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1237 lines (996 loc) · 34.3 KB
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""" Basic functions for manipulating 2d arrays
"""
import functools
import operator
import os
import warnings
from numpy._core import iinfo, overrides
from numpy._core._multiarray_umath import _array_converter
from numpy._core.numeric import (
arange,
asanyarray,
asarray,
diagonal,
empty,
greater_equal,
indices,
int8,
int16,
int32,
int64,
intp,
multiply,
nonzero,
ones,
promote_types,
where,
zeros,
)
from numpy._core.overrides import finalize_array_function_like, set_module
from numpy.lib._stride_tricks_impl import broadcast_to
__all__ = [
'diag', 'diagflat', 'eye', 'fliplr', 'flipud', 'tri', 'triu',
'tril', 'vander', 'histogram2d', 'mask_indices', 'tril_indices',
'tril_indices_from', 'triu_indices', 'triu_indices_from', ]
array_function_dispatch = functools.partial(
overrides.array_function_dispatch, module='numpy')
i1 = iinfo(int8)
i2 = iinfo(int16)
i4 = iinfo(int32)
def _min_int(low, high):
""" get small int that fits the range """
if high <= i1.max and low >= i1.min:
return int8
if high <= i2.max and low >= i2.min:
return int16
if high <= i4.max and low >= i4.min:
return int32
return int64
def _flip_dispatcher(m):
return (m,)
@array_function_dispatch(_flip_dispatcher)
def fliplr(m):
"""
Reverse the order of elements along axis 1 (left/right).
For a 2-D array, this flips the entries in each row in the left/right
direction. Columns are preserved, but appear in a different order than
before.
Parameters
----------
m : array_like
Input array, must be at least 2-D.
Returns
-------
f : ndarray
A view of `m` with the columns reversed. Since a view
is returned, this operation is :math:`\\mathcal O(1)`.
See Also
--------
flipud : Flip array in the up/down direction.
flip : Flip array in one or more dimensions.
rot90 : Rotate array counterclockwise.
Notes
-----
Equivalent to ``m[:,::-1]`` or ``np.flip(m, axis=1)``.
Requires the array to be at least 2-D.
Examples
--------
>>> import numpy as np
>>> A = np.diag([1.,2.,3.])
>>> A
array([[1., 0., 0.],
[0., 2., 0.],
[0., 0., 3.]])
>>> np.fliplr(A)
array([[0., 0., 1.],
[0., 2., 0.],
[3., 0., 0.]])
>>> rng = np.random.default_rng()
>>> A = rng.normal(size=(2,3,5))
>>> np.all(np.fliplr(A) == A[:,::-1,...])
True
"""
m = asanyarray(m)
if m.ndim < 2:
raise ValueError("Input must be >= 2-d.")
return m[:, ::-1]
@array_function_dispatch(_flip_dispatcher)
def flipud(m):
"""
Reverse the order of elements along axis 0 (up/down).
For a 2-D array, this flips the entries in each column in the up/down
direction. Rows are preserved, but appear in a different order than before.
Parameters
----------
m : array_like
Input array.
Returns
-------
out : array_like
A view of `m` with the rows reversed. Since a view is
returned, this operation is :math:`\\mathcal O(1)`.
See Also
--------
fliplr : Flip array in the left/right direction.
flip : Flip array in one or more dimensions.
rot90 : Rotate array counterclockwise.
Notes
-----
Equivalent to ``m[::-1, ...]`` or ``np.flip(m, axis=0)``.
Requires the array to be at least 1-D.
Examples
--------
>>> import numpy as np
>>> A = np.diag([1.0, 2, 3])
>>> A
array([[1., 0., 0.],
[0., 2., 0.],
[0., 0., 3.]])
>>> np.flipud(A)
array([[0., 0., 3.],
[0., 2., 0.],
[1., 0., 0.]])
>>> rng = np.random.default_rng()
>>> A = rng.normal(size=(2,3,5))
>>> np.all(np.flipud(A) == A[::-1,...])
True
>>> np.flipud([1,2])
array([2, 1])
"""
m = asanyarray(m)
if m.ndim < 1:
raise ValueError("Input must be >= 1-d.")
return m[::-1, ...]
