title: How to select some rows with specific rownames from a dataframe?
body: <p>I have a data frame with several rows. I want to select some rows with specific rownames (such as <code>stu2,stu3,stu5,stu9</code>) from this dataframe. The input example dataframe is as follows:</p>
  
  <pre><code>        attr1 attr2 attr3 attr4
    stu1      0     0     1     0
    stu2     -1     1    -1     1
    stu3      1    -1     0    -1
    stu4      1    -1     1    -1
    stu5     -1     1     0     1
    stu6      1    -1     1     0
    stu7     -1    -1    -1     1
    stu8      1    -1     0    -1
    stu9     -1    -1     1    -1
    stu10    -1     1     0     1
  </code></pre>
  
  <p>Expected output:</p>
  
  <pre><code>        attr1 attr2 attr3 attr4
    stu2     -1     1    -1     1
    stu3      1    -1     0    -1
    stu5     -1     1     0     1
    stu9     -1    -1     1    -1
  </code></pre>