title: R: Finding similiar/ "duplicate" PAIRS of entries based on multiple variables/ columns/ constraints
body: <p>I am relatively new to R and absolutely new to stackoverflow (having researched a lot here anyway as I have some prior experience in Stata, Excel, VBA and little C).</p>
  
  <p>I have a R dataframe df1 that looks like the following example, just with a few thousand rows:</p>
  
  <pre><code>ID       Date         Value  Class  ZIP 
  TRA0001  2007-09-25   150    1      75019
  TRA0002  2002-08-09   200    2      30152
  TRA0003  2010-08-31   500    3      12451
  TRA0004  2005-06-17   75     1      45242
  TRA0005  2010-08-26   410    3      14618
  TRA0006  2008-07-07   155    1      70139
  TRA0007  2010-01-15   450    3      12883
  TRA0008  2000-11-03   80     4      45242
  TRA0009  2003-05-01   120    2      63017
  TRA0010  2000-10-01   85     5      23712
  </code></pre>
  
  <p>Each row stands for one transaction. What I need to find are similar transactions to each transaction based on the following combination of "matching criteria" (AND connected):</p>
  
  <ol>
  <li><strong>Date</strong> must be within +/- 18 months, e.g. for TRA0001 the only match would be TRA 0006</li>
  <li><strong>Value</strong> must be within +/- 20% of the original row's value, e.g. matches for TRA0001 would be TRA0006 and TRA0009</li>
  <li><strong>Class</strong> must be an exact match, e.g. the matches for TRA0001 upon this criterion would be TRA0004 and TRA0006</li>
  </ol>
  
  <p>Note that there can be no match, one match or multiple matches for each transaction/ row. What I need in the end is a list of matches in respect of the combination of the three criteria mentioned above.</p>
  
  <p>For the given example, a result df2 would look like this:</p>
  
  <pre><code>ID       ID_Match   ZIP_Match
  TRA0001  TRA0006    70139
  TRA0003  TRA0005    14618
  TRA0003  TRA0007    12883
  TRA0005  TRA0007    12883
  TRA0006  TRA0001    75019
  TRA0007  TRA0003    12451
  TRA0007  TRA0005    14618
  </code></pre>
  
  <p>So far, I tried various combinations of duplicate search to get closer to my desired outcome by fulfilling at least one matching criteria and next "filtering down" this result according to the other constraints. I started with the <strong>Class</strong> condition, as this seemed to me to be the easiest criterion (and probably also the most selective). All I came up in the end was e.g a list of all classes that have duplicates and there respective index positions where the duplicates can be found. For that I used the following code (found on stackoverflow, credits to user "eddi"):</p>
  
  <pre><code>dups = duplicated(df1$Class) | duplicated(d1$Class, fromLast = T)
  split(which(dups), df1$Class[dups])
  </code></pre>
  
  <p>However, this still leaves me miles away from my desired result and I have no idea how to "integrate" the other conditions. Hope I could provide all the necessary information and could make clear my problem. Any kind of hints, suggestions or solutions is more than welcome! Thanks in advance!</p>
  
  <p>Additionally: If someone comes up with an idea how to do the required work with Stata, this would also be welcome - I have slightly slightly more knowledge on Stata than on R.</p>