@finalize_array_function_like
@set_module('numpy')
def eye(N, M=None, k=0, dtype=float, order='C', *, device=None, like=None):
"""
Return a 2-D array with ones on the diagonal and zeros elsewhere.
Parameters
----------
N : int
Number of rows in the output.
M : int, optional
Number of columns in the output. If None, defaults to `N`.
k : int, optional
Index of the diagonal: 0 (the default) refers to the main diagonal,
a positive value refers to an upper diagonal, and a negative value
to a lower diagonal.
dtype : data-type, optional
Data-type of the returned array.
order : {'C', 'F'}, optional
Whether the output should be stored in row-major (C-style) or
column-major (Fortran-style) order in memory.
device : str, optional
The device on which to place the created array. Default: None.
For Array-API interoperability only, so must be ``"cpu"`` if passed.
.. versionadded:: 2.0.0
${ARRAY_FUNCTION_LIKE}
Returns
-------
I : ndarray of shape (N,M)
An array where all elements are equal to zero, except for the `k`-th
diagonal, whose values are equal to one.
See Also
--------
identity : (almost) equivalent function
diag : diagonal 2-D array from a 1-D array specified by the user.
Examples
--------
>>> import numpy as np
>>> np.eye(2, dtype=np.int_)
array([[1, 0],
[0, 1]])
>>> np.eye(3, k=1)
array([[0., 1., 0.],
[0., 0., 1.],
[0., 0., 0.]])
"""
if like is not None:
return _eye_with_like(
like, N, M=M, k=k, dtype=dtype, order=order, device=device
)
if M is None:
M = N
m = zeros((N, M), dtype=dtype, order=order, device=device)
if k >= M:
return m
# Ensure M and k are integers, so we don't get any surprise casting
# results in the expressions `M-k` and `M+1` used below. This avoids
# a problem with inputs with type (for example) np.uint64.
M = operator.index(M)
k = operator.index(k)
if k >= 0:
i = k
else:
i = (-k) * M
m[:M - k].flat[i::M + 1] = 1
return m
_eye_with_like = array_function_dispatch()(eye)
def _diag_dispatcher(v, k=None):
return (v,)
@array_function_dispatch(_diag_dispatcher)
def diag(v, k=0):
"""
Extract a diagonal or construct a diagonal array.
See the more detailed documentation for ``numpy.diagonal`` if you use this
function to extract a diagonal and wish to write to the resulting array;
whether it returns a copy or a view depends on what version of numpy you
are using.
Parameters
----------
v : array_like
If `v` is a 2-D array, return a copy of its `k`-th diagonal.
If `v` is a 1-D array, return a 2-D array with `v` on the `k`-th
diagonal.
k : int, optional
Diagonal in question. The default is 0. Use `k>0` for diagonals
above the main diagonal, and `k<0` for diagonals below the main
diagonal.
Returns
-------
out : ndarray
The extracted diagonal or constructed diagonal array.
See Also
--------
diagonal : Return specified diagonals.
diagflat : Create a 2-D array with the flattened input as a diagonal.
trace : Sum along diagonals.
triu : Upper triangle of an array.
tril : Lower triangle of an array.
Examples
--------
>>> import numpy as np
>>> x = np.arange(9).reshape((3,3))
>>> x
array([[0, 1, 2],
[3, 4, 5],
[6, 7, 8]])
>>> np.diag(x)
array([0, 4, 8])
>>> np.diag(x, k=1)
array([1, 5])
>>> np.diag(x, k=-1)
array([3, 7])
>>> np.diag(np.diag(x))
array([[0, 0, 0],
[0, 4, 0],
[0, 0, 8]])
"""
v = asanyarray(v)
s = v.shape
if len(s) == 1:
n = s[0] + abs(k)
res = zeros((n, n), v.dtype)
if k >= 0:
i = k
else:
i = (-k) * n
res[:n - k].flat[i::n + 1] = v
return res
elif len(s) == 2:
return diagonal(v, k)
else:
raise ValueError("Input must be 1- or 2-d.")
