title: R: is.na() does not pick up NA value
body: <p>So I have a dataset and just by looking at it there are clear NA's in the dataset. </p>
  
  <pre><code> &gt; dput(bmi.cig)
  structure(list(MSI.subset.BMI = structure(c(4L, 4L, 4L, 4L, 4L, 
  4L, 4L, 4L, 4L, 4L, 4L, 1L, 2L, 3L, 3L, 1L, 3L, 3L, 1L, 4L, 4L, 
  4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L), .Label = c("0", "1", "2", 
  "NA"), class = "factor"), MSI.subset.Cigarette = structure(c(3L, 
  3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 2L, 2L, 1L, 2L, 1L, 2L, 
  2L, 2L, 2L, 1L, 2L, 1L, 2L, 2L, 2L, 1L, 1L, 1L, 1L), .Label = c("1", 
  "2", "NA"), class = "factor")), .Names = c("MSI.subset.BMI", 
  "MSI.subset.Cigarette"), row.names = c(NA, 30L), class = "data.frame")
  
  
  &gt; head(bmi.cig)
    MSI.subset.BMI MSI.subset.Cigarette
  1             NA                   NA
  2             NA                   NA
  3             NA                   NA
  4             NA                   NA
  5             NA                   NA
  6             NA                   NA
  </code></pre>
  
  <p>I want to remove any row that contains an NA in either column, so I'm using the list-wise deletion function <code>ld</code> in the <code>ForImp</code> package. However, R is not recognizing the NA values. </p>
  
  <pre><code>is.na(bmi.cig$MSI.subset.BMI)
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>    &gt; is.na(bmi.cig$MSI.subset.BMI)
   [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
  [26] FALSE FALSE FALSE FALSE FALSE
  </code></pre>
  
  <p>So once I use the <code>ld</code> function I just get an empty dataset in return.</p>