4.13. Array Axis

4.13.1. About Axis

../../_images/array-axis.png

Figure 4.8. Visualizing Multi-Dimensional Arrays [NP2]

4.13.2. One Dimensional

import numpy

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

a.shape                 # (3,)

a[0]                    # 1
a[1]                    # 2
a[2]                    # 3

a.take(0, axis=0)       # 1
a.take(1, axis=0)       # 2
a.take(2, axis=0)       # 3

a.take(0, axis=-1)      # 1
a.take(1, axis=-1)      # 2
a.take(2, axis=-1)      # 3

a[:, 1]                 # IndexError: too many indices for array
a.take(0, axis=1)       # AxisError: axis 1 is out of bounds for array of dimension 1

4.13.3. Two Dimensional

4.13.3.1. Rows

import numpy


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

a.shape                 # (3, 3)

a[0, :]                 # array([1, 2, 3])
a[1, :]                 # array([4, 5, 6])
a[2, :]                 # array([7, 8, 9])

a.take(0, axis=0)       # array([1, 2, 3])
a.take(1, axis=0)       # array([4, 5, 6])
a.take(2, axis=0)       # array([7, 8, 9])

4.13.3.2. Columns

import numpy


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

a.shape                 # (3, 3)

a[:, 0]                 # array([1, 4, 7])
a[:, 1]                 # array([2, 5, 8])
a[:, 2]                 # array([3, 6, 9])

a.take(0, axis=1)       # array([1, 4, 7])
a.take(1, axis=1)       # array([2, 5, 8])
a.take(2, axis=1)       # array([3, 6, 9])

a.take(0, axis=-1)      # array([1, 4, 7])
a.take(1, axis=-1)      # array([2, 5, 8])
a.take(2, axis=-1)      # array([3, 6, 9])

4.13.4. Three Dimensional

4.13.4.1. Depth

import numpy

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[0,:,:]
# array([[1, 2, 3],
#        [4, 5, 6],
#        [5, 6, 7]])

a[1,:,:]
# array([[11, 22, 33],
#        [44, 55, 66],
#        [77, 88, 99]])

a[2,:,:]
# IndexError: index 2 is out of bounds for axis 0 with size 2

a.take(0, axis=0)
# array([[1, 2, 3],
#        [4, 5, 6],
#        [5, 6, 7]])

a.take(1, axis=0)
# array([[11, 22, 33],
#        [44, 55, 66],
#        [77, 88, 99]])

a.take(2, axis=0)
# IndexError: index 2 is out of bounds for size 2

4.13.4.2. Rows

import numpy

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[:,0,:]
# array([[ 1,  2,  3],
#        [11, 22, 33]])

a[:,1,:]
# array([[ 4,  5,  6],
#        [44, 55, 66]])

a[:,2,:]
# array([[ 5,  6,  7],
#        [77, 88, 99]])

a.take(0, axis=1)
# array([[ 1,  2,  3],
#        [11, 22, 33]])

a.take(1, axis=1)
# array([[ 4,  5,  6],
#        [44, 55, 66]])

a.take(2, axis=1)
# array([[ 5,  6,  7],
#        [77, 88, 99]])

4.13.4.3. Columns

import numpy

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[:,:,0]
# array([[ 1,  4,  5],
#        [11, 44, 77]])

a[:,:,1]
# array([[ 2,  5,  6],
#        [22, 55, 88]])

a[:,:,2]
# array([[ 3,  6,  7],
#        [33, 66, 99]])

a.take(0, axis=2)
# array([[ 1,  4,  5],
#        [11, 44, 77]])

a.take(1, axis=2)
# array([[ 2,  5,  6],
#        [22, 55, 88]])

a.take(2, axis=2)
# array([[ 3,  6,  7],
#        [33, 66, 99]])

a.take(0, axis=-1)
# array([[ 1,  4,  5],
#        [11, 44, 77]])

a.take(1, axis=-1)
# array([[ 2,  5,  6],
#        [22, 55, 88]])

a.take(2, axis=-1)
# array([[ 3,  6,  7],
#        [33, 66, 99]])

4.13.5. n Dimensional

  • axis is an index in a.shape

  • Columns are always last