@array_function_dispatch(_diag_dispatcher)
def diagflat(v, k=0):
"""
Create a two-dimensional array with the flattened input as a diagonal.
Parameters
----------
v : array_like
Input data, which is flattened and set as the `k`-th
diagonal of the output.
k : int, optional
Diagonal to set; 0, the default, corresponds to the "main" diagonal,
a positive (negative) `k` giving the number of the diagonal above
(below) the main.
Returns
-------
out : ndarray
The 2-D output array.
See Also
--------
diag : MATLAB work-alike for 1-D and 2-D arrays.
diagonal : Return specified diagonals.
trace : Sum along diagonals.
Examples
--------
>>> import numpy as np
>>> np.diagflat([[1,2], [3,4]])
array([[1, 0, 0, 0],
[0, 2, 0, 0],
[0, 0, 3, 0],
[0, 0, 0, 4]])
>>> np.diagflat([1,2], 1)
array([[0, 1, 0],
[0, 0, 2],
[0, 0, 0]])
"""
conv = _array_converter(v)
v, = conv.as_arrays(subok=False)
v = v.ravel()
s = len(v)
n = s + abs(k)
res = zeros((n, n), v.dtype)
if (k >= 0):
i = arange(0, n - k, dtype=intp)
fi = i + k + i * n
else:
i = arange(0, n + k, dtype=intp)
fi = i + (i - k) * n
res.flat[fi] = v
return conv.wrap(res)
@finalize_array_function_like
@set_module('numpy')
def tri(N, M=None, k=0, dtype=float, *, like=None):
"""
An array with ones at and below the given diagonal and zeros elsewhere.
Parameters
----------
N : int
Number of rows in the array.
M : int, optional
Number of columns in the array.
By default, `M` is taken equal to `N`.
k : int, optional
The sub-diagonal at and below which the array is filled.
`k` = 0 is the main diagonal, while `k` < 0 is below it,
and `k` > 0 is above. The default is 0.
dtype : dtype, optional
Data type of the returned array. The default is float.
${ARRAY_FUNCTION_LIKE}
Returns
-------
tri : ndarray of shape (N, M)
Array with its lower triangle filled with ones and zero elsewhere;
in other words ``T[i,j] == 1`` for ``j <= i + k``, 0 otherwise.
Examples
--------
>>> import numpy as np
>>> np.tri(3, 5, 2, dtype=np.int_)
array([[1, 1, 1, 0, 0],
[1, 1, 1, 1, 0],
[1, 1, 1, 1, 1]])
>>> np.tri(3, 5, -1)
array([[0., 0., 0., 0., 0.],
[1., 0., 0., 0., 0.],
[1., 1., 0., 0., 0.]])
"""
warning_for_type = None
try:
N = operator.index(N)
except TypeError:
warning_for_type = warning_for_type or type(N)
if like is not None:
return _tri_with_like(like, N, M=M, k=k, dtype=dtype)
if M is None:
M = N
else:
try:
M = operator.index(M)
except TypeError:
warning_for_type = warning_for_type or type(M)
try:
k = operator.index(k)
except TypeError:
warning_for_type = warning_for_type or type(k)
m = greater_equal.outer(arange(N, dtype=_min_int(0, N)),
arange(-k, M - k, dtype=_min_int(-k, M - k)))
# Avoid making a copy if the requested type is already bool
m = m.astype(dtype, copy=False)
# Deprecation in NumPy 2.5, 2026-03
if warning_for_type:
warnings.warn(
(f"Cannot convert {(warning_for_type).__name__} safely to an integer. "
"This will raise an error in future versions (Deprecated NumPy 2.5)"),
DeprecationWarning,
skip_file_prefixes=(os.path.dirname(__file__),),
)
return m
_tri_with_like = array_function_dispatch()(tri)
def _trilu_dispatcher(m, k=None):
return (m,)
@array_function_dispatch(_trilu_dispatcher)
def tril(m, k=0):
"""
Lower triangle of an array.
Return a copy of an array with elements above the `k`-th diagonal zeroed.
For arrays with ``ndim`` exceeding 2, `tril` will apply to the final two
axes.
