How to increment counter?

[blueowlgreenfish]

I would like to increment a counter inside for loop, but it doesn't work.
What I tried:

{% let mut counter = 0 %}
{% for value in some_values.iter() %}
{{ counter += 1 }}
{% endfor %}
<p>counter number is now {{ counter }}</p>

This doesn't work.
Could someone tell me what I'm doing wrong?

[GuillaumeGomez]

You can't. Use a function for that which would expect a &mut usize argument to increment it.