Как найти значение в массиве numpy

Single element indexing#

Single element indexing works
exactly like that for other standard Python sequences. It is 0-based,
and accepts negative indices for indexing from the end of the array.

>>> x = np.arange(10)
>>> x[2]
2
>>> x[-2]
8

It is not necessary to
separate each dimension’s index into its own set of square brackets.

>>> x.shape = (2, 5)  # now x is 2-dimensional
>>> x[1, 3]
8
>>> x[1, -1]
9

Note that if one indexes a multidimensional array with fewer indices
than dimensions, one gets a subdimensional array. For example:

>>> x[0]
array([0, 1, 2, 3, 4])

That is, each index specified selects the array corresponding to the
rest of the dimensions selected. In the above example, choosing 0
means that the remaining dimension of length 5 is being left unspecified,
and that what is returned is an array of that dimensionality and size.
It must be noted that the returned array is a view, i.e., it is not a
copy of the original, but points to the same values in memory as does the
original array.
In this case, the 1-D array at the first position (0) is returned.
So using a single index on the returned array, results in a single
element being returned. That is:

So note that x[0, 2] == x[0][2] though the second case is more
inefficient as a new temporary array is created after the first index
that is subsequently indexed by 2.

Slicing and striding#

Basic slicing extends Python’s basic concept of slicing to N
dimensions. Basic slicing occurs when obj is a slice object
(constructed by start:stop:step notation inside of brackets), an
integer, or a tuple of slice objects and integers. Ellipsis
and newaxis objects can be interspersed with these as
well.

The simplest case of indexing with N integers returns an array
scalar representing the corresponding item. As in
Python, all indices are zero-based: for the i-th index (n_i),
the valid range is (0 le n_i < d_i) where (d_i) is the
i-th element of the shape of the array. Negative indices are
interpreted as counting from the end of the array (i.e., if
(n_i < 0), it means (n_i + d_i)).

All arrays generated by basic slicing are always views
of the original array.

The standard rules of sequence slicing apply to basic slicing on a
per-dimension basis (including using a step index). Some useful
concepts to remember include:

• The basic slice syntax is i:j:k where i is the starting index,
  j is the stopping index, and k is the step ((kneq0)).
  This selects the m elements (in the corresponding dimension) with
  index values i, i + k, …, i + (m — 1) k where
  (m = q + (rneq0)) and q and r are the quotient and remainder
  obtained by dividing j — i by k: j — i = q k + r, so that
  i + (m — 1) k < j.
  For example:

  >>> x = np.array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
  >>> x[1:7:2]
  array([1, 3, 5])
  

• Negative i and j are interpreted as n + i and n + j where
  n is the number of elements in the corresponding dimension.
  Negative k makes stepping go towards smaller indices.
  From the above example:

  >>> x[-2:10]
  array([8, 9])
  >>> x[-3:3:-1]
  array([7, 6, 5, 4])
  

• Assume n is the number of elements in the dimension being
  sliced. Then, if i is not given it defaults to 0 for k > 0 and
  n — 1 for k < 0 . If j is not given it defaults to n for k > 0
  and -n-1 for k < 0 . If k is not given it defaults to 1. Note that
  :: is the same as : and means select all indices along this
  axis.
  From the above example:

  >>> x[5:]
  array([5, 6, 7, 8, 9])
  

• If the number of objects in the selection tuple is less than
  N, then : is assumed for any subsequent dimensions.
  For example:

  >>> x = np.array([[[1],[2],[3]], [[4],[5],[6]]])
  >>> x.shape
  (2, 3, 1)
  >>> x[1:2]
  array([[[4],
          [5],
          [6]]])
  

• An integer, i, returns the same values as i:i+1
  except the dimensionality of the returned object is reduced by
  1. In particular, a selection tuple with the p-th
  element an integer (and all other entries :) returns the
  corresponding sub-array with dimension N — 1. If N = 1
  then the returned object is an array scalar. These objects are
  explained in Scalars.

