:orphan:

.. _variables:

****************
Global variables
****************

A simulation may use custom variables using the ``@variables`` keyword. 
We call these *global variables* to distinguish them from
:ref:`local phase variables<local-phase-variables>`.

Global variables can be useful
if the same value occurs at several places in the script, and this value need to be changed.
Then only one single change in the script is necessary. For example, if a script defines
several ``@phase`` descriptions, and each has the same stop condition, this stop
condition may include a global variable.

To set a global variable, use the ``@variables`` keyword::

  @variables x=1, y=2

creates a variable ``x`` with value 1, and a variable ``y`` with value 2.

A variable name must be a :ref:`valid name<valid-names>`.

Variables can be used when setting values to parameters or other variables, for example::

  @variables x = 10
  n_subjects = 100 + x

It is possible to change the value of a variable (outside a ``@phase``) by simply
reassigning the new value::

  @variables x = 10
  n_subjects = 100 + x

  x = 1
  beta = x

The ``@variables`` keyword is only needed when initializing a global variable. You can change the
value of a global variable after the ``@variables`` statement::

  @variables x=1
  ...
  x=2

Global variables can also be used in a :doc:`phase block<phase>`, but their value cannot be changed within a phase.
:ref:`local-phase-variables` can change values within a phase.

Note that the value of parameters do not work as variables: It is not possible to
refer to a parameter to access its value, like it is possible with variables:

::

  alpha_v = 0.5
  alpha_w = alpha_v / 10  # Does not work

::

  @variables alpha = 0.5
  alpha_v = alpha
  alpha_w = alpha / 10  # Works