Setting element of array from Twig

Question

How can I set member of an already existing array from Twig?

I tried doing it next way:

{% set arr[\'element\'] = \'value\' %}

but

Answer 1

Just use this like {% set arr={'key':'value'} %} (with no blank space after the :), it works well.

But when I use it inside a for loop, to make it an array, it does not work outside of the for scope.

{% for group in user.groups %}
  {% set foo={'loop.index0':'group.id'} %}
  {% set title={'loop.index0':'group.title'} %}
  {{ title }} //it work 
{% else %}
  {% set foo={'0':'-1'} %}
  {% set title={'0':'未分组'} %}
{% endfor %}
{{ title }}  //it does not work, saying title is not defined