Javascript looping through Page elements to change states?

This is relatively simple, but I'm missing something. I have 10 checkboxes on a page, in a table in a form. They all have names and id's of add0, add1, add2, etc. I wanted to build a "check/uncheck all" checkbox, but my code doesn't seem to be working. On firefox, using firebug, but it can't seem to follow the script execution.

function checkboxprocess(current)
{
    for (i = 0; i < 10; i++)
    {
        if (current.checked)
        {
            document.getElementById("add" + i).checked = true;
        }
        else
        {
            document.getElementById("add" + i]).checked = false;
        }
    }
}

Checkbox:

echo "Select All: <input type=\"checkbox\" name=\"add\" id=\"add\" value=1 onChange=\"checkboxprocess(this)\"><br>";