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475 lines
14 KiB
Python
475 lines
14 KiB
Python
"""
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========
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Glossary
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========
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.. glossary::
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along an axis
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Axes are defined for arrays with more than one dimension. A
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2-dimensional array has two corresponding axes: the first running
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vertically downwards across rows (axis 0), and the second running
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horizontally across columns (axis 1).
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Many operations can take place along one of these axes. For example,
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we can sum each row of an array, in which case we operate along
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columns, or axis 1::
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>>> x = np.arange(12).reshape((3,4))
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>>> x
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array([[ 0, 1, 2, 3],
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[ 4, 5, 6, 7],
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[ 8, 9, 10, 11]])
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>>> x.sum(axis=1)
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array([ 6, 22, 38])
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array
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A homogeneous container of numerical elements. Each element in the
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array occupies a fixed amount of memory (hence homogeneous), and
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can be a numerical element of a single type (such as float, int
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or complex) or a combination (such as ``(float, int, float)``). Each
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array has an associated data-type (or ``dtype``), which describes
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the numerical type of its elements::
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>>> x = np.array([1, 2, 3], float)
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>>> x
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array([ 1., 2., 3.])
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>>> x.dtype # floating point number, 64 bits of memory per element
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dtype('float64')
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# More complicated data type: each array element is a combination of
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# and integer and a floating point number
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>>> np.array([(1, 2.0), (3, 4.0)], dtype=[('x', int), ('y', float)])
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array([(1, 2.0), (3, 4.0)],
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dtype=[('x', '<i4'), ('y', '<f8')])
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Fast element-wise operations, called a :term:`ufunc`, operate on arrays.
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array_like
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Any sequence that can be interpreted as an ndarray. This includes
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nested lists, tuples, scalars and existing arrays.
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attribute
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A property of an object that can be accessed using ``obj.attribute``,
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e.g., ``shape`` is an attribute of an array::
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>>> x = np.array([1, 2, 3])
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>>> x.shape
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(3,)
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big-endian
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When storing a multi-byte value in memory as a sequence of bytes, the
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sequence addresses/sends/stores the most significant byte first (lowest
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address) and the least significant byte last (highest address). Common in
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micro-processors and used for transmission of data over network protocols.
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BLAS
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`Basic Linear Algebra Subprograms <https://en.wikipedia.org/wiki/Basic_Linear_Algebra_Subprograms>`_
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broadcast
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NumPy can do operations on arrays whose shapes are mismatched::
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>>> x = np.array([1, 2])
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>>> y = np.array([[3], [4]])
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>>> x
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array([1, 2])
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>>> y
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array([[3],
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[4]])
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>>> x + y
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array([[4, 5],
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[5, 6]])
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See `numpy.doc.broadcasting` for more information.
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C order
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See `row-major`
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column-major
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A way to represent items in a N-dimensional array in the 1-dimensional
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computer memory. In column-major order, the leftmost index "varies the
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fastest": for example the array::
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[[1, 2, 3],
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[4, 5, 6]]
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is represented in the column-major order as::
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[1, 4, 2, 5, 3, 6]
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Column-major order is also known as the Fortran order, as the Fortran
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programming language uses it.
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decorator
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An operator that transforms a function. For example, a ``log``
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decorator may be defined to print debugging information upon
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function execution::
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>>> def log(f):
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... def new_logging_func(*args, **kwargs):
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... print("Logging call with parameters:", args, kwargs)
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... return f(*args, **kwargs)
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...
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... return new_logging_func
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Now, when we define a function, we can "decorate" it using ``log``::
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>>> @log
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... def add(a, b):
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... return a + b
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Calling ``add`` then yields:
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>>> add(1, 2)
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Logging call with parameters: (1, 2) {}
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3
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dictionary
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Resembling a language dictionary, which provides a mapping between
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words and descriptions thereof, a Python dictionary is a mapping
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between two objects::
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>>> x = {1: 'one', 'two': [1, 2]}
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Here, `x` is a dictionary mapping keys to values, in this case
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the integer 1 to the string "one", and the string "two" to
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the list ``[1, 2]``. The values may be accessed using their
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corresponding keys::
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>>> x[1]
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'one'
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>>> x['two']
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[1, 2]
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Note that dictionaries are not stored in any specific order. Also,
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most mutable (see *immutable* below) objects, such as lists, may not
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be used as keys.
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For more information on dictionaries, read the
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`Python tutorial <https://docs.python.org/tutorial/>`_.
