title: Matching Columns, Creating Loop in R
body: <p>I have the following question:</p>
  
  <p>I have data frame which looks like this. I have prices, 3 X's and 2 R's.</p>
  
  <pre><code>Date    Name  Price  Interest
  01.02.10 X  120     0.2
  01.02.10 R  120     0.3
  01.02.10 X  130     0.8
  01.02.10 X  140     0.4
  01.02.10 R  130     0.2
  etc.
  </code></pre>
  
  <p>I would like to tell R to look for pairs of X&amp;Rs with the same price, and delete the rest. So this should result: 2 X's and 2'Rs (in this case).</p>
  
  <pre><code>Date    Name  Price  Interest
  01.02.10 X  120     0.2
  01.02.10 R  120     0.3
  01.02.10 X  130     0.8
  01.02.10 R  130     0.2
  etc.
  </code></pre>
  
  <p>To make it clearer (hopefully): I have a lot of different prices for each date. Each row either has an X or an R in it. There are a lot of pairs on each date, i.e. for example X, Price = 120 &amp; R, Price = 120 on Date 1. But there are also Prices which only match one Name, for example there is a Price = 140 only for Name = X. So what i would like R to do is: check for machting Names for one Price (i.e. there exists the same Price for one X and one R) and delete the rest. What actually would result is the same number of X's and R's because I'm looking for pairs.</p>
  
  <p>I'm sorry not to be able to post something I tried. I just couldn't think of anything.</p>
  
  <p>Now, to the next problem:
  If the pairs are there, I would like to tell R to check each line. If the Name is X, I want it to calculate a new price, if not just print the existing price.
  I tried</p>
  
  <pre><code>xx &lt;- if(Name == "X"){Price + 100*interest} else print{Price}
  </code></pre>
  
  <p>but it didn't work.</p>
  
  <p>Thanks for help </p>
  
  <p>Cheers
  Dani</p>