Parameters
----------
m : array_like, shape (..., M, N)
Input array.
k : int, optional
Diagonal above which to zero elements. `k = 0` (the default) is the
main diagonal, `k < 0` is below it and `k > 0` is above.
Returns
-------
tril : ndarray, shape (..., M, N)
Lower triangle of `m`, of same shape and data-type as `m`.
See Also
--------
triu : same thing, only for the upper triangle
Examples
--------
>>> import numpy as np
>>> np.tril([[1,2,3],[4,5,6],[7,8,9],[10,11,12]], -1)
array([[ 0, 0, 0],
[ 4, 0, 0],
[ 7, 8, 0],
[10, 11, 12]])
>>> np.tril(np.arange(3*4*5).reshape(3, 4, 5))
array([[[ 0, 0, 0, 0, 0],
[ 5, 6, 0, 0, 0],
[10, 11, 12, 0, 0],
[15, 16, 17, 18, 0]],
[[20, 0, 0, 0, 0],
[25, 26, 0, 0, 0],
[30, 31, 32, 0, 0],
[35, 36, 37, 38, 0]],
[[40, 0, 0, 0, 0],
[45, 46, 0, 0, 0],
[50, 51, 52, 0, 0],
[55, 56, 57, 58, 0]]])
"""
m = asanyarray(m)
mask = tri(*m.shape[-2:], k=k, dtype=bool)
return where(mask, m, zeros(1, m.dtype))
@array_function_dispatch(_trilu_dispatcher)
def triu(m, k=0):
"""
Upper triangle of an array.
Return a copy of an array with the elements below the `k`-th diagonal
zeroed. For arrays with ``ndim`` exceeding 2, `triu` will apply to the
final two axes.
Please refer to the documentation for `tril` for further details.
See Also
--------
tril : lower triangle of an array
Examples
--------
>>> import numpy as np
>>> np.triu([[1,2,3],[4,5,6],[7,8,9],[10,11,12]], -1)
array([[ 1, 2, 3],
[ 4, 5, 6],
[ 0, 8, 9],
[ 0, 0, 12]])
>>> np.triu(np.arange(3*4*5).reshape(3, 4, 5))
array([[[ 0, 1, 2, 3, 4],
[ 0, 6, 7, 8, 9],
[ 0, 0, 12, 13, 14],
[ 0, 0, 0, 18, 19]],
[[20, 21, 22, 23, 24],
[ 0, 26, 27, 28, 29],
[ 0, 0, 32, 33, 34],
[ 0, 0, 0, 38, 39]],
[[40, 41, 42, 43, 44],
[ 0, 46, 47, 48, 49],
[ 0, 0, 52, 53, 54],
[ 0, 0, 0, 58, 59]]])
"""
m = asanyarray(m)
mask = tri(*m.shape[-2:], k=k - 1, dtype=bool)
return where(mask, zeros(1, m.dtype), m)
def _vander_dispatcher(x, N=None, increasing=None):
return (x,)
# Originally borrowed from John Hunter and matplotlib
@array_function_dispatch(_vander_dispatcher)
def vander(x, N=None, increasing=False):
"""
Generate a Vandermonde matrix.
The columns of the output matrix are powers of the input vector. The
order of the powers is determined by the `increasing` boolean argument.
Specifically, when `increasing` is False, the `i`-th output column is
the input vector raised element-wise to the power of ``N - i - 1``. Such
a matrix with a geometric progression in each row is named for Alexandre-
Theophile Vandermonde.
Parameters
----------
x : array_like
1-D input array.
N : int, optional
Number of columns in the output. If `N` is not specified, a square
array is returned (``N = len(x)``).
increasing : bool, optional
Order of the powers of the columns. If True, the powers increase
from left to right, if False (the default) they are reversed.
Returns
-------
out : ndarray
Vandermonde matrix. If `increasing` is False, the first column is
``x^(N-1)``, the second ``x^(N-2)`` and so forth. If `increasing` is
True, the columns are ``x^0, x^1, ..., x^(N-1)``.
