Compute, fixes, variables
*************************

This section documents accessing or modifying data from objects like
computes, fixes, or variables in LAMMPS using the :py:mod:`lammps` module.

For :py:meth:`lammps.extract_compute() <lammps.lammps.extract_compute()>` and
:py:meth:`lammps.extract_fix() <lammps.lammps.extract_fix()>`, the global, per-atom,
or local data calculated by the compute or fix can be accessed. What is returned
depends on whether the compute or fix calculates a scalar or vector or array.
For a scalar, a single double value is returned.  If the compute or fix calculates
a vector or array, a pointer to the internal LAMMPS data is returned, which you can
use via normal Python subscripting.

The one exception is that for a fix that calculates a
global vector or array, a single double value from the vector or array
is returned, indexed by I (vector) or I and J (array).  I,J are
zero-based indices.
See the :doc:`Howto output <Howto_output>` page for a discussion of
global, per-atom, and local data, and of scalar, vector, and array
data types.  See the doc pages for individual :doc:`computes <compute>`
and :doc:`fixes <fix>` for a description of what they calculate and
store.

For :py:meth:`lammps.extract_variable() <lammps.lammps.extract_variable()>`,
an :doc:`equal-style or atom-style variable <variable>` is evaluated and
its result returned.

For equal-style variables a single ``c_double`` value is returned and the
group argument is ignored.  For atom-style variables, a vector of
``c_double`` is returned, one value per atom, which you can use via normal
Python subscripting. The values will be zero for atoms not in the
specified group.

:py:meth:`lammps.numpy.extract_compute() <lammps.numpy_wrapper.numpy_wrapper.extract_compute()>`,
:py:meth:`lammps.numpy.extract_fix() <lammps.numpy_wrapper.numpy_wrapper.extract_fix()>`, and
:py:meth:`lammps.numpy.extract_variable() <lammps.numpy_wrapper.numpy_wrapper.extract_variable()>` are
equivalent NumPy implementations that return NumPy arrays instead of ``ctypes`` pointers.

The :py:meth:`lammps.set_variable() <lammps.lammps.set_variable()>` method sets an
existing string-style variable to a new string value, so that subsequent LAMMPS
commands can access the variable.

**Methods**:

* :py:meth:`lammps.extract_compute() <lammps.lammps.extract_compute()>`: extract value(s) from a compute
* :py:meth:`lammps.extract_fix() <lammps.lammps.extract_fix()>`: extract value(s) from a fix
* :py:meth:`lammps.extract_variable() <lammps.lammps.extract_variable()>`: extract value(s) from a variable
* :py:meth:`lammps.set_variable() <lammps.lammps.set_variable()>`: set existing named string-style variable to value

**NumPy Methods**:

* :py:meth:`lammps.numpy.extract_compute() <lammps.numpy_wrapper.numpy_wrapper.extract_compute()>`: extract value(s) from a compute, return arrays as numpy arrays
* :py:meth:`lammps.numpy.extract_fix() <lammps.numpy_wrapper.numpy_wrapper.extract_fix()>`: extract value(s) from a fix, return arrays as numpy arrays
* :py:meth:`lammps.numpy.extract_variable() <lammps.numpy_wrapper.numpy_wrapper.extract_variable()>`: extract value(s) from a variable, return arrays as numpy arrays