text field and select field dependence

Hi guys! I want to create a form whereby whenever a value is selected from a list of select field it's corresponding value should appear on the text field.
example,if the select field has a list of file numbers,then if a certain file number is selected the corresponding file name should apppear on the text field

 

Let us know if you have any experience in javascript.

 

I am a begginer to javascript

 

Before you get your hand on the script, i suggest you read the following few references:

• 
  [1]getElementById() http://msdn.microsoft.com/en-us/library/ms536437(VS.85).aspx
  
  [2]getElementsBytagName() http://msdn.microsoft.com/en-us/library/ms536439(VS.85).aspx
  
  [3]selectedIndex property of SELECT element http://msdn.microsoft.com/en-us/library/ms534624(VS.85).aspx

The javascript:

<script type="text/javascript">
function set_text_field()
{
var select_box=document.getElementById("select_box");
var selectionOptionIndex=select_box.selectedIndex;
var optionSet=select_box.getElementsByTagName("option");
var selectedOption=optionSet[selectionOptionIndex];
var value_to_copy=selectedOption.value;
document.getElementById("text_field").value=value_to_copy;
}
</script>

Code (javascript):

the HTML:

<select onchange="set_text_field()" id="select_box">
<option value="1">File 1</option>
<option value="2">File 2</option>
<option value="3">File 3</option>
<option value="4">File 4</option>
<option value="5">File 5</option>
</select>
<input type="text" id="text_field">

HTML: