in HowTo/gRPC/Linux/OpenAI/LangChain/PyServer/venv/Lib/numpy/lib/function_base.py [0:0]
def gradient(f, *varargs, axis=None, edge_order=1):
"""
Return the gradient of an N-dimensional array.
The gradient is computed using second order accurate central differences
in the interior points and either first or second order accurate one-sides
(forward or backwards) differences at the boundaries.
The returned gradient hence has the same shape as the input array.
Parameters
----------
f : array_like
An N-dimensional array containing samples of a scalar function.
varargs : list of scalar or array, optional
Spacing between f values. Default unitary spacing for all dimensions.
Spacing can be specified using:
1. single scalar to specify a sample distance for all dimensions.
2. N scalars to specify a constant sample distance for each dimension.
i.e. `dx`, `dy`, `dz`, ...
3. N arrays to specify the coordinates of the values along each
dimension of F. The length of the array must match the size of
the corresponding dimension
4. Any combination of N scalars/arrays with the meaning of 2. and 3.
If `axis` is given, the number of varargs must equal the number of axes.
Default: 1.
edge_order : {1, 2}, optional
Gradient is calculated using N-th order accurate differences
at the boundaries. Default: 1.
.. versionadded:: 1.9.1
axis : None or int or tuple of ints, optional
Gradient is calculated only along the given axis or axes
The default (axis = None) is to calculate the gradient for all the axes
of the input array. axis may be negative, in which case it counts from
the last to the first axis.
.. versionadded:: 1.11.0
Returns
-------
gradient : ndarray or list of ndarray
A list of ndarrays (or a single ndarray if there is only one dimension)
corresponding to the derivatives of f with respect to each dimension.
Each derivative has the same shape as f.
Examples
--------
>>> f = np.array([1, 2, 4, 7, 11, 16], dtype=float)
>>> np.gradient(f)
array([1. , 1.5, 2.5, 3.5, 4.5, 5. ])
>>> np.gradient(f, 2)
array([0.5 , 0.75, 1.25, 1.75, 2.25, 2.5 ])
Spacing can be also specified with an array that represents the coordinates
of the values F along the dimensions.
For instance a uniform spacing:
>>> x = np.arange(f.size)
>>> np.gradient(f, x)
array([1. , 1.5, 2.5, 3.5, 4.5, 5. ])
Or a non uniform one:
>>> x = np.array([0., 1., 1.5, 3.5, 4., 6.], dtype=float)
>>> np.gradient(f, x)
array([1. , 3. , 3.5, 6.7, 6.9, 2.5])
For two dimensional arrays, the return will be two arrays ordered by
axis. In this example the first array stands for the gradient in
rows and the second one in columns direction:
>>> np.gradient(np.array([[1, 2, 6], [3, 4, 5]], dtype=float))
[array([[ 2., 2., -1.],
[ 2., 2., -1.]]), array([[1. , 2.5, 4. ],
[1. , 1. , 1. ]])]
In this example the spacing is also specified:
uniform for axis=0 and non uniform for axis=1
>>> dx = 2.
>>> y = [1., 1.5, 3.5]
>>> np.gradient(np.array([[1, 2, 6], [3, 4, 5]], dtype=float), dx, y)
[array([[ 1. , 1. , -0.5],
[ 1. , 1. , -0.5]]), array([[2. , 2. , 2. ],
[2. , 1.7, 0.5]])]
It is possible to specify how boundaries are treated using `edge_order`
>>> x = np.array([0, 1, 2, 3, 4])
>>> f = x**2
>>> np.gradient(f, edge_order=1)
array([1., 2., 4., 6., 7.])
>>> np.gradient(f, edge_order=2)
array([0., 2., 4., 6., 8.])
The `axis` keyword can be used to specify a subset of axes of which the
gradient is calculated
>>> np.gradient(np.array([[1, 2, 6], [3, 4, 5]], dtype=float), axis=0)
array([[ 2., 2., -1.],
[ 2., 2., -1.]])
