How to dinamically change the href attribute on links

//this is the few javascript that i have...
<script type="text/javascript">
user = new Array();
user[0]= 'mark';
user[1]= 'tom';
activeUser = 0;
</script>


<input type="radio" checked="true" onclick="redirect(this.value);" name="radio1" value="0" id="mark"/>
<label for="mark">mark</label>
<input type="radio" onclick="redirect(this.value);" name="radio1" value="1" id="tom"/>
<label for="tom">tom</label>
<br />

<a href="mark/intro.htm">intro</a><br />
<a href="mark/home.htm">home</a><br />
<a href="mark/account.htm">account</a><br />

<script type="text/javascript">
user = new Array();
user[0]= 'mark';
user[1]= 'tom';
activeUser = 0;

function redirect(userSelected){
for(i=0; i<document.getElementById('section').getElementsByTagName('a').length; i++){
s=document.getElementsByTagName('a')[i].href;
document.getElementsByTagName('a')[i].href = s.replace(user[activeUser], user[userSelected]);
}activeUser = userSelected;}
</script>

<div id="section">
<input type="radio" checked="true" onclick="redirect(this.value);" name="radio1" value="0" id="mark"/>
<label for="mark">mark</label>
<input type="radio" onclick="redirect(this.value);" name="radio1" value="1" id="tom"/>
<label for="tom">tom</label>
<br />

<a href="mark/intro.htm">intro</a><br />
<a href="mark/home.htm">home</a><br />
<a href="mark/account.htm">account</a><br />

</div>