• If the selection tuple has all entries : except the
  p-th entry which is a slice object i:j:k,
  then the returned array has dimension N formed by
  concatenating the sub-arrays returned by integer indexing of
  elements i, i+k, …, i + (m — 1) k < j,

• Basic slicing with more than one non-: entry in the slicing
  tuple, acts like repeated application of slicing using a single
  non-: entry, where the non-: entries are successively taken
  (with all other non-: entries replaced by :). Thus,
  x[ind1, ..., ind2,:] acts like x[ind1][..., ind2, :] under basic
  slicing.

  Warning

  The above is not true for advanced indexing.

• You may use slicing to set values in the array, but (unlike lists) you
  can never grow the array. The size of the value to be set in
  x[obj] = value must be (broadcastable to) the same shape as
  x[obj].

• A slicing tuple can always be constructed as obj
  and used in the x[obj] notation. Slice objects can be used in
  the construction in place of the [start:stop:step]
  notation. For example, x[1:10:5, ::-1] can also be implemented
  as obj = (slice(1, 10, 5), slice(None, None, -1)); x[obj] . This
  can be useful for constructing generic code that works on arrays
  of arbitrary dimensions. See Dealing with variable numbers of indices within programs
  for more information.

Dimensional indexing tools#

There are some tools to facilitate the easy matching of array shapes with
expressions and in assignments.

Ellipsis expands to the number of : objects needed for the
selection tuple to index all dimensions. In most cases, this means that the
length of the expanded selection tuple is x.ndim. There may only be a
single ellipsis present.
From the above example:

>>> x[..., 0]
array([[1, 2, 3],
      [4, 5, 6]])

This is equivalent to:

>>> x[:, :, 0]
array([[1, 2, 3],
      [4, 5, 6]])

Each newaxis object in the selection tuple serves to expand
the dimensions of the resulting selection by one unit-length
dimension. The added dimension is the position of the newaxis
object in the selection tuple. newaxis is an alias for
None, and None can be used in place of this with the same result.
From the above example:

>>> x[:, np.newaxis, :, :].shape
(2, 1, 3, 1)
>>> x[:, None, :, :].shape
(2, 1, 3, 1)

This can be handy to combine two
arrays in a way that otherwise would require explicit reshaping
operations. For example:

>>> x = np.arange(5)
>>> x[:, np.newaxis] + x[np.newaxis, :]
array([[0, 1, 2, 3, 4],
      [1, 2, 3, 4, 5],
      [2, 3, 4, 5, 6],
      [3, 4, 5, 6, 7],
      [4, 5, 6, 7, 8]])

Integer array indexing#

Integer array indexing allows selection of arbitrary items in the array
based on their N-dimensional index. Each integer array represents a number
of indices into that dimension.

Negative values are permitted in the index arrays and work as they do with
single indices or slices:

>>> x = np.arange(10, 1, -1)
>>> x
array([10,  9,  8,  7,  6,  5,  4,  3,  2])
>>> x[np.array([3, 3, 1, 8])]
array([7, 7, 9, 2])
>>> x[np.array([3, 3, -3, 8])]
array([7, 7, 4, 2])

If the index values are out of bounds then an IndexError is thrown:

>>> x = np.array([[1, 2], [3, 4], [5, 6]])
>>> x[np.array([1, -1])]
array([[3, 4],
      [5, 6]])
>>> x[np.array([3, 4])]
Traceback (most recent call last):
  ...
IndexError: index 3 is out of bounds for axis 0 with size 3

When the index consists of as many integer arrays as dimensions of the array
being indexed, the indexing is straightforward, but different from slicing.

Advanced indices always are broadcast and
iterated as one:

result[i_1, ..., i_M] == x[ind_1[i_1, ..., i_M], ind_2[i_1, ..., i_M],
                           ..., ind_N[i_1, ..., i_M]]

Note that the resulting shape is identical to the (broadcast) indexing array
shapes ind_1, ..., ind_N. If the indices cannot be broadcast to the
same shape, an exception IndexError: shape mismatch: indexing arrays could
not be broadcast together with shapes... is raised.

