Units with Numpy¶
For high-performance computation, Scimath.units includes two objects for adding
units to Numpy ndarray objects: the
UnitScalar and the
UnitArray. UnitScalars and UnitArrays
can be used directly in computations but are best handled with unitted
functions constructed using the
Working with UnitScalars and UnitArrays¶
As a basic example using scalar values, let’s create two UnitScalars and add them. Say we were averaging the wing spans of African swallows (a) with that of European swallows (e).
>>> from scimath.units.api import UnitScalar >>> a = UnitScalar(5, units="inches") >>> e = UnitScalar(15, units="cm") >>> (a + e) / 2 UnitScalar(5.452755905511811, units='0.025400000000000002*m')
Note that the result is a UnitScalar whose default units are inches, but it displays as a derivation from the SI fundamental unit meters. This is because UnitScalar values are stored internally as derivations of the SI system, thus if the value is assigned in a non-SI unit, part of the value may appear in the units. (See the section on internal representation for more detail.)
>>> UnitScalar(1, units="inch") UnitScalar(1, units='0.025400000000000002*m')
A UnitScalar assumes the units that are first assigned to it. From the example above,
>>> (e + a) / 2 UnitScalar(13.85, units='0.01*m')
This does not fundamentally change the value of the variable, but it can lead to rounding errors and unexpected results:
>>> (e + a) == (a + e) UnitScalar(False, units='None')
>>> (a + e) - (e + a) UnitScalar(1.7763568394002505e-15, units='0.025400000000000002*m')
which is awfully close but not quite equal to zero.
A UnitArray uses a Numpy ndarray as its value.
>>> from numpy import linspace >>> a = UnitArray(linspace(0, 5, 6), units="cm") >>> a UnitArray([ 0., 1., 2., 3., 4., 5.], units='0.01*m')
UnitArrays can be multiplied by UnitScalars or other UnitArrays, as in NumPy.:
>>> a * b UnitArray([0., 1., 2., 3., 4., 5.], units='0.0030480000000000004*m**2')
Note that part of the value is contained in the unit string.
For high-performance computation with UnitArrays use unitted functions.