title: Reading file with missing values in R
body: <p>I have a file with filename = 'fn', which I am reading as follows:</p>
  
  <pre><code>age CALCIUM CREATININE  GLUCOSE
  64.3573     1.1 488
  69.9043 8.1 1.1 472
  65.6633 8.6 0.8 461
  50.3693 8.1 1.3 418
  57.0334 8.7 0.8 NEG
  81.4939     1.1 NEG
  56.954  9.8 1   
  76.9298 9.1 0.8 NEG
  
  
  &gt; tmpData = read.table(fn, header = TRUE,  sep= "\t" , na.strings = c('', 'NA', '&lt;NA&gt;'),  blank.lines.skip = TRUE)
  &gt; tmpData
        age CALCIUM CREATININE GLUCOSE
  1 64.3573            NA        1.1     488
  2 69.9043           8.1        1.1     472
  3 65.6633           8.6        0.8     461
  4 50.3693           8.1        1.3     418
  5 57.0334           8.7        0.8     NEG
  6 81.4939            NA        1.1     NEG
  7 56.9540           9.8        1.0    &lt;NA&gt;
  8 76.9298           9.1        0.8     NEG
  </code></pre>
  
  <p>The file is read as above with missing values replaced as NA and  &lt; NA >.  I guess that the 'glucose' column is treated as factor. Is there an easy way to interpret  &lt; NA > as real NA and convert any non-numeric values into NA (in this example  NEG into NA)  </p>