See Also
--------
polynomial.polynomial.polyvander
Examples
--------
>>> import numpy as np
>>> x = np.array([1, 2, 3, 5])
>>> N = 3
>>> np.vander(x, N)
array([[ 1, 1, 1],
[ 4, 2, 1],
[ 9, 3, 1],
[25, 5, 1]])
>>> np.column_stack([x**(N-1-i) for i in range(N)])
array([[ 1, 1, 1],
[ 4, 2, 1],
[ 9, 3, 1],
[25, 5, 1]])
>>> x = np.array([1, 2, 3, 5])
>>> np.vander(x)
array([[ 1, 1, 1, 1],
[ 8, 4, 2, 1],
[ 27, 9, 3, 1],
[125, 25, 5, 1]])
>>> np.vander(x, increasing=True)
array([[ 1, 1, 1, 1],
[ 1, 2, 4, 8],
[ 1, 3, 9, 27],
[ 1, 5, 25, 125]])
The determinant of a square Vandermonde matrix is the product
of the differences between the values of the input vector:
>>> np.linalg.det(np.vander(x))
48.000000000000043 # may vary
>>> (5-3)*(5-2)*(5-1)*(3-2)*(3-1)*(2-1)
48
"""
x = asarray(x)
if x.ndim != 1:
raise ValueError("x must be a one-dimensional array or sequence.")
if N is None:
N = len(x)
v = empty((len(x), N), dtype=promote_types(x.dtype, int))
tmp = v[:, ::-1] if not increasing else v
if N > 0:
tmp[:, 0] = 1
if N > 1:
tmp[:, 1:] = x[:, None]
multiply.accumulate(tmp[:, 1:], out=tmp[:, 1:], axis=1)
return v
def _histogram2d_dispatcher(x, y, bins=None, range=None, density=None,
weights=None):
yield x
yield y
# This terrible logic is adapted from the checks in histogram2d
try:
N = len(bins)
except TypeError:
N = 1
if N == 2:
yield from bins # bins=[x, y]
else:
yield bins
yield weights
@array_function_dispatch(_histogram2d_dispatcher)
def histogram2d(x, y, bins=10, range=None, density=None, weights=None):
"""
Compute the bi-dimensional histogram of two data samples.
Parameters
----------
x : array_like, shape (N,)
An array containing the x coordinates of the points to be
histogrammed.
y : array_like, shape (N,)
An array containing the y coordinates of the points to be
histogrammed.
bins : int or array_like or [int, int] or [array, array], optional
The bin specification:
* If int, the number of bins for the two dimensions (nx=ny=bins).
* If array_like, the bin edges for the two dimensions
(x_edges=y_edges=bins).
* If [int, int], the number of bins in each dimension
(nx, ny = bins).
* If [array, array], the bin edges in each dimension
(x_edges, y_edges = bins).
* A combination [int, array] or [array, int], where int
is the number of bins and array is the bin edges.
range : array_like, shape(2,2), optional
The leftmost and rightmost edges of the bins along each dimension
(if not specified explicitly in the `bins` parameters):
``[[xmin, xmax], [ymin, ymax]]``. All values outside of this range
will be considered outliers and not tallied in the histogram.
density : bool, optional
If False, the default, returns the number of samples in each bin.
If True, returns the probability *density* function at the bin,
``bin_count / sample_count / bin_area``.
weights : array_like, shape(N,), optional
An array of values ``w_i`` weighing each sample ``(x_i, y_i)``.
Weights are normalized to 1 if `density` is True. If `density` is
False, the values of the returned histogram are equal to the sum of
the weights belonging to the samples falling into each bin.
Returns
-------
H : ndarray, shape(nx, ny)
The bi-dimensional histogram of samples `x` and `y`. Values in `x`
are histogrammed along the first dimension and values in `y` are
histogrammed along the second dimension.
xedges : ndarray, shape(nx+1,)
The bin edges along the first dimension.
yedges : ndarray, shape(ny+1,)
The bin edges along the second dimension.
See Also
--------
histogram : 1D histogram
histogramdd : Multidimensional histogram
Notes
-----
When `density` is True, then the returned histogram is the sample
density, defined such that the sum over bins of the product
``bin_value * bin_area`` is 1.
