title: Extract a part of a data frame by selecting specific observations of a column in R
body: <p>I have a data frame </p>
  
  <pre><code>item_i  a   b   c
  1   0   0   2
  1   0   0   0
  1   0   0   0
  1   0   0   0
  4   0.25    0   0.625
  9   0.166666667 0   0.666666667
  9   0.166666667 0   0.666666667
  9   0   0   0.5
  </code></pre>
  
  <p>the rows i have are 1000 and item_i column has at least 20 different values. Lets say i want to extract the part of the data frame where item_i = 1,9,13,etc. By extract i mean that i also want columns a b and c. Do you have any idea?</p>