I got the $as_index variable to work.  Here is an example:

<tr t-foreach="o.order_line" t-as="l">
                        <td>
                            <span t-esc="l_index + 1" />
. . .

The index is zero-based, hence the addition of 1.  Hope this helps.

This is pretty simple and straight forward.

When we define our loop like this:

<code>

<t t-foreach="people" t-as="person">

<tr>

<th scope="row"><span t-raw="person_index"/></th>

<td><span t-field="person.name"/></td>

<td><span t-field="person.age"/></td>

<td><span t-raw="person.weight"/></td>

</tr>

</t>

</code>

To get the current index/position of the person with the loop you can do person_index. i.e your variable_index or any other available variable withing the context of qweb loop.

I have tryed this:

<t t-set="i" t-value="1"/>
<tr t-foreach="o.order_line" t-as="l">
                        <td> <span t-esc="i"/> </td>
<t t-set="i" t-value="i+1"/>
</tr>

When the loop finishes, the ounter resets to 1. Does anyone kow a way to update a variable outside the loop? I would like to define a variable before the loop and set it to false. If I find what I am looking for inside the loop I would like to set the value of the variable to True. This var would later be used as a condition if a div is visible or not.

Any help is appreciated. Thx

var_index is automatically defined

Check https://www.odoo.com/documentation/8.0/reference/qweb.html#loops