Please note that the histogram does not follow the Cartesian convention
where `x` values are on the abscissa and `y` values on the ordinate
axis. Rather, `x` is histogrammed along the first dimension of the
array (vertical), and `y` along the second dimension of the array
(horizontal). This ensures compatibility with `histogramdd`.
Examples
--------
>>> import numpy as np
>>> from matplotlib.image import NonUniformImage
>>> import matplotlib.pyplot as plt
Construct a 2-D histogram with variable bin width. First define the bin
edges:
>>> xedges = [0, 1, 3, 5]
>>> yedges = [0, 2, 3, 4, 6]
Next we create a histogram H with random bin content:
>>> x = np.random.normal(2, 1, 100)
>>> y = np.random.normal(1, 1, 100)
>>> H, xedges, yedges = np.histogram2d(x, y, bins=(xedges, yedges))
>>> # Histogram does not follow Cartesian convention (see Notes),
>>> # therefore transpose H for visualization purposes.
>>> H = H.T
:func:`imshow <matplotlib.pyplot.imshow>` can only display square bins:
>>> fig = plt.figure(figsize=(7, 3))
>>> ax = fig.add_subplot(131, title='imshow: square bins')
>>> plt.imshow(H, interpolation='nearest', origin='lower',
... extent=[xedges[0], xedges[-1], yedges[0], yedges[-1]])
<matplotlib.image.AxesImage object at 0x...>
:func:`pcolormesh <matplotlib.pyplot.pcolormesh>` can display actual edges:
>>> ax = fig.add_subplot(132, title='pcolormesh: actual edges',
... aspect='equal')
>>> X, Y = np.meshgrid(xedges, yedges)
>>> ax.pcolormesh(X, Y, H)
<matplotlib.collections.QuadMesh object at 0x...>
:class:`NonUniformImage <matplotlib.image.NonUniformImage>` can be used to
display actual bin edges with interpolation:
>>> ax = fig.add_subplot(133, title='NonUniformImage: interpolated',
... aspect='equal', xlim=xedges[[0, -1]], ylim=yedges[[0, -1]])
>>> im = NonUniformImage(ax, interpolation='bilinear')
>>> xcenters = (xedges[:-1] + xedges[1:]) / 2
>>> ycenters = (yedges[:-1] + yedges[1:]) / 2
>>> im.set_data(xcenters, ycenters, H)
>>> ax.add_image(im)
>>> plt.show()
It is also possible to construct a 2-D histogram without specifying bin
edges:
>>> # Generate non-symmetric test data
>>> n = 10000
>>> x = np.linspace(1, 100, n)
>>> y = 2*np.log(x) + np.random.rand(n) - 0.5
>>> # Compute 2d histogram. Note the order of x/y and xedges/yedges
>>> H, yedges, xedges = np.histogram2d(y, x, bins=20)
Now we can plot the histogram using
:func:`pcolormesh <matplotlib.pyplot.pcolormesh>`, and a
:func:`hexbin <matplotlib.pyplot.hexbin>` for comparison.
>>> # Plot histogram using pcolormesh
>>> fig, (ax1, ax2) = plt.subplots(ncols=2, sharey=True)
>>> ax1.pcolormesh(xedges, yedges, H, cmap='rainbow')
>>> ax1.plot(x, 2*np.log(x), 'k-')
>>> ax1.set_xlim(x.min(), x.max())
>>> ax1.set_ylim(y.min(), y.max())
>>> ax1.set_xlabel('x')
>>> ax1.set_ylabel('y')
>>> ax1.set_title('histogram2d')
>>> ax1.grid()
>>> # Create hexbin plot for comparison
>>> ax2.hexbin(x, y, gridsize=20, cmap='rainbow')
>>> ax2.plot(x, 2*np.log(x), 'k-')
>>> ax2.set_title('hexbin')
>>> ax2.set_xlim(x.min(), x.max())
>>> ax2.set_xlabel('x')
>>> ax2.grid()
>>> plt.show()
"""
from numpy import histogramdd
if len(x) != len(y):
raise ValueError('x and y must have the same length.')
try:
N = len(bins)
except TypeError:
N = 1
if N not in {1, 2}:
bins = [bins, bins]
hist, edges = histogramdd([x, y], bins, range, density, weights)
return hist, edges[0], edges[1]
@set_module('numpy')
def mask_indices(n, mask_func, k=0):
"""
Return the indices to access (n, n) arrays, given a masking function.
