# Quantity¶

class simtk.unit.quantity.Quantity(value=None, unit=None)

Physical quantity, such as 1.3 meters per second.

Quantities contain both a value, such as 1.3; and a unit, such as ‘meters per second’.

Supported value types include:

1 - numbers (float, int, long) 2 - lists of numbers, e.g. [1,2,3] 3 - tuples of numbers, e.g. (1,2,3)

Note - unit conversions will cause tuples to be converted to lists

4 - lists of tuples of numbers, lists of lists of ... etc. of numbers 5 - numpy.arrays

__init__(value=None, unit=None)

Create a new Quantity from a value and a unit.

Parameters
• value: (any type, usually a number) Measure of this quantity
• unit: (Unit) the physical unit, e.g. simtk.unit.meters.

Methods

 __init__([value, unit]) Create a new Quantity from a value and a unit. append(item) count(item) extend(rhs) format(format_spec) in_unit_system(system) Returns a new Quantity equal to this one, expressed in a particular unit system. in_units_of(other_unit) Returns an equal Quantity expressed in different units. index(item) insert(index, item) max(*args, **kwargs) Computes the maximum value of the sequence, with the result having the same unit as the current sequence. mean(*args, **kwargs) Computes the mean of a sequence, with the result having the same unit as the current sequence. min(*args, **kwargs) Computes the minimum value of the sequence, with the result having the same unit as the current sequence. pop(*args) reduce_unit([guide_unit]) Combine similar component units and scale, to form an equal Quantity in simpler units. remove(item) reshape(shape[, order]) Same as numpy.ndarray.reshape, except the result is a Quantity with the sqrt() Returns square root of a Quantity. std(*args, **kwargs) Computes the square root of the variance of a sequence, with the result having the same unit as the current sequence. sum(*args, **kwargs) Computes the sum of a sequence, with the result having the same unit as the current sequence. value_in_unit(unit) Returns underlying value, in the specified units. value_in_unit_system(system) Returns the underlying value type, after conversion to a particular unit system.
__copy__()

Shallow copy produces a new Quantity with the shallow copy of value and the same unit. Because we want copy operations to work just the same way they would on the underlying value.

__deepcopy__(memo)

Deep copy produces a new Quantity with a deep copy of the value, and the same unit. Because we want copy operations to work just the same way they would on the underlying value.

__getattr__(attribute)

Delegate unrecognized attribute calls to the underlying value type.

__str__()

Printable string version of this Quantity.

Returns a string consisting of quantity number followed by unit abbreviation.

__repr__()
__add__(other)

Only Quantities with the same dimensions (e.g. length) can be added. Raises TypeError otherwise.

Parameters
• self: left hand member of sum
• other: right hand member of sum

Returns a new Quantity that is the sum of the two arguments.

__sub__(other)

Subtract two Quantities.

Only Quantities with the same dimensions (e.g. length) can be subtracted. Raises TypeError otherwise.

Parameters
• self: left hand member (a) of a - b.
• other: right hand member (b) of a - b.

Returns a new Quantity that is the difference of the two arguments.

__eq__(other)
__ne__(other)
reduce_unit(guide_unit=None)

Combine similar component units and scale, to form an equal Quantity in simpler units.

Returns underlying value type if unit is dimensionless.

__mul__(other)

Multiply a quantity by another object

Returns a new Quantity that is the product of the self * other, unless the resulting unit is dimensionless, in which case the underlying value type is returned, instead of a Quantity.

__rmul__(other)

Multiply a scalar by a Quantity

Returns a new Quantity with the same units as self, but with the value multiplied by other.

__truediv__(other)

Divide a Quantity by another object

Returns a new Quantity, unless the resulting unit type is dimensionless, in which case the underlying value type is returned.

__div__(other)

Divide a Quantity by another object

Returns a new Quantity, unless the resulting unit type is dimensionless, in which case the underlying value type is returned.

__rtruediv__(other)

Divide a scalar by a quantity.

Returns a new Quantity. The resulting units are the inverse of the self argument units.

__rdiv__(other)

Divide a scalar by a quantity.

Returns a new Quantity. The resulting units are the inverse of the self argument units.

__pow__(exponent)

Raise a Quantity to a power.

Generally both the value and the unit of the Quantity are affected by this operation.

Returns a new Quantity equal to self**exponent.

sqrt()

Returns square root of a Quantity.

Raises ArithmeticError if component exponents are not even. This behavior can be changed if you present a reasonable real life case to me.

sum(*args, **kwargs)

Computes the sum of a sequence, with the result having the same unit as the current sequence.

If the value is not iterable, it raises a TypeError (same behavior as if you tried to iterate over, for instance, an integer).

This function can take as arguments any arguments recognized by numpy.sum. If arguments are passed to a non-numpy array, a TypeError is raised

mean(*args, **kwargs)

Computes the mean of a sequence, with the result having the same unit as the current sequence.

If the value is not iterable, it raises a TypeError

This function can take as arguments any arguments recognized by numpy.mean. If arguments are passed to a non-numpy array, a TypeError is raised

std(*args, **kwargs)

Computes the square root of the variance of a sequence, with the result having the same unit as the current sequence.

If the value is not iterable, it raises a TypeError

This function can take as arguments any arguments recognized by numpy.std. If arguments are passed to a non-numpy array, a TypeError is raised

max(*args, **kwargs)

Computes the maximum value of the sequence, with the result having the same unit as the current sequence.

If the value is not iterable, it raises a TypeError

This function can take as arguments any arguments recognized by numpy.max. If arguments are passed to a non-numpy array, a TypeError is raised

min(*args, **kwargs)

Computes the minimum value of the sequence, with the result having the same unit as the current sequence.

If the value is not iterable, it raises a TypeError

This function can take as arguments any arguments recognized by numpy.min. If arguments are passed to a non-numpy array, a TypeError is raised

reshape(shape, order='C')

Same as numpy.ndarray.reshape, except the result is a Quantity with the same units as the current object rather than a plain numpy.ndarray

__abs__()

Return absolute value of a Quantity.

The unit is unchanged. A negative value of self will result in a positive value in the result.

__pos__()

Returns a reference to self.

__neg__()

Negate a Quantity.

Returns a new Quantity with a different sign on the value.

__nonzero__()

Returns True if value underlying Quantity is zero, False otherwise.

value_in_unit(unit)

Returns underlying value, in the specified units.

value_in_unit_system(system)

Returns the underlying value type, after conversion to a particular unit system.

in_unit_system(system)

Returns a new Quantity equal to this one, expressed in a particular unit system.

in_units_of(other_unit)

Returns an equal Quantity expressed in different units.

If the units are the same as those in self, a reference to self is returned. Raises a TypeError if the new unit is not compatible with the original unit.

The post_multiply argument is used in case the multiplication operation is not commutative.
i.e. result = factor * value when post_multiply is False and result = value * factor when post_multiply is True
__len__()

Return size of internal value type.

__getitem__(key)

Keep the same units on contained elements.