``if``
======

The ``if`` statement in Twig is comparable with the if statements of PHP.

In the simplest form you can use it to test if an expression evaluates to
``true``:

.. code-block:: jinja

    {% if online == false %}
        <p>Our website is in maintenance mode. Please, come back later.</p>
    {% endif %}

You can also test if an array is not empty:

.. code-block:: jinja

    {% if users %}
        <ul>
            {% for user in users %}
                <li>{{ user.username|e }}</li>
            {% endfor %}
        </ul>
    {% endif %}

.. note::

    If you want to test if the variable is defined, use ``if users is
    defined`` instead.

You can also use ``not`` to check for values that evaluate to ``false``:

.. code-block:: jinja

    {% if not user.subscribed %}
        <p>You are not subscribed to our mailing list.</p>
    {% endif %}

For multiple conditions, ``and`` and ``or`` can be used:

.. code-block:: jinja

    {% if temperature > 18 and temperature < 27 %}
        <p>It's a nice day for a walk in the park.</p>
    {% endif %}

For multiple branches ``elseif`` and ``else`` can be used like in PHP. You can
use more complex ``expressions`` there too:

.. code-block:: jinja

    {% if kenny.sick %}
        Kenny is sick.
    {% elseif kenny.dead %}
        You killed Kenny! You bastard!!!
    {% else %}
        Kenny looks okay --- so far
    {% endif %}

.. note::

    The rules to determine if an expression is ``true`` or ``false`` are the
    same as in PHP; here are the edge cases rules:

    ====================== ====================
    Value                  Boolean evaluation
    ====================== ====================
    empty string           false
    numeric zero           false
    NAN (Not A Number)     true
    INF (Infinity)         true
    whitespace-only string true
    string "0" or '0'      false
    empty array            false
    null                   false
    non-empty array        true
    object                 true
    ====================== ====================
