numpy Arrays Reshaping an array


Example

The numpy.reshape (same as numpy.ndarray.reshape) method returns an array of the same total size, but in a new shape:

print(np.arange(10).reshape((2, 5)))   
# [[0 1 2 3 4]
#  [5 6 7 8 9]]

It returns a new array, and doesn't operate in place:

a = np.arange(12)
a.reshape((3, 4))
print(a)
# [ 0  1  2  3  4  5  6  7  8  9 10 11]

However, it is possible to overwrite the shape attribute of an ndarray:

a = np.arange(12)
a.shape = (3, 4)
print(a)
# [[ 0  1  2  3]
#  [ 4  5  6  7]
#  [ 8  9 10 11]]

This behavior might be surprising at first, but ndarrays are stored in contiguous blocks of memory, and their shape only specifies how this stream of data should be interpreted as a multidimensional object.

Up to one axis in the shape tuple can have a value of -1. numpy will then infer the length of this axis for you:

a = np.arange(12)
print(a.reshape((3, -1)))
# [[ 0  1  2  3]
#  [ 4  5  6  7]
#  [ 8  9 10 11]]

Or:

a = np.arange(12)
print(a.reshape((3, 2, -1)))

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

#  [[ 4  5]
#   [ 6  7]]

#  [[ 8  9]
#   [10 11]]]

Multiple unspecified dimensions, e.g. a.reshape((3, -1, -1)) are not allowed and will throw a ValueError.