How to create an array in a for loop in Liquid?

You can directly create a new empty array controllers and concat to it your controllerName converted into an array using the workaround split:''. The result is directly an array, without the extra string manipulations.

{% assign controllers = '' | split: '' %}
{% for operation in menuItems %}
    {% assign controllerName = operation.Title | split: '_' | first | split: '' %}
    {% assign controllers = controllers | concat: controllerName %}
{% endfor %}

What worked for me

{% assign otherarticles = "" | split: ',' %}
{% assign software_engineering = "" | split: ',' %}

{% for file in site.static_files %}
  {% if file.extname == ".html" %}
    {% if file.path contains "software_engineering" %}
       {% assign software_engineering = software_engineering | push: file %}
    {% else %}
      {% assign otherarticles = otherarticles | push: file %}
    {% endif %}
  {% endif %}
{% endfor %}