Function To Dynamically Calculate?

writing a function that will dynamically calculate filed values.I have a PHP-generated form which may have a varying number of fields. I need to:

1. calculate the line total for each row -- unitprice * units = linetotal

2. calculate total of all linetotals.

I have named my fields as follows:

unitprice[1], unitprice[2]... , units[1], units[2], ...

I have the following calculate function:

function calculateOld() {
// get both values
unitprice = document.forms["invoice"].unitprice.value;[code].....

This does calculate what I need but only if I have up to two rows. I need to make the function dynamically count how many rows there are, and calculate the linetotal for each row. And here's where my limited JS knowledge brings me to a halt.I have been thinking about what this new and dynamic function should look like, but that's the best I could produce...

function calculate() {
var unitprice[i]= document.forms["invoice"].unitprice[i].value;
var units[i]= document.forms["invoice"].units[i].value;[code]....

how things are done, don't just give me the code.