NumPy - Splitting Arrays

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Splitting NumPy Array

Splitting arrays in NumPy is a way to divide a single array into multiple sub-arrays. This can be done along any axis, depending on how you want to partition the data. NumPy provides several functions to split arrays in different ways. They are as follows −

  • Using numpy.split() Function
  • Using numpy.array_split() Function
  • Using numpy.hsplit() Function
  • Using numpy.vsplit() Function
  • Using numpy.dsplit() Function

Splitting Arrays Using split() Function

We can use the split() function in NumPy to split an array into multiple sub-arrays along a specified axis. The array is divided based on the provided indices. Following is the syntax −

numpy.split(array, indices_or_sections, axis=0)

Where,

  • array − The input array to be split.

  • indices_or_sections − This can be either an integer or a 1D array of sorted integers.

    If an integer, it specifies the number of equal-sized sub-arrays to create. The array must be divisible evenly into this number of sections.

    If a 1D array of sorted integers, it specifies the points at which to split the array.

  • axis − The axis along which to split the array. The default is 0 (split along rows for 2D arrays).

Example: Splitting into Equal-sized Sub-arrays

In the below example, we are splitting an array "arr" into 3 equal sub-arrays along the columns (axis=1) using the numpy.split() function−

import numpy as np

# array
arr = np.arange(9).reshape(3, 3)

# Split into 3 equal sub-arrays 
split_arr = np.split(arr, 3, axis=1)

print("Original Array:")
print(arr)
print("\nSplit into 3 equal sub-arrays along axis 1:")
for sub_arr in split_arr:
   print(sub_arr)

Following is the output obtained −

Original Array:
[[0 1 2]
 [3 4 5]
 [6 7 8]]

Split into 3 equal sub-arrays along axis 1:
[[0]
 [3]
 [6]]
[[1]
 [4]
 [7]]
[[2]
 [5]
 [8]]

Example: Splitting at Specific Indices

Here, we are splitting an array at indices [1, 2] along the rows (axis=0) using the split() function in NumPy −

import numpy as np

# array
arr = np.arange(9).reshape(3, 3)

# Split array at specified indices
split_arr = np.split(arr, [1, 2], axis=0)

print("\nSplit at indices [1, 2] along axis 0:")
for sub_arr in split_arr:
   print(sub_arr)

This will produce the following result −

Split at indices [1, 2] along axis 0:
[[0 1 2]]
[[3 4 5]]
[[6 7 8]]

Splitting Arrays Using array_split() Function

We can also use the array_split() function in NumPy to split an array into multiple sub-arrays along a specified axis. Unlike numpy.split() function, the array_split() function allows for unequal splits if the array cannot be evenly divided.

When the array does not evenly divide into the specified number of sections, the numpy.array_split() function ensures that the resulting sub-arrays are as equal in size as possible, distributing any extra elements to the earlier sub-arrays. Following is the syntax −

numpy.array_split(array, indices_or_sections, axis=0)

Where,

  • array − The input array to be split.

  • indices_or_sections − This can be either an integer or a 1D array of sorted integers.

    If an integer, it specifies the number of equal-sized sub-arrays to create. The array must be divided as equally as possible.

    If a 1D array of sorted integers, it specifies the points at which to split the array.

  • axis − The axis along which to split the array. The default is 0 (split along rows for 2D arrays).

Example

In the example below, we are splitting a 1D array with "10" elements into 3 unequal sub-arrays using numpy.array_split() function −

import numpy as np

# array
arr = np.arange(10)

# Split into 3 sub-arrays along axis 0
split_arr = np.array_split(arr, 3)

print("Original Array:")
print(arr)
print("\nSplit into 3 unequal sub-arrays:")
for sub_arr in split_arr:
   print(sub_arr)

Following is the output of the above code −

Original Array:
[0 1 2 3 4 5 6 7 8 9]

Split into 3 unequal sub-arrays:
[0 1 2 3]
[4 5 6]
[7 8 9]

Horizontal Splitting

We can split an array along the horizontal axis, which is axis = 1, for 2D arrays using the hsplit() function in NumPy. This function divides the array into sub-arrays horizontally, effectively separating columns of data. Following is the syntax −

numpy.hsplit(array, indices_or_sections)

Where,

  • array − The input array to be split.

  • indices_or_sections − Either an integer or a 1D array of indices that indicate how to split the array.

Example

In this example, we are splitting a 2D array "arr" along its columns into 2 equal parts using the numpy.hsplit() function −

import numpy as np

# 2D array
arr = np.array([[1, 2, 3, 4],
                [5, 6, 7, 8]])

# Split into 2 equal parts along axis 1
split_arr = np.hsplit(arr, 2)

print("Original Array:")
print(arr)
print("\nSplit into 2 equal parts along axis 1:")
for sub_arr in split_arr:
   print(sub_arr)

After executing the above code, we get the following output −

Original Array:[[1 2 3 4]
 [5 6 7 8]]

Split into 2 equal parts along axis 1:
[[1 2]
 [5 6]]
[[3 4]
 [7 8]]

Vertical Splitting

We can also split an array along the vertical axis, which is axis = 0, for 2D arrays using the vsplit() function in NumPy. This function divides the array into sub-arrays vertically, effectively separating rows of data. Following is the syntax −

numpy.vsplit(array, indices_or_sections)

Where,

  • array − The input array to be split.

  • indices_or_sections − Either an integer or a 1D array of indices that indicate how to split the array.

Example

In the example below, we are splitting a 2D array "arr" along its rows into 3 equal parts using the numpy.vsplit() function −

import numpy as np

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

# Split into 3 equal parts along axis 0
split_arr = np.vsplit(arr, 3)

print("Original Array:")
print(arr)
print("\nSplit into 3 equal parts along axis 0:")
for sub_arr in split_arr:
   print(sub_arr)

The output obtained is as shown below −

Original Array:
[[1 2 3]
 [4 5 6]
 [7 8 9]]

Split into 3 equal parts along axis 0:
[[1 2 3]]
[[4 5 6]]
[[7 8 9]]

Depth Splitting

The numpy.dsplit() function is used to split a 3D array along its third dimension. This dimension is commonly referred to as the depth dimension, corresponding to axis=2. Following is the syntax −

numpy.dsplit(array, indices_or_sections)

Example

In this example, using the numpy.dsplit() function to split a 3D array "arr" into four equal parts along its third dimension −

import numpy as np

# Example 3D array
arr = np.arange(24).reshape((2, 3, 4))

# Split into 4 equal parts along axis 2 (depth)
split_arr = np.dsplit(arr, 4)

print("Original Array:")
print(arr)
print("\nSplit into 4 equal parts along axis 2 (depth):")
for sub_arr in split_arr:
   print(sub_arr)
   print()

The result produced is as follows −

Original Array:
[[[ 0  1  2  3]
  [ 4  5  6  7]
  [ 8  9 10 11]]

 [[12 13 14 15]
  [16 17 18 19]
  [20 21 22 23]]]

Split into 4 equal parts along axis 2 (depth):
[[[ 0]
  [ 4]
  [ 8]]

 [[12]
  [16]
  [20]]]

[[[ 1]
  [ 5]
  [ 9]]

 [[13]
  [17]
  [21]]]

[[[ 2]
  [ 6]
  [10]]

 [[14]
  [18]
  [22]]]

[[[ 3]
  [ 7]
  [11]]

 [[15]
  [19]
  [23]]]
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