title: R: Filtering out non numerical values in dataframe
body: <p>I have the following dataframe from a .csv file which consists of more rows but in order to keep it simple I've narrowed it down to those three. You can also access the csv file here: <a href="https://dl.dropboxusercontent.com/u/16277659/filter.csv" rel="nofollow">https://dl.dropboxusercontent.com/u/16277659/filter.csv</a></p>
  
  <pre><code>NAME;       YEAR;   VALUE
  SAMPLE1;    1969;   6
  SAMPLE1;    1970;   -6
  SAMPLE1;    1971;   -7
  SAMPLE1;    1972;   =-X
  SAMPLE1;    1972;   ST
  SAMPLE1;    1972;   3
  SAMPLE1;    1975;   -7
  SAMPLE1;    1976;   3
  SAMPLE1;    1977;   3
  SAMPLE1;    1978;   0
  SAMPLE2;    1991;   -15
  SAMPLE2;    1992;   =X
  SAMPLE2;    1992;   -58
  SAMPLE2;    1994;   -40
  </code></pre>
  
  <p>What I'd like to do is the following: I sometimes have qualitative values (like =-X, ST etc) which I don't necessarily want to loose but if there is a numerical value for the same year (in SAMPLE1 1972 =-X and ST), I would like to keep only the numerical value and get rid of the other values. </p>
  
  <p>How would you do this? Thanks for your help.</p>