title: Recode a subset of column and row
body: <p>I have a data.frame and i want to recode some specific column after selecting for some row value.
  My data.frame look like this. First Column ID, second and third columns are value i would like to use to select specific rows, column 4 and above are the value store that i want to recode.</p>
  
  <p>I have store the column name I want to recode in a vector -> list1</p>
  
  <pre><code>df &lt;- data.frame(letters[1:10],letters[1:2],letters[3:4],rep(0:1,each=5),rep(1:0,each=5),rep(1:0,each=5),rep(1:0,each=5))
  
  names(df)&lt;-c("Sample","cond1","cond2","ge1","ge2","ge3","ge4")
  df
  
  list1  &lt;-  c("ge1","ge3")
  </code></pre>
  
  <p>What i want to get:
  when I select for cond1 == a and cond2 == c I can recode the value of specific column (listed in a vector) in this case column ge1 and ge3.</p>
  
  <p>a.e if conditions are meet then ge1 and ge3 are recode as NA. </p>
  
  <p>I have tried things like : 
  <code>df$cond1[df$cond1 == "a" &amp; df$cond2 =="c"]</code>
  and many other but I always fail!</p>