.. _measurement:
Using Measurements
==================
If you have the `Uncertainties package`_ installed, you can use Pint to keep
track of measurements with specified uncertainty, and not just exact physical
quantities.
Measurements are the combination of two quantities: the mean value and the error
(or uncertainty). The easiest ways to generate a measurement object is from a
quantity using the ``plus_minus()`` method.
.. doctest::
:skipif: not_installed['uncertainties']
>>> import numpy as np
>>> from pint import UnitRegistry
>>> ureg = UnitRegistry()
>>> book_length = (20. * ureg.centimeter).plus_minus(2.)
>>> print(book_length)
(20.0 +/- 2.0) centimeter
You can inspect the mean value, the absolute error and the relative error:
.. doctest::
:skipif: not_installed['uncertainties']
>>> print(book_length.value)
20.0 centimeter
>>> print(book_length.error)
2.0 centimeter
>>> print(book_length.rel)
0.1
You can also create a Measurement object giving the relative error:
.. doctest::
:skipif: not_installed['uncertainties']
>>> book_length = (20. * ureg.centimeter).plus_minus(.1, relative=True)
>>> print(book_length)
(20.0 +/- 2.0) centimeter
Measurements support the same formatting codes as Quantity. For example, to pretty
print a measurement with 2 decimal positions:
.. doctest::
:skipif: not_installed['uncertainties']
>>> print('{:.02fP}'.format(book_length))
(20.00 ± 2.00) centimeter
Mathematical operations with Measurements, return new measurements following
the `Propagation of uncertainty`_ rules.
.. doctest::
:skipif: not_installed['uncertainties']
>>> print(2 * book_length)
(40 +/- 4) centimeter
>>> width = (10 * ureg.centimeter).plus_minus(1)
>>> print('{:.02f}'.format(book_length + width))
(30.00 +/- 2.24) centimeter
.. note:: Only linear combinations are currently supported.
.. _`Propagation of uncertainty`: http://en.wikipedia.org/wiki/Propagation_of_uncertainty
.. _`Uncertainties package`: https://uncertainties-python-package.readthedocs.io/en/latest/