Creating multidimensional arrays

Arrays need not be limited to lists. In fact, they can have an arbitrary number of dimensions. In machine learning, we will often deal with at least 2D arrays, where the column index stands for the values of a particular feature and the rows contain the actual feature values.

With NumPy, it is easy to create multidimensional arrays from scratch. Let's say that we want to create an array with three rows and five columns, with all the elements initialized to zero. If we don't specify a data type, NumPy will default to using floats:

In [23]: arr_2d = np.zeros((3, 5))
...      arr_2d
Out[23]: array([[ 0., 0., 0., 0., 0.],
...             [ 0., 0., 0., 0., 0.],
...             [ 0., 0., 0., 0., 0.]])

As you probably know from your OpenCV days, this could be interpreted as a 3 x 5 grayscale image with all pixels set to 0 (black). Analogously, if we wanted to create a tiny 2 x 4 pixel image with three color channels (R, G, B), but all pixels set to white, we would use NumPy to create a 3D array with the dimensions, 3 x 2 x 4:

In [24]: arr_float_3d = np.ones((3, 2, 4))
...      arr_float_3d
Out[24]: array([[[ 1., 1., 1., 1.],
...              [ 1., 1., 1., 1.]],
...
...             [[ 1., 1., 1., 1.],
...              [ 1., 1., 1., 1.]],
...
...             [[ 1., 1., 1., 1.],
...              [ 1., 1., 1., 1.]]])

Here, the first dimension defines the color channel (red, green, blue, green, and red in OpenCV). Thus, if this was real image data, we could easily grab the color information in the first channel by slicing the array:

In [25]: arr_float_3d[0, :, :]
Out[25]: array([[ 1., 1., 1., 1.],
...             [ 1., 1., 1., 1.]])

In OpenCV, images either come as 32-bit float arrays with values between 0 and 1 or they come as 8-bit integer arrays with values between 0 and 255. Hence, we can also create a 2 x 4 pixel, all-white RGB image using 8-bit integers by specifying the dtype attribute of the NumPy array and multiplying all the ones in the array by 255:

In [26]: arr_uint_3d = np.ones((3, 2, 4), dtype=np.uint8) * 255
...      arr_unit_3d
Out[26]: array([[[255, 255, 255, 255],
...              [255, 255, 255, 255]],
...
...             [[255, 255, 255, 255],
...              [255, 255, 255, 255]],
...
...             [[255, 255, 255, 255],
...              [255, 255, 255, 255]]], dtype=uint8)

We will look at more advanced array manipulations in later chapters.