2.4. Array Attributes

2.4.1. Size

  • Number of elements

import numpy as np


a = np.array([1, 2, 3])

b = np.array([[1, 2, 3],
              [4, 5, 6]])

c = np.array([[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]])

d = np.array([[[ 1,  2,  3],
               [ 4,  5,  6],
               [ 5,  6,  7]],

              [[11, 22, 33],
               [44, 55, 66],
               [77, 88, 99]]])

a.size
# 3
b.size
# 6
c.size
# 9
d.size
# 18

2.4.2. Shape

import numpy as np


a = np.array([1, 2, 3])

b = np.array([[1, 2, 3],
              [4, 5, 6]])

c = np.array([[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]])

d = np.array([[[ 1,  2,  3],
               [ 4,  5,  6],
               [ 5,  6,  7]],

              [[11, 22, 33],
               [44, 55, 66],
               [77, 88, 99]]])

a.shape
# (3,)
b.shape
# (2, 3)
c.shape
# (3, 3)
d.shape
# (2, 3, 3)

2.4.3. NDim

  • Number of Dimensions

  • len(ndarray.shape)

import numpy as np


a = np.array([1, 2, 3])

b = np.array([[1, 2, 3],
              [4, 5, 6]])

c = np.array([[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]])

d = np.array([[[ 1,  2,  3],
               [ 4,  5,  6],
               [ 5,  6,  7]],

              [[11, 22, 33],
               [44, 55, 66],
               [77, 88, 99]]])

a.ndim
# 1
b.ndim
# 2
c.ndim
# 2
d.ndim
# 3

2.4.4. Length

  • Number of elements in first dimension

  • ndarray.shape[0]

import numpy as np


a = np.array([1, 2, 3])

b = np.array([[1, 2, 3],
              [4, 5, 6]])

c = np.array([[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]])

d = np.array([[[ 1,  2,  3],
               [ 4,  5,  6],
               [ 5,  6,  7]],

              [[11, 22, 33],
               [44, 55, 66],
               [77, 88, 99]]])

len(a)
# 3
len(b)
# 2
len(c)
# 3
len(d)
# 2

2.4.5. Itemsize

  • int64 takes 64 bits (8 bytes of memory)

import numpy as np

a = np.array([1, 2, 3], dtype=np.int16)
b = np.array([1, 2, 3], dtype=np.int32)
c = np.array([1, 2, 3], dtype=np.int64)

a.itemsize
# 2
b.itemsize
# 4
c.itemsize
# 8

import numpy as np

a = np.array([1, 2, 3], dtype=np.float16)
b = np.array([1, 2, 3], dtype=np.float32)
c = np.array([1, 2, 3], dtype=np.float64)

a.itemsize
# 2
b.itemsize
# 4
c.itemsize
# 8

2.4.6. Strides

  • int64 takes 64 bits (8 bytes of memory)

  • Strides inform how many bytes numpy has to jump to access values in each dimensions

import numpy as np


a = np.array([1, 2, 3])

b = np.array([[1, 2, 3],
              [4, 5, 6]])

c = np.array([[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]])

d = np.array([[[ 1,  2,  3],
               [ 4,  5,  6],
               [ 5,  6,  7]],

              [[11, 22, 33],
               [44, 55, 66],
               [77, 88, 99]]])

a.strides
# (8,)
b.strides
# (24, 8)
c.strides
# (24, 8)
d.strides
# (72, 24, 8)

2.4.7. Data

import numpy as np


a = np.array([1, 2, 3])

a.shape
# (3,)
a.itemsize
# 8
a.strides
# (8,)
a.data
# <memory at 0x10cdfaa10>

import numpy as np


a = np.array([[1, 2, 3],
              [4, 5, 6]])

a.shape
# (2, 3)
a.itemsize
# 8
a.strides
# (24, 8)
a.data
# <memory at 0x10caefbb0>

import numpy as np


a = np.array([[[ 1,  2,  3],
               [ 4,  5,  6],
               [ 5,  6,  7]],

              [[11, 22, 33],
               [44, 55, 66],
               [77, 88, 99]]])

a.shape
# (2, 3, 3)
a.itemsize
# 8
a.strides
# (72, 24, 8)
a.data
# <memory at 0x107933c70>
../_images/array-attributes-data.png

2.4.8. Recap

../_images/array-attributes-recap.png

2.4.9. Assignments

Code 2.34. Solution
"""
* Assignment: Numpy Attributes
* Complexity: easy
* Lines of code: 7 lines
* Time: 5 min

English:
    1. Define `result: dict` with:
        a. number of dimensions;
        b. number of elements;
        c. data type;
        d. element size;
        e. shape;
        f. strides.
    2. Run doctests - all must succeed

Polish:
    1. Zdefiniuj `result: dict` z:
        a. liczbę wymiarów,
        b. liczbę elementów,
        c. typ danych,
        d. rozmiar elementu,
        e. kształt,
        f. przeskoki (strides).
    2. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> type(result) is dict
    True

    >>> result  # doctest: +NORMALIZE_WHITESPACE
    {'number of dimensions': 2,
     'number of elements': 6,
     'data type': dtype('float64'),
     'element size': 8,
     'shape': (2, 3),
     'strides': (24, 8)}
"""

import numpy as np

DATA = np.array([[-1.1, 0.0, 1.1],
                 [2.2, 3.3, 4.4]])

result = {
    'number of dimensions': ...,
    'number of elements': ...,
    'data type': ...,
    'element size': ...,
    'shape': ...,
    'strides': ...,
}