change select options on the basis of previously selected option in another select

Question

I wish to display options in a select on the basis of what was selected in previous select, I have tried using createOption in javascript, but it keeps on creating options as many times as I change the option in parent select, hence I need a way to accomplish that, my code is as follow:

function addsem(deg)
{
  var semnum,i;
  switch(deg)
  {
    case 'var1':numsem=8;
              break;
case 'var2':numsem=6;
              break;
    case 'var3':numsem=10;
              break;

case 'var4':numsem=4;
}
select1=document.getElementById('selsem');
var sem=new Array('I','II','III','IV','V','VI','VII','VIII','IX','X'); 

 for(i=0;i<numsem;i++)
 {
   var opt = new Option(sem[i],sem[i]);
 select.options.add(opt);
 }
}

my html code:

  <select name="seldeg" id="seldeg" style="width:180px onChange="addsem(this.value);">
                <option value="deg">Select</option>
                <option value="var1">var1</option>
                <option value="var2">var2</option>
                <option value="var3">var3</option>
                <option value="var4">var4</option>
                </select>
<select name="selsem" id="selsem" style="width:180px;" onchange="" >
        <option  value="sem">Select</option>
</select>

I wish select to show different range of sems ie I,II...IV for var4, I,II....VI for var2

everytime I change deg value.

Thaknyou in advance

Answer 1

In your Javacript, select.options.add should be select1.options.add . There's a syntax error in the HTML, you didn't close your style="width:180px attribute. Also you need to remove the items from the select so that you don't get duplicate items each time you change the first select.

jsFiddle Demo

function addsem(deg) {
    var semnum, i;
    switch (deg) {
        case 'var1':
            numsem = 8;
            break;
        case 'var2':
            numsem = 6;
            break;
        case 'var3':
            numsem = 10;
            break;

        case 'var4':
            numsem = 4;
    }
    select1 = document.getElementById('selsem');

    // remove existing items
    while (select1.options.length) {
        select1.remove(0);
    }

    var sem = new Array('I', 'II', 'III', 'IV', 'V', 'VI', 'VII', 'VIII', 'IX', 'X');

    for (i = 0; i < numsem; i++) {
        var opt = new Option(sem[i], sem[i]);
        select1.options.add(opt); // changed to select1
    }
}