I was trying to put a simple html code with javascript but i can't find the solution. Following is my basic html code:

<form name="student" id="student" action="submit.php" method="post" >

<label>1. Enter Student Roll Number</label>
<input type="text" name="rollno" id="rollno" />

<label>2. Choose Subject</label>
<select name="mysubject" id="mysubject">
<option value="1">English</option>
<option value="2">Math</option>
<option value="3">Science</option>

<input type="submit" value="Submit" />

I am trying this:
1. When visitor enter his roll number in field 1, suppose he enter 1
2. Then subject field automatically changes to English subject for roll no.1 and similarly if visitor enter roll no.2 then subject auto changes to Math. and so on.

In this way i want visitor a simple way to deal with this form by avoiding selection of subject from dropdown manually. I know this can be possible with javascript, so i need help here.