title: How to use loop in R for big data set
body: <p>Folks,
  Today I have joined this community and I have a problem in using loop in R 
   My sample data set is as follows:-</p>
  
  <pre><code>Roll No     Marks
     1          60
     2          78
     3          84
  </code></pre>
  
  <p>I want a column to be added in this data set which will calculate grade of student.For eg If marks is more than 90 then A, 70-90 then B , less than 70 c.
  I want my out as:-</p>
  
  <pre><code>Roll No       Marks     Grade
     1            60        C
     2            78        B
     3            84        B
  </code></pre>
  
  <p>Is it possible to do it In R using loop , because i have around 60 thousand of data</p>
  
  <p>Any help will be highly appreciated </p>