title: Attempting to subset dataframe
body: <p>Using the mtcars dataframe:</p>
  
  <pre><code>head(mtcars)
                     mpg cyl disp  hp drat    wt  qsec vs am gear carb
  Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
  Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
  Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
  Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
  Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
  Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1
  </code></pre>
  
  <p>and while learning subsetting (now I know how to do it by using indexing correctly or <code>subset</code>), I was experimenting and used the code</p>
  
  <pre><code>head(mtcars[,-mtcars$drat])
                     mpg drat    wt  qsec vs am gear carb
  Mazda RX4         21.0 3.90 2.620 16.46  0  1    4    4
  Mazda RX4 Wag     21.0 3.90 2.875 17.02  0  1    4    4
  Datsun 710        22.8 3.85 2.320 18.61  1  1    4    1
  Hornet 4 Drive    21.4 3.08 3.215 19.44  1  0    3    1
  Hornet Sportabout 18.7 3.15 3.440 17.02  0  0    3    2
  Valiant           18.1 2.76 3.460 20.22  1  0    3    1
  </code></pre>
  
  <p>What is the logic behind this output?</p>