Javascript: Changing a textbox value with an array as a name

[The Letter J]

I have a textbox with the following name: list_data[2][cost]

How would I change the value of that text box? I have tried the following and it doesn't work:

document.list.list_data[2][cost].value = '3.50';

It must be something stupid that I'm missing.

[Dr. Script]

Use it as an array:document.list.elements['list_data[2][cost]'].value = '3.50';If you want to loop thru:document.list.elements['list_data['+i+'][cost]'].value = '3.50';Dr. Script