Notes
-----
Assuming that :math:`f\\in C^{3}` (i.e., :math:`f` has at least 3 continuous
derivatives) and let :math:`h_{*}` be a non-homogeneous stepsize, we
minimize the "consistency error" :math:`\\eta_{i}` between the true gradient
and its estimate from a linear combination of the neighboring grid-points:
.. math::
\\eta_{i} = f_{i}^{\\left(1\\right)} -
\\left[ \\alpha f\\left(x_{i}\\right) +
\\beta f\\left(x_{i} + h_{d}\\right) +
\\gamma f\\left(x_{i}-h_{s}\\right)
\\right]
By substituting :math:`f(x_{i} + h_{d})` and :math:`f(x_{i} - h_{s})`
with their Taylor series expansion, this translates into solving
the following the linear system:
.. math::
\\left\\{
\\begin{array}{r}
\\alpha+\\beta+\\gamma=0 \\\\
\\beta h_{d}-\\gamma h_{s}=1 \\\\
\\beta h_{d}^{2}+\\gamma h_{s}^{2}=0
\\end{array}
\\right.
The resulting approximation of :math:`f_{i}^{(1)}` is the following:
.. math::
\\hat f_{i}^{(1)} =
\\frac{
h_{s}^{2}f\\left(x_{i} + h_{d}\\right)
+ \\left(h_{d}^{2} - h_{s}^{2}\\right)f\\left(x_{i}\\right)
- h_{d}^{2}f\\left(x_{i}-h_{s}\\right)}
{ h_{s}h_{d}\\left(h_{d} + h_{s}\\right)}
+ \\mathcal{O}\\left(\\frac{h_{d}h_{s}^{2}
+ h_{s}h_{d}^{2}}{h_{d}
+ h_{s}}\\right)
It is worth noting that if :math:`h_{s}=h_{d}`
(i.e., data are evenly spaced)
we find the standard second order approximation:
.. math::
\\hat f_{i}^{(1)}=
\\frac{f\\left(x_{i+1}\\right) - f\\left(x_{i-1}\\right)}{2h}
+ \\mathcal{O}\\left(h^{2}\\right)
With a similar procedure the forward/backward approximations used for
boundaries can be derived.
References
----------
.. [1] Quarteroni A., Sacco R., Saleri F. (2007) Numerical Mathematics
(Texts in Applied Mathematics). New York: Springer.
.. [2] Durran D. R. (1999) Numerical Methods for Wave Equations
in Geophysical Fluid Dynamics. New York: Springer.
.. [3] Fornberg B. (1988) Generation of Finite Difference Formulas on
Arbitrarily Spaced Grids,
Mathematics of Computation 51, no. 184 : 699-706.
`PDF <http://www.ams.org/journals/mcom/1988-51-184/
S0025-5718-1988-0935077-0/S0025-5718-1988-0935077-0.pdf>`_.
"""
f = np.asanyarray(f)
N = f.ndim # number of dimensions
if axis is None:
axes = tuple(range(N))
else:
axes = _nx.normalize_axis_tuple(axis, N)
len_axes = len(axes)
n = len(varargs)
if n == 0:
# no spacing argument - use 1 in all axes
dx = [1.0] * len_axes
elif n == 1 and np.ndim(varargs[0]) == 0:
# single scalar for all axes
dx = varargs * len_axes
elif n == len_axes:
# scalar or 1d array for each axis
dx = list(varargs)
for i, distances in enumerate(dx):
distances = np.asanyarray(distances)
if distances.ndim == 0:
continue
elif distances.ndim != 1:
raise ValueError("distances must be either scalars or 1d")
if len(distances) != f.shape[axes[i]]:
raise ValueError("when 1d, distances must match "
"the length of the corresponding dimension")
if np.issubdtype(distances.dtype, np.integer):
# Convert numpy integer types to float64 to avoid modular
# arithmetic in np.diff(distances).
distances = distances.astype(np.float64)
diffx = np.diff(distances)
# if distances are constant reduce to the scalar case
# since it brings a consistent speedup
if (diffx == diffx[0]).all():
diffx = diffx[0]
dx[i] = diffx
else:
raise TypeError("invalid number of arguments")
if edge_order > 2:
raise ValueError("'edge_order' greater than 2 not supported")
# use central differences on interior and one-sided differences on the
# endpoints. This preserves second order-accuracy over the full domain.
outvals = []
# create slice objects --- initially all are [:, :, ..., :]
slice1 = [slice(None)]*N
slice2 = [slice(None)]*N
slice3 = [slice(None)]*N
slice4 = [slice(None)]*N
otype = f.dtype
if otype.type is np.datetime64:
# the timedelta dtype with the same unit information
otype = np.dtype(otype.name.replace('datetime', 'timedelta'))
# view as timedelta to allow addition
f = f.view(otype)
elif otype.type is np.timedelta64:
pass
elif np.issubdtype(otype, np.inexact):
pass
else:
# All other types convert to floating point.
# First check if f is a numpy integer type; if so, convert f to float64
# to avoid modular arithmetic when computing the changes in f.
if np.issubdtype(otype, np.integer):
f = f.astype(np.float64)
otype = np.float64
for axis, ax_dx in zip(axes, dx):
if f.shape[axis] < edge_order + 1:
raise ValueError(
"Shape of array too small to calculate a numerical gradient, "
"at least (edge_order + 1) elements are required.")