Indexing with multidimensional index arrays tend
to be more unusual uses, but they are permitted, and they are useful for some
problems. We’ll start with the simplest multidimensional case:

>>> y = np.arange(35).reshape(5, 7)
>>> y
array([[ 0,  1,  2,  3,  4,  5,  6],
       [ 7,  8,  9, 10, 11, 12, 13],
       [14, 15, 16, 17, 18, 19, 20],
       [21, 22, 23, 24, 25, 26, 27],
       [28, 29, 30, 31, 32, 33, 34]])
>>> y[np.array([0, 2, 4]), np.array([0, 1, 2])]
array([ 0, 15, 30])

In this case, if the index arrays have a matching shape, and there is an
index array for each dimension of the array being indexed, the resultant
array has the same shape as the index arrays, and the values correspond
to the index set for each position in the index arrays. In this example,
the first index value is 0 for both index arrays, and thus the first value
of the resultant array is y[0, 0]. The next value is y[2, 1], and
the last is y[4, 2].

If the index arrays do not have the same shape, there is an attempt to
broadcast them to the same shape. If they cannot be broadcast to the same
shape, an exception is raised:

>>> y[np.array([0, 2, 4]), np.array([0, 1])]
Traceback (most recent call last):
  ...
IndexError: shape mismatch: indexing arrays could not be broadcast
together with shapes (3,) (2,)

The broadcasting mechanism permits index arrays to be combined with
scalars for other indices. The effect is that the scalar value is used
for all the corresponding values of the index arrays:

>>> y[np.array([0, 2, 4]), 1]
array([ 1, 15, 29])

Jumping to the next level of complexity, it is possible to only partially
index an array with index arrays. It takes a bit of thought to understand
what happens in such cases. For example if we just use one index array
with y:

>>> y[np.array([0, 2, 4])]
array([[ 0,  1,  2,  3,  4,  5,  6],
      [14, 15, 16, 17, 18, 19, 20],
      [28, 29, 30, 31, 32, 33, 34]])

It results in the construction of a new array where each value of the
index array selects one row from the array being indexed and the resultant
array has the resulting shape (number of index elements, size of row).

In general, the shape of the resultant array will be the concatenation of
the shape of the index array (or the shape that all the index arrays were
broadcast to) with the shape of any unused dimensions (those not indexed)
in the array being indexed.

Example

From each row, a specific element should be selected. The row index is just
[0, 1, 2] and the column index specifies the element to choose for the
corresponding row, here [0, 1, 0]. Using both together the task
can be solved using advanced indexing:

>>> x = np.array([[1, 2], [3, 4], [5, 6]])
>>> x[[0, 1, 2], [0, 1, 0]]
array([1, 4, 5])

To achieve a behaviour similar to the basic slicing above, broadcasting can be
used. The function ix_ can help with this broadcasting. This is best
understood with an example.

Example

From a 4×3 array the corner elements should be selected using advanced
indexing. Thus all elements for which the column is one of [0, 2] and
the row is one of [0, 3] need to be selected. To use advanced indexing
one needs to select all elements explicitly. Using the method explained
previously one could write:

>>> x = np.array([[ 0,  1,  2],
...               [ 3,  4,  5],
...               [ 6,  7,  8],
...               [ 9, 10, 11]])
>>> rows = np.array([[0, 0],
...                  [3, 3]], dtype=np.intp)
>>> columns = np.array([[0, 2],
...                     [0, 2]], dtype=np.intp)
>>> x[rows, columns]
array([[ 0,  2],
       [ 9, 11]])

However, since the indexing arrays above just repeat themselves,
broadcasting can be used (compare operations such as
rows[:, np.newaxis] + columns) to simplify this:

>>> rows = np.array([0, 3], dtype=np.intp)
>>> columns = np.array([0, 2], dtype=np.intp)
>>> rows[:, np.newaxis]
array([[0],
       [3]])
>>> x[rows[:, np.newaxis], columns]
array([[ 0,  2],
       [ 9, 11]])

This broadcasting can also be achieved using the function ix_:

>>> x[np.ix_(rows, columns)]
array([[ 0,  2],
       [ 9, 11]])

Note that without the np.ix_ call, only the diagonal elements would
be selected:

>>> x[rows, columns]
array([ 0, 11])

This difference is the most important thing to remember about
indexing with multiple advanced indices.

Example

A real-life example of where advanced indexing may be useful is for a color
lookup table where we want to map the values of an image into RGB triples for
display. The lookup table could have a shape (nlookup, 3). Indexing
such an array with an image with shape (ny, nx) with dtype=np.uint8
(or any integer type so long as values are with the bounds of the
lookup table) will result in an array of shape (ny, nx, 3) where a
triple of RGB values is associated with each pixel location.

