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authorStefan van der Walt <stefan@sun.ac.za>2008-08-23 23:17:23 +0000
committerStefan van der Walt <stefan@sun.ac.za>2008-08-23 23:17:23 +0000
commit5c86844c34674e3d580ac2cd12ef171e18130b13 (patch)
tree2fdf1150706c07c7e193eb7483ce58a5074e5774 /numpy/doc/reference/glossary.py
parent376d483d31c4c5427510cf3a8c69fc795aef63aa (diff)
downloadnumpy-5c86844c34674e3d580ac2cd12ef171e18130b13.tar.gz
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-"""
-=================
-Glossary
-=================
-
-along an axis
- Axes are defined for arrays with more than one dimension. A
- 2-dimensional array has two corresponding axes: the first running
- vertically downwards across rows (axis 0), and the second running
- horizontally across columns (axis 1).
-
- Many operation can take place along one of these axes. For example,
- we can sum each row of an array, in which case we operate along
- columns, or axis 1::
-
- >>> x = np.arange(12).reshape((3,4))
-
- >>> x
- array([[ 0, 1, 2, 3],
- [ 4, 5, 6, 7],
- [ 8, 9, 10, 11]])
-
- >>> x.sum(axis=1)
- array([ 6, 22, 38])
-
-array or ndarray
- A homogeneous container of numerical elements. Each element in the
- array occupies a fixed amount of memory (hence homogeneous), and
- can be a numerical element of a single type (such as float, int
- or complex) or a combination (such as ``(float, int, float)``). Each
- array has an associated data-type (or ``dtype``), which describes
- the numerical type of its elements::
-
- >>> x = np.array([1, 2, 3], float)
-
- >>> x
- array([ 1., 2., 3.])
-
- >>> x.dtype # floating point number, 64 bits of memory per element
- dtype('float64')
-
-
- # More complicated data type: each array element is a combination of
- # and integer and a floating point number
- >>> np.array([(1, 2.0), (3, 4.0)], dtype=[('x', int), ('y', float)])
- array([(1, 2.0), (3, 4.0)],
- dtype=[('x', '<i4'), ('y', '<f8')])
-
- Fast element-wise operations, called `ufuncs`_, operate on arrays.
-
-array_like
- Any sequence that can be interpreted as an ndarray. This includes
- nested lists, tuples, scalars and existing arrays.
-
-attribute
- A property of an object that can be accessed using ``obj.attribute``,
- e.g., ``shape`` is an attribute of an array::
-
- >>> x = np.array([1, 2, 3])
- >>> x.shape
- (3,)
-
-broadcast
- NumPy can do operations on arrays whose shapes are mismatched::
-
- >>> x = np.array([1, 2])
- >>> y = np.array([[3], [4]])
-
- >>> x
- array([1, 2])
-
- >>> y
- array([[3],
- [4]])
-
- >>> x + y
- array([[4, 5],
- [5, 6]])
-
- See `doc.broadcasting`_ for more information.
-
-decorator
- An operator that transforms a function. For example, a ``log``
- decorator may be defined to print debugging information upon
- function execution::
-
- >>> def log(f):
- ... def new_logging_func(*args, **kwargs):
- ... print "Logging call with parameters:", args, kwargs
- ... return f(*args, **kwargs)
- ...
- ... return new_logging_func
-
- Now, when we define a function, we can "decorate" it using ``log``::
-
- >>> @log
- ... def add(a, b):
- ... return a + b
-
- Calling ``add`` then yields:
-
- >>> add(1, 2)
- Logging call with parameters: (1, 2) {}
- 3
-
-dictionary
- Resembling a language dictionary, which provides a mapping between
- words and descriptions thereof, a Python dictionary is a mapping
- between two objects::
-
- >>> x = {1: 'one', 'two': [1, 2]}
-
- Here, `x` is a dictionary mapping keys to values, in this case
- the integer 1 to the string "one", and the string "two" to
- the list ``[1, 2]``. The values may be accessed using their
- corresponding keys::
-
- >>> x[1]
- 'one'
-
- >>> x['two']
- [1, 2]
-
- Note that dictionaries are not stored in any specific order. Also,
- most mutable (see *immutable* below) objects, such as lists, may not
- be used as keys.
-
- For more information on dictionaries, read the
- `Python tutorial <http://docs.python.org/tut>`_.
-
-immutable
- An object that cannot be modified after execution is called
- immutable. Two common examples are strings and tuples.
-
-instance
- A class definition gives the blueprint for constructing an object::
-
- >>> class House(object):
- ... wall_colour = 'white'
-
- Yet, we have to *build* a house before it exists::
-
- >>> h = House() # build a house
-
- Now, ``h`` is called a ``House`` instance. An instance is therefore
- a specific realisation of a class.
-
-iterable
- A sequence that allows "walking" (iterating) over items, typically
- using a loop such as::
-
- >>> x = [1, 2, 3]
- >>> [item**2 for item in x]
- [1, 4, 9]
-
- It is often used in combintion with ``enumerate``::
-
- >>> for n, k in enumerate(keys):
- ... print "Key %d: %s" % (n, k)
- ...
- Key 0: a
- Key 1: b
- Key 2: c
-
-list
- A Python container that can hold any number of objects or items.