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field
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In a :term:`structured data type`, each sub-type is called a `field`.
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The `field` has a name (a string), a type (any valid dtype), and
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an optional `title`. See :ref:`arrays.dtypes`
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Fortran order
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See `column-major`
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flattened
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Collapsed to a one-dimensional array. See `numpy.ndarray.flatten`
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for details.
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homogenous
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Describes a block of memory comprised of blocks, each block comprised of
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items and of the same size, and blocks are interpreted in exactly the
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same way. In the simplest case each block contains a single item, for
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instance int32 or float64.
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immutable
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An object that cannot be modified after execution is called
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immutable. Two common examples are strings and tuples.
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instance
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A class definition gives the blueprint for constructing an object::
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>>> class House:
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... wall_colour = 'white'
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Yet, we have to *build* a house before it exists::
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>>> h = House() # build a house
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Now, ``h`` is called a ``House`` instance. An instance is therefore
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a specific realisation of a class.
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iterable
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A sequence that allows "walking" (iterating) over items, typically
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using a loop such as::
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>>> x = [1, 2, 3]
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>>> [item**2 for item in x]
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[1, 4, 9]
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It is often used in combination with ``enumerate``::
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>>> keys = ['a','b','c']
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>>> for n, k in enumerate(keys):
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... print("Key %d: %s" % (n, k))
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...
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Key 0: a
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Key 1: b
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Key 2: c
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itemsize
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The size of the dtype element in bytes.
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list
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A Python container that can hold any number of objects or items.
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The items do not have to be of the same type, and can even be
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lists themselves::
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>>> x = [2, 2.0, "two", [2, 2.0]]
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The list `x` contains 4 items, each which can be accessed individually::
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>>> x[2] # the string 'two'
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'two'
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>>> x[3] # a list, containing an integer 2 and a float 2.0
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[2, 2.0]
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It is also possible to select more than one item at a time,
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using *slicing*::
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>>> x[0:2] # or, equivalently, x[:2]
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[2, 2.0]
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In code, arrays are often conveniently expressed as nested lists::
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>>> np.array([[1, 2], [3, 4]])
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array([[1, 2],
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[3, 4]])
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For more information, read the section on lists in the `Python
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tutorial <https://docs.python.org/tutorial/>`_. For a mapping
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type (key-value), see *dictionary*.
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little-endian
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When storing a multi-byte value in memory as a sequence of bytes, the
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sequence addresses/sends/stores the least significant byte first (lowest
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address) and the most significant byte last (highest address). Common in
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x86 processors.
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mask
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A boolean array, used to select only certain elements for an operation::
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>>> x = np.arange(5)
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>>> x
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array([0, 1, 2, 3, 4])
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>>> mask = (x > 2)
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>>> mask
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array([False, False, False, True, True])
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>>> x[mask] = -1
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>>> x
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array([ 0, 1, 2, -1, -1])
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masked array
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Array that suppressed values indicated by a mask::
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>>> x = np.ma.masked_array([np.nan, 2, np.nan], [True, False, True])
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>>> x
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masked_array(data = [-- 2.0 --],
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mask = [ True False True],
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fill_value = 1e+20)
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>>> x + [1, 2, 3]
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masked_array(data = [-- 4.0 --],
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mask = [ True False True],
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fill_value = 1e+20)
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Masked arrays are often used when operating on arrays containing
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missing or invalid entries.
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matrix
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A 2-dimensional ndarray that preserves its two-dimensional nature
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throughout operations. It has certain special operations, such as ``*``
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(matrix multiplication) and ``**`` (matrix power), defined::
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>>> x = np.mat([[1, 2], [3, 4]])
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>>> x
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matrix([[1, 2],
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[3, 4]])
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>>> x**2
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matrix([[ 7, 10],
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[15, 22]])
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method
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A function associated with an object. For example, each ndarray has a
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method called ``repeat``::
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>>> x = np.array([1, 2, 3])
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>>> x.repeat(2)
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array([1, 1, 2, 2, 3, 3])
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ndarray
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See *array*.
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record array
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An :term:`ndarray` with :term:`structured data type` which has been
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subclassed as ``np.recarray`` and whose dtype is of type ``np.record``,
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making the fields of its data type to be accessible by attribute.
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reference
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If ``a`` is a reference to ``b``, then ``(a is b) == True``. Therefore,
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``a`` and ``b`` are different names for the same Python object.
