Combobox force position

[KahL]

Hi there,
here's my problem :
i have a combobox in wich i can change the value with selectedIndex attribute, but how can i change the label or the selected position, because the value changes but the position don't ?
Thanx in advance

[KahL]

I've found the true mistake but i still don't understand why it doesn't work :

cb.selectedIndex = value;

This doesn't work, neither the value nor the index change !!!
And when i "trace" cb.selectedIndex, the result is "undefined" !
And i'm sure of the path to reach the combobox.
Help please :'(