Dropdown list and value selection

Hi,

I am new to CI and grocery CRUD. Need help with this.

I have created a dropdown list:

for ($i=0; $i<$info["count"]; $i++)
{
array_push($userName, $info[$i]["clientname"][0]);
}

$crud->field_type('name','dropdown', $userName);

$output = $crud->render();
$this->_example_output($output);

$userName is an array.

Based on the value selected in this dropdown, I want to set another field below it with an associated value based on the array index.

How do I get the array index of the value selected in this dropdown list.

Appreciate it if someone come provide guidance and examples. :)

Thank You.