title: Using ifelse() to replace NAs in one data frame by referencing another data frame of different length
body: <p>I already reviewed the following two posts and think they <em>might</em> answer my question, although I'm struggling to see how:</p>
  
  <p>1) <a href="https://stackoverflow.com/questions/8214303/conditional-replacement-of-values-in-a-data-frame">Conditional replacement of values in a data.frame</a>
  2) <a href="https://stackoverflow.com/questions/8349909/creating-a-function-to-replace-nas-from-one-data-frame-with-values-from-another">Creating a function to replace NAs from one data.frame with values from another</a></p>
  
  <p>With that said, I'm trying to replace NAs in one data frame by referencing another data frame of a different (shorter) length and pulling in replacement values from column "B" where the values for column "A" in each data frame match.</p>
  
  <p>I've modified the data, below, for simplicity and illustration, although the concept is the same in the actual data.  FYI, in the real second data frame, there are also no duplicates in column "A".</p>
  
  <p>Here's the first data frame (df1):</p>
  
  <pre><code>&gt; df1
      B          C  A
  1  NA 2012-10-01  0
  2  NA 2012-10-01  5
  3   4 2012-10-01 10
  4  NA 2012-10-01 15
  5  NA 2012-10-01 20
  6  20 2012-10-01 25
  7  NA 2012-10-01  0
  8  NA 2012-10-01  5
  9   5 2012-10-01 10
  10  5 2012-10-01 15
  
  &gt; str(df1)
  'data.frame':   10 obs. of  3 variables:
   $ B: num  NA NA 4 NA NA 20 NA NA 5 5
   $ C: Factor w/ 1 level "2012-10-01": 1 1 1 1 1 1 1 1 1 1
   $ A: num  0 5 10 15 20 25 0 5 10 15
  </code></pre>
  
  <p>And the second data frame (df2).</p>
  
  <pre><code>&gt; df2
     A         B
  1  0 1.7169811
  2  5 0.3396226
  3 10 0.1320755
  4 15 0.1509434
  5 20 0.0754717
  6 25 2.0943396
  
  &gt; str(df2)
  'data.frame':   6 obs. of  2 variables:
   $ A: int  0 5 10 15 20 25
   $ B: num  1.717 0.3396 0.1321 0.1509 0.0755 ...
  </code></pre>
  
  <p>I think I'm pretty close with the following code:</p>
  
  <pre><code>&gt; ifelse(is.na(df1$B) == TRUE, df2$B[df2$A == df1$A], df1$B)
   [1]  1.7169811  0.3396226  4.0000000  0.1509434  0.0754717 20.0000000         NA         NA
   [9]  5.0000000  5.0000000
  Warning message:
  In df2$A == df1$A :
    longer object length is not a multiple of shorter object length
  </code></pre>
  
  <p>Obviously, I want the 7th and 8th output elements to be 1.7169811 and 0.3396226, rather than NAs . . .</p>
  
  <p>Thanks, in advance, for any help, and, once again, thanks for your patience!</p>