- The items do not have to be of the same type, and can even be
- lists themselves::
-
- >>> x = [2, 2.0, "two", [2, 2.0]]
-
- The list `x` contains 4 items, each which can be accessed individually::
-
- >>> x[2] # the string 'two'
- 'two'
-
- >>> x[3] # a list, containing an integer 2 and a float 2.0
- [2, 2.0]
-
- It is also possible to select more than one item at a time,
- using *slicing*::
-
- >>> x[0:2] # or, equivalently, x[:2]
- [2, 2.0]
-
- In code, arrays are often conveniently expressed as nested lists::
-
-
- >>> np.array([[1, 2], [3, 4]])
- array([[1, 2],
- [3, 4]])
-
- For more information, read the section on lists in the `Python
- tutorial <http://docs.python.org/tut>`_. For a mapping
- type (key-value), see *dictionary*.
-
-mask
- A boolean array, used to select only certain elements for an operation::
-
- >>> x = np.arange(5)
- >>> x
- array([0, 1, 2, 3, 4])
-
- >>> mask = (x > 2)
- >>> mask
- array([False, False, False, True, True], dtype=bool)
-
- >>> x[mask] = -1
- >>> x
- array([ 0, 1, 2, -1, -1])
-
-masked array
- Array that suppressed values indicated by a mask::
-
- >>> x = np.ma.masked_array([np.nan, 2, np.nan], [True, False, True])
- >>> x
- masked_array(data = [-- 2.0 --],
- mask = [ True False True],
- fill_value=1e+20)
-
- >>> x + [1, 2, 3]
- masked_array(data = [-- 4.0 --],
- mask = [ True False True],
- fill_value=1e+20)
-
- Masked arrays are often used when operating on arrays containing
- missing or invalid entries.
-
-matrix
- A 2-dimensional ndarray that preserves its two-dimensional nature
- throughout operations. It has certain special operations, such as ``*``
- (matrix multiplication) and ``**`` (matrix power), defined::
-
- >>> x = np.mat([[1, 2], [3, 4]])
-
- >>> x
- matrix([[1, 2],
- [3, 4]])
-
- >>> x**2
- matrix([[ 7, 10],
- [15, 22]])
-
-method
- A function associated with an object. For example, each ndarray has a
- method called ``repeat``::
-
- >>> x = np.array([1, 2, 3])
-
- >>> x.repeat(2)
- array([1, 1, 2, 2, 3, 3])
-
-reference
- If ``a`` is a reference to ``b``, then ``(a is b) == True``. Therefore,
- ``a`` and ``b`` are different names for the same Python object.
-
-self
- Often seen in method signatures, ``self`` refers to the instance
- of the associated class. For example:
-
- >>> class Paintbrush(object):
- ... color = 'blue'
- ...
- ... def paint(self):
- ... print "Painting the city %s!" % self.color
- ...
- >>> p = Paintbrush()
- >>> p.color = 'red'
- >>> p.paint() # self refers to 'p'
- Painting the city red!
-
-slice
- Used to select only certain elements from a sequence::
-
- >>> x = range(5)
- >>> x
- [0, 1, 2, 3, 4]
-
- >>> x[1:3] # slice from 1 to 3 (excluding 3 itself)
- [1, 2]
-
- >>> x[1:5:2] # slice from 1 to 5, but skipping every second element
- [1, 3]
-
- >>> x[::-1] # slice a sequence in reverse
- [4, 3, 2, 1, 0]
-
- Arrays may have more than one dimension, each which can be sliced
- individually::
-
- >>> x = np.array([[1, 2], [3, 4]])
- >>> x
- array([[1, 2],
- [3, 4]])
-
- >>> x[:, 1]
- array([2, 4])
-
-tuple
- A sequence that may contain a variable number of types of any
- kind. A tuple is immutable, i.e., once constructed it cannot be
- changed. Similar to a list, it can be indexed and sliced::
-
- >>> x = (1, 'one', [1, 2])
-
- >>> x
- (1, 'one', [1, 2])
-
- >>> x[0]
- 1
-
- >>> x[:2]
- (1, 'one')
-
- A useful concept is "tuple unpacking", which allows variables to
- be assigned to the contents of a tuple::
-
- >>> x, y = (1, 2)
- >>> x, y = 1, 2
-
- This is often used when a function returns multiple values:
-
- >>> def return_many():
- ... return 1, 'alpha'
-
- >>> a, b, c = return_many()
- >>> a, b, c
- (1, 'alpha', None)
-
- >>> a
- 1
- >>> b
- 'alpha'
-
-ufunc
- Universal function. A fast element-wise array operation. Examples include
- ``add``, ``sin`` and ``logical_or``.
-
-view
- An array that does not own its data, but refers to another array's
- data instead. For example, we may create a view that only shows
- every second element of another array::
-
- >>> x = np.arange(5)
- >>> x
- array([0, 1, 2, 3, 4])
-
- >>> y = x[::2]
- >>> y
- array([0, 2, 4])
-
- >>> x[0] = 3 # changing x changes y as well, since y is a view on x
- >>> y
- array([3, 2, 4])
-
-wrapper
- Python is a high-level (highly abstracted, or English-like) language.
- This abstraction comes at a price in execution speed, and sometimes
- it becomes necessary to use lower level languages to do fast
- computations. A wrapper is code that provides a bridge between
- high and the low level languages, allowing, e.g., Python to execute
- code written in C or Fortran.
-
- Examples include ctypes, SWIG and Cython (which wraps C and C++)
- and f2py (which wraps Fortran).
-
-"""