Boolean array indexing#

This advanced indexing occurs when obj is an array object of Boolean
type, such as may be returned from comparison operators. A single
boolean index array is practically identical to x[obj.nonzero()] where,
as described above, obj.nonzero() returns a
tuple (of length obj.ndim) of integer index
arrays showing the True elements of obj. However, it is
faster when obj.shape == x.shape.

If obj.ndim == x.ndim, x[obj] returns a 1-dimensional array
filled with the elements of x corresponding to the True
values of obj. The search order will be row-major,
C-style. If obj has True values at entries that are outside
of the bounds of x, then an index error will be raised. If obj is
smaller than x it is identical to filling it with False.

A common use case for this is filtering for desired element values.
For example, one may wish to select all entries from an array which
are not NaN:

>>> x = np.array([[1., 2.], [np.nan, 3.], [np.nan, np.nan]])
>>> x[~np.isnan(x)]
array([1., 2., 3.])

Or wish to add a constant to all negative elements:

>>> x = np.array([1., -1., -2., 3])
>>> x[x < 0] += 20
>>> x
array([ 1., 19., 18., 3.])

In general if an index includes a Boolean array, the result will be
identical to inserting obj.nonzero() into the same position
and using the integer array indexing mechanism described above.
x[ind_1, boolean_array, ind_2] is equivalent to
x[(ind_1,) + boolean_array.nonzero() + (ind_2,)].

If there is only one Boolean array and no integer indexing array present,
this is straightforward. Care must only be taken to make sure that the
boolean index has exactly as many dimensions as it is supposed to work
with.

In general, when the boolean array has fewer dimensions than the array being
indexed, this is equivalent to x[b, ...], which means x is indexed by b
followed by as many : as are needed to fill out the rank of x. Thus the
shape of the result is one dimension containing the number of True elements of
the boolean array, followed by the remaining dimensions of the array being
indexed:

>>> x = np.arange(35).reshape(5, 7)
>>> b = x > 20
>>> b[:, 5]
array([False, False, False,  True,  True])
>>> x[b[:, 5]]
array([[21, 22, 23, 24, 25, 26, 27],
      [28, 29, 30, 31, 32, 33, 34]])

Here the 4th and 5th rows are selected from the indexed array and
combined to make a 2-D array.

Example

From an array, select all rows which sum up to less or equal two:

>>> x = np.array([[0, 1], [1, 1], [2, 2]])
>>> rowsum = x.sum(-1)
>>> x[rowsum <= 2, :]
array([[0, 1],
       [1, 1]])

Combining multiple Boolean indexing arrays or a Boolean with an integer
indexing array can best be understood with the
obj.nonzero() analogy. The function ix_
also supports boolean arrays and will work without any surprises.

Example

Use boolean indexing to select all rows adding up to an even
number. At the same time columns 0 and 2 should be selected with an
advanced integer index. Using the ix_ function this can be done
with:

>>> x = np.array([[ 0,  1,  2],
...               [ 3,  4,  5],
...               [ 6,  7,  8],
...               [ 9, 10, 11]])
>>> rows = (x.sum(-1) % 2) == 0
>>> rows
array([False,  True, False,  True])
>>> columns = [0, 2]
>>> x[np.ix_(rows, columns)]
array([[ 3,  5],
       [ 9, 11]])

Without the np.ix_ call, only the diagonal elements would be
selected.

Or without np.ix_ (compare the integer array examples):

>>> rows = rows.nonzero()[0]
>>> x[rows[:, np.newaxis], columns]
array([[ 3,  5],
       [ 9, 11]])

Example

Use a 2-D boolean array of shape (2, 3)
with four True elements to select rows from a 3-D array of shape
(2, 3, 5) results in a 2-D result of shape (4, 5):

>>> x = np.arange(30).reshape(2, 3, 5)
>>> x
array([[[ 0,  1,  2,  3,  4],
        [ 5,  6,  7,  8,  9],
        [10, 11, 12, 13, 14]],
      [[15, 16, 17, 18, 19],
        [20, 21, 22, 23, 24],
        [25, 26, 27, 28, 29]]])
>>> b = np.array([[True, True, False], [False, True, True]])
>>> x[b]
array([[ 0,  1,  2,  3,  4],
      [ 5,  6,  7,  8,  9],
      [20, 21, 22, 23, 24],
      [25, 26, 27, 28, 29]])

Combining advanced and basic indexing#

When there is at least one slice (:), ellipsis (...) or newaxis
in the index (or the array has more dimensions than there are advanced indices),
then the behaviour can be more complicated. It is like concatenating the
indexing result for each advanced index element.

