Multiple Select

I have (as an example) an array of values as follows:
arrayvalues=new Array("0001","0003","0005") where each is the value
of an option in a select statement:

<select id="usertypes" multiple="multiple">
<option value="0033">data1</option>
<option value="0025">data2</option>
<option value="0001">data3</option>
<option value="0003">data4</option>
<option value="1234">data5</option>
<option value="0005">data6</option>
</select>

Based on my array values I would like to highlight each option whose
value cooresponds to an element of my array. I have the following
javascript code:

usertypes=document.getElementById("usertypes");
for(var j=0; j<arrayvalues.length; j++)
for(var i=0; i<usertypes.options.length; i++)
{
if(usertypes.options[i].value==arrayvalues[j])
usertypes.options[i].selected=true;
}

Problem: Script will not highlight all options associated with my
array values. However, if I add an alert statement before or after
the if statement, the script will highlight each entry as needed.
Does anyone have any ideas on why this is happening?