.d
  1     7     j     k     z         2     f     g     h
  2     4     j     k     l         7     j     k     z
  3     2     f     g     h         NA    NA    NA    NA
  </code></pre>
  
  <p>Thanks in advance!</p>