Assume `mask_func` is a function that, for a square array a of size
``(n, n)`` with a possible offset argument `k`, when called as
``mask_func(a, k)`` returns a new array with zeros in certain locations
(functions like `triu` or `tril` do precisely this). Then this function
returns the indices where the non-zero values would be located.
Parameters
----------
n : int
The returned indices will be valid to access arrays of shape (n, n).
mask_func : callable
A function whose call signature is similar to that of `triu`, `tril`.
That is, ``mask_func(x, k)`` returns a boolean array, shaped like `x`.
`k` is an optional argument to the function.
k : scalar, optional
An optional argument which is passed through to `mask_func`. Functions
like `triu`, `tril` take a second argument that is interpreted as an
offset.
Returns
-------
indices : tuple of arrays.
The `n` arrays of indices corresponding to the locations where
``mask_func(np.ones((n, n)), k)`` is True.
See Also
--------
triu, tril, triu_indices, tril_indices
Examples
--------
>>> import numpy as np
These are the indices that would allow you to access the upper triangular
part of any 3x3 array:
>>> iu = np.mask_indices(3, np.triu)
For example, if `a` is a 3x3 array:
>>> a = np.arange(9).reshape(3, 3)
>>> a
array([[0, 1, 2],
[3, 4, 5],
[6, 7, 8]])
>>> a[iu]
array([0, 1, 2, 4, 5, 8])
An offset can be passed also to the masking function. This gets us the
indices starting on the first diagonal right of the main one:
>>> iu1 = np.mask_indices(3, np.triu, 1)
with which we now extract only three elements:
>>> a[iu1]
array([1, 2, 5])
"""
m = ones((n, n), int)
a = mask_func(m, k)
return nonzero(a != 0)
@set_module('numpy')
def tril_indices(n, k=0, m=None):
"""
Return the indices for the lower-triangle of an (n, m) array.
Parameters
----------
n : int
The row dimension of the arrays for which the returned
indices will be valid.
k : int, optional
Diagonal offset (see `tril` for details).
m : int, optional
The column dimension of the arrays for which the returned
arrays will be valid.
By default `m` is taken equal to `n`.
Returns
-------
inds : tuple of arrays
The row and column indices, respectively. The row indices are sorted
in non-decreasing order, and the corresponding column indices are
strictly increasing for each row.
See also
--------
triu_indices : similar function, for upper-triangular.
mask_indices : generic function accepting an arbitrary mask function.
tril, triu
Examples
--------
>>> import numpy as np
Compute two different sets of indices to access 4x4 arrays, one for the
lower triangular part starting at the main diagonal, and one starting two
diagonals further right:
>>> il1 = np.tril_indices(4)
>>> il1
(array([0, 1, 1, 2, 2, 2, 3, 3, 3, 3]), array([0, 0, 1, 0, 1, 2, 0, 1, 2, 3]))
Note that row indices (first array) are non-decreasing, and the corresponding
column indices (second array) are strictly increasing for each row.
Here is how they can be used with a sample array:
>>> a = np.arange(16).reshape(4, 4)
>>> a
array([[ 0, 1, 2, 3],
[ 4, 5, 6, 7],
[ 8, 9, 10, 11],
[12, 13, 14, 15]])
Both for indexing:
>>> a[il1]
array([ 0, 4, 5, ..., 13, 14, 15])
And for assigning values:
>>> a[il1] = -1
>>> a
array([[-1, 1, 2, 3],
[-1, -1, 6, 7],
[-1, -1, -1, 11],
[-1, -1, -1, -1]])
These cover almost the whole array (two diagonals right of the main one):
>>> il2 = np.tril_indices(4, 2)
>>> a[il2] = -10