# result allocation
out = np.empty_like(f, dtype=otype)
# spacing for the current axis
uniform_spacing = np.ndim(ax_dx) == 0
# Numerical differentiation: 2nd order interior
slice1[axis] = slice(1, -1)
slice2[axis] = slice(None, -2)
slice3[axis] = slice(1, -1)
slice4[axis] = slice(2, None)
if uniform_spacing:
out[tuple(slice1)] = (f[tuple(slice4)] - f[tuple(slice2)]) / (2. * ax_dx)
else:
dx1 = ax_dx[0:-1]
dx2 = ax_dx[1:]
a = -(dx2)/(dx1 * (dx1 + dx2))
b = (dx2 - dx1) / (dx1 * dx2)
c = dx1 / (dx2 * (dx1 + dx2))
# fix the shape for broadcasting
shape = np.ones(N, dtype=int)
shape[axis] = -1
a.shape = b.shape = c.shape = shape
# 1D equivalent -- out[1:-1] = a * f[:-2] + b * f[1:-1] + c * f[2:]
out[tuple(slice1)] = a * f[tuple(slice2)] + b * f[tuple(slice3)] + c * f[tuple(slice4)]
# Numerical differentiation: 1st order edges
if edge_order == 1:
slice1[axis] = 0
slice2[axis] = 1
slice3[axis] = 0
dx_0 = ax_dx if uniform_spacing else ax_dx[0]
# 1D equivalent -- out[0] = (f[1] - f[0]) / (x[1] - x[0])
out[tuple(slice1)] = (f[tuple(slice2)] - f[tuple(slice3)]) / dx_0
slice1[axis] = -1
slice2[axis] = -1
slice3[axis] = -2
dx_n = ax_dx if uniform_spacing else ax_dx[-1]
# 1D equivalent -- out[-1] = (f[-1] - f[-2]) / (x[-1] - x[-2])
out[tuple(slice1)] = (f[tuple(slice2)] - f[tuple(slice3)]) / dx_n
# Numerical differentiation: 2nd order edges
else:
slice1[axis] = 0
slice2[axis] = 0
slice3[axis] = 1
slice4[axis] = 2
if uniform_spacing:
a = -1.5 / ax_dx
b = 2. / ax_dx
c = -0.5 / ax_dx
else:
dx1 = ax_dx[0]
dx2 = ax_dx[1]
a = -(2. * dx1 + dx2)/(dx1 * (dx1 + dx2))
b = (dx1 + dx2) / (dx1 * dx2)
c = - dx1 / (dx2 * (dx1 + dx2))
# 1D equivalent -- out[0] = a * f[0] + b * f[1] + c * f[2]
out[tuple(slice1)] = a * f[tuple(slice2)] + b * f[tuple(slice3)] + c * f[tuple(slice4)]
slice1[axis] = -1
slice2[axis] = -3
slice3[axis] = -2
slice4[axis] = -1
if uniform_spacing:
a = 0.5 / ax_dx
b = -2. / ax_dx
c = 1.5 / ax_dx
else:
dx1 = ax_dx[-2]
dx2 = ax_dx[-1]
a = (dx2) / (dx1 * (dx1 + dx2))
b = - (dx2 + dx1) / (dx1 * dx2)
c = (2. * dx2 + dx1) / (dx2 * (dx1 + dx2))
# 1D equivalent -- out[-1] = a * f[-3] + b * f[-2] + c * f[-1]
out[tuple(slice1)] = a * f[tuple(slice2)] + b * f[tuple(slice3)] + c * f[tuple(slice4)]
outvals.append(out)
# reset the slice object in this dimension to ":"
slice1[axis] = slice(None)
slice2[axis] = slice(None)
slice3[axis] = slice(None)
slice4[axis] = slice(None)
if len_axes == 1:
return outvals[0]
elif np._using_numpy2_behavior():
return tuple(outvals)
else:
return outvals