In the simplest case, there is only a single advanced index combined with
a slice. For example:

>>> y = np.arange(35).reshape(5,7)
>>> y[np.array([0, 2, 4]), 1:3]
array([[ 1,  2],
       [15, 16],
       [29, 30]])

In effect, the slice and index array operation are independent. The slice
operation extracts columns with index 1 and 2, (i.e. the 2nd and 3rd columns),
followed by the index array operation which extracts rows with index 0, 2 and 4
(i.e the first, third and fifth rows). This is equivalent to:

>>> y[:, 1:3][np.array([0, 2, 4]), :]
array([[ 1,  2],
       [15, 16],
       [29, 30]])

A single advanced index can, for example, replace a slice and the result array
will be the same. However, it is a copy and may have a different memory layout.
A slice is preferable when it is possible.
For example:

>>> x = np.array([[ 0,  1,  2],
...               [ 3,  4,  5],
...               [ 6,  7,  8],
...               [ 9, 10, 11]])
>>> x[1:2, 1:3]
array([[4, 5]])
>>> x[1:2, [1, 2]]
array([[4, 5]])

The easiest way to understand a combination of multiple advanced indices may
be to think in terms of the resulting shape. There are two parts to the indexing
operation, the subspace defined by the basic indexing (excluding integers) and
the subspace from the advanced indexing part. Two cases of index combination
need to be distinguished:

• The advanced indices are separated by a slice, Ellipsis or
  newaxis. For example x[arr1, :, arr2].

• The advanced indices are all next to each other.
  For example x[..., arr1, arr2, :] but not x[arr1, :, 1]
  since 1 is an advanced index in this regard.

In the first case, the dimensions resulting from the advanced indexing
operation come first in the result array, and the subspace dimensions after
that.
In the second case, the dimensions from the advanced indexing operations
are inserted into the result array at the same spot as they were in the
initial array (the latter logic is what makes simple advanced indexing
behave just like slicing).

Example

Suppose x.shape is (10, 20, 30) and ind is a (2, 3, 4)-shaped
indexing intp array, then result = x[..., ind, :] has
shape (10, 2, 3, 4, 30) because the (20,)-shaped subspace has been
replaced with a (2, 3, 4)-shaped broadcasted indexing subspace. If
we let i, j, k loop over the (2, 3, 4)-shaped subspace then
result[..., i, j, k, :] = x[..., ind[i, j, k], :]. This example
produces the same result as x.take(ind, axis=-2).

Example

Let x.shape be (10, 20, 30, 40, 50) and suppose ind_1
and ind_2 can be broadcast to the shape (2, 3, 4). Then
x[:, ind_1, ind_2] has shape (10, 2, 3, 4, 40, 50) because the
(20, 30)-shaped subspace from X has been replaced with the
(2, 3, 4) subspace from the indices. However,
x[:, ind_1, :, ind_2] has shape (2, 3, 4, 10, 30, 50) because there
is no unambiguous place to drop in the indexing subspace, thus
it is tacked-on to the beginning. It is always possible to use
.transpose() to move the subspace
anywhere desired. Note that this example cannot be replicated
using take.

Example

Slicing can be combined with broadcasted boolean indices:

>>> x = np.arange(35).reshape(5, 7)
>>> b = x > 20
>>> b
array([[False, False, False, False, False, False, False],
      [False, False, False, False, False, False, False],
      [False, False, False, False, False, False, False],
      [ True,  True,  True,  True,  True,  True,  True],
      [ True,  True,  True,  True,  True,  True,  True]])
>>> x[b[:, 5], 1:3]
array([[22, 23],
      [29, 30]])