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row-major
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A way to represent items in a N-dimensional array in the 1-dimensional
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computer memory. In row-major order, the rightmost index "varies
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the fastest": for example the array::
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[[1, 2, 3],
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[4, 5, 6]]
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is represented in the row-major order as::
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[1, 2, 3, 4, 5, 6]
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Row-major order is also known as the C order, as the C programming
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language uses it. New NumPy arrays are by default in row-major order.
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self
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Often seen in method signatures, ``self`` refers to the instance
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of the associated class. For example:
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>>> class Paintbrush:
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... color = 'blue'
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...
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... def paint(self):
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... print("Painting the city %s!" % self.color)
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...
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>>> p = Paintbrush()
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>>> p.color = 'red'
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>>> p.paint() # self refers to 'p'
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Painting the city red!
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slice
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Used to select only certain elements from a sequence:
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>>> x = range(5)
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>>> x
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[0, 1, 2, 3, 4]
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>>> x[1:3] # slice from 1 to 3 (excluding 3 itself)
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[1, 2]
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>>> x[1:5:2] # slice from 1 to 5, but skipping every second element
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[1, 3]
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>>> x[::-1] # slice a sequence in reverse
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[4, 3, 2, 1, 0]
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Arrays may have more than one dimension, each which can be sliced
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individually:
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>>> x = np.array([[1, 2], [3, 4]])
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>>> x
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array([[1, 2],
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[3, 4]])
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>>> x[:, 1]
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array([2, 4])
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structure
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See :term:`structured data type`
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structured data type
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A data type composed of other datatypes
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subarray data type
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A :term:`structured data type` may contain a :term:`ndarray` with its
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own dtype and shape:
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>>> dt = np.dtype([('a', np.int32), ('b', np.float32, (3,))])
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>>> np.zeros(3, dtype=dt)
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array([(0, [0., 0., 0.]), (0, [0., 0., 0.]), (0, [0., 0., 0.])],
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dtype=[('a', '<i4'), ('b', '<f4', (3,))])
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title
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In addition to field names, structured array fields may have an
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associated :ref:`title <titles>` which is an alias to the name and is
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commonly used for plotting.
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tuple
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A sequence that may contain a variable number of types of any
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kind. A tuple is immutable, i.e., once constructed it cannot be
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changed. Similar to a list, it can be indexed and sliced::
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>>> x = (1, 'one', [1, 2])
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>>> x
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(1, 'one', [1, 2])
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>>> x[0]
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1
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>>> x[:2]
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(1, 'one')
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A useful concept is "tuple unpacking", which allows variables to
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be assigned to the contents of a tuple::
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>>> x, y = (1, 2)
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>>> x, y = 1, 2
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This is often used when a function returns multiple values:
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>>> def return_many():
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... return 1, 'alpha', None
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>>> a, b, c = return_many()
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>>> a, b, c
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(1, 'alpha', None)
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>>> a
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1
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>>> b
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'alpha'
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ufunc
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Universal function. A fast element-wise, :term:`vectorized
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<vectorization>` array operation. Examples include ``add``, ``sin`` and
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``logical_or``.
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vectorization
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Optimizing a looping block by specialized code. In a traditional sense,
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vectorization performs the same operation on multiple elements with
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fixed strides between them via specialized hardware. Compilers know how
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to take advantage of well-constructed loops to implement such
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optimizations. NumPy uses :ref:`vectorization <whatis-vectorization>`
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to mean any optimization via specialized code performing the same
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operations on multiple elements, typically achieving speedups by
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avoiding some of the overhead in looking up and converting the elements.
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view
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An array that does not own its data, but refers to another array's
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data instead. For example, we may create a view that only shows
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every second element of another array::
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>>> x = np.arange(5)
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>>> x
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array([0, 1, 2, 3, 4])
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>>> y = x[::2]
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>>> y
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array([0, 2, 4])
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>>> x[0] = 3 # changing x changes y as well, since y is a view on x
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>>> y
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array([3, 2, 4])
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wrapper
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Python is a high-level (highly abstracted, or English-like) language.
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This abstraction comes at a price in execution speed, and sometimes
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it becomes necessary to use lower level languages to do fast
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computations. A wrapper is code that provides a bridge between
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high and the low level languages, allowing, e.g., Python to execute
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code written in C or Fortran.
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Examples include ctypes, SWIG and Cython (which wraps C and C++)
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and f2py (which wraps Fortran).
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"""
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