How to handle twig view and bootstrap 3 rows/columns? - symfony

Question

I have an Article entity and I made a findAll() in the controller.
I rendered each article in a div with the col-md-6 class.
But foreach 2 articles I must wrap these divs in a row div.
How can I do this with twig ?
Thanks.
EDIT :
I tried your code (NHG) like this:
{% for article in articles %}
{% if loop.index % 2 == 0 %}
<div class="row"></div>
{% endif %}
<div class="col-md-6">
<article class="well well-sm">
<img src="{{ article.image }}" alt="{{ article.title }}" class="img-thumbnail">
<h2 class="h3 text-center">{{ article.title }}</h2>
<div class="alert alert-success well-sm">
{{ article.content|striptags|slice(0, 235) }}...
</div>
<a class="btn btn-default btn-sm pull-right" href="#">{{ article.comments|length }} Comments</a>
<div class="btn-group btn-group-sm">
{% for tag in article.tags %}
<a class="btn btn-default">{{ tag.name }}</a>
{% endfor %}
</div>
</article>
</div>
{% endfor %}
But it doesn't work.
I want to have something like this :
<div class="row">
<div class="col-md-6"></div>
<div class="col-md-6"></div>
</div>
<div class="row">
<div class="col-md-6"></div>
<div class="col-md-6"></div>
</div>
<div class="row">
<div class="col-md-6"></div>
<div class="col-md-6"></div>
</div>

Answer 1

UPDATED:
As #Maerlyn suggested:
{% for row in articles|batch(2) %}
<div class="row">
{% for article in row %}
<div class="col-md-6">
// your content
</div>
{% endfor %}
</div>
{% endfor %}
OLD way:
Use loop.index (doc: The loop variable), modulo (doc: Math operators) and if (doc: if statement )
{% for article in articles %}
{% if loop.index % 2 == 1 %}
<div class="row">
{% endif %}
<div class="col-md-6">
// your content
</div>
{% if (loop.index % 2 == 0 or loop.last) %}
</div>
{% endif %}
{% endfor %}

Answer 2

{% for row in articles|batch(2) %}
<div class="row">
{% for article in row %}
<div class="col-md-6">
// your content
{% if loop.index is divisibleby(3) %}
</div>
{% endif %}
{% endfor %}
</div>
{% endfor %}

Answer 3

I suggest :
<div class="row">
{% for article in articles %}
{% if (loop.index0 % 2 == 0) and not loop.first %}
</div>
<div class="row">
{% endif %}
<div class="col-xs-12 col-sm-6 col-md-6 col-lg-6">
// your content
</div>
{% endfor %}
</div>