.. include:: ../../../_includes/_warning_banner.rst
.. |abs| replace:: ``<abs>``

=====
|abs|
=====

.. note::
    The |abs| element used inside the FlightGear ``<expression>`` tag is distinct from the
    |abs|
    `element used in JSBSim <https://jsbsim-team.github.io/jsbsim-reference-manual/user/concepts/math/>`_
    for XML configuration files - although they are closely related and both perform the same
    mathematical operation.

Overview
--------

The |abs| element takes the absolute value of a ``<property>`` child element. It is the same
principle as functions of the same name in many programming languages.

.. tip::
    For more information about absolute values in mathematics, see the
    `Wikipedia article <https://wikipedia.org/wiki/Absolute_value>`_ on the subject.

Syntax
------

.. todo:: Does abs support more than one argument?

The |abs| element must be a child element of an ``<expression>`` element, and the |abs| element
 must have a ``<property>`` child element.

Example
-------

One use case for the |abs| element is when a numerical value needs to be checked or used in a
calculation, but whether or not the value is positive or negative is irrelevant.

In this example, passing the ``/velocities/vertical-speed-fps`` as a value to the |abs| element
will return the vertical speed as a positive number, regardless of whether its value is positive
or negative.

.. code-block:: xml

    <abs>
        <property>/velocities/vertical-speed-fps</property>
    </abs>