Python Program to get the flattened 1D array

An array is a data structure, which is used to store a set of homogeneous data elements. And it can have more than one dimension.

1D array −

[1 2 3 4 5 6]

2D array −

[[1 2 3]
   [4 5 6]
   [7 8 9]]

Flattening an array means reducing the dimensionality of a Multi-dimensional array.

In the article below we will discuss the python program to get the flattened 1D array. Here we will use List and NumPy arrays to represent a normal array. Because python does not have a native data structure for arrays.

Input Output Scenarios

Assume we have a 2-Dimensional array as an input. And the output will be the flattened array.

Input array:  
[[1 2 3]
 [4 5 6]
 [7 8 9]]
Output:
Flattened array:  [1 2 3 4 5 6 7 8 9]

Using a nested list

From the python functools module we can use the reduce function to flatten the 2-dimensional array. reduce() function is used to apply a specified function to the items of the sequence and it will return a reduced sequence. Following is the syntax to do so –

reduce(function, iterable[, initializer])

Where,

• function is a predefined function that applies on items of iterable.

• Iterable any python iterables ex: list, tuple, string, and dictionary.

To use the reduce function then we need to import it from the functools module.

Example

In this example, we will get the flattened 1d array by using lambda and reduce () function.

from functools import reduce 

arr_2d = [[1, 2, 3],
   [3, 6, 7],
   [7, 5, 4]]            
# print initial array
print("Original array: ", arr_2d)

# flattening the 2d array into 1d array
# using reduce function
flattened_arr = reduce(lambda x,y:x+y, arr_2d)

print("Flattened array: ", flattened_arr)

Output

Original array:  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened array:  [1, 2, 3, 3, 6, 7, 7, 5, 4]

The reduce function successfully flattened the 2D array with the help of lambda function.

Example

Also, we can use list comprehension to get the flattened array. Let's see the example below.

arr_2d = [[1, 2, 3],
   [3, 6, 7],
   [7, 5, 4]]            

# print initial array
print("Original array: ", arr_2d)

# flattening the 2d array into 1d array
# using list comprehension
flattened_arr = [j for sub in arr_2d for j in sub]
print("Flattened array: ", flattened_arr)

Output

Original array:  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened array:  [1, 2, 3, 3, 6, 7, 7, 5, 4]

With the help of list comprehension, we have iterated the array and its sub array elements then created the flattened array which is stored in a flattened_arr variable.

Example

In this example we will using sum() function we will get the 1d array.

arr_2d = [[1, 2, 3],
   [3, 6, 7],
   [7, 5, 4]]            

# print initial array
print("Original array: ", arr_2d)

# flattening the 2d array into 1d array
# using list sum function
flattened_arr = sum(arr_2d, [])

print("Flattened array: ", flattened_arr)

Output

Original array:  [[1, 2, 3], [3, 6, 7], [7, 5, 4]]
Flattened array:  [1, 2, 3, 3, 6, 7, 7, 5, 4]

The syntax sum(arr_2d, []) flatten the 2D array, here the inbuilt sum() function performs the concatenation of the inner arrays which is like [1, 2] + [3, 4].

Note − This method is not recommended as it takes more time to perform the task.

Use numpy.flatten() function

We can easily get the flattened array with the help of NumPy flatten() functions. Following is the syntax of this function –

ndarray.flatten(order='C')

The method returns a flattened array from the input N-Dimensional array. Here the parameter order is an optional parameter, and the default value is C.

Example

In this example we will flatten the 2-dimensional numpy array into a 1-dimensional array using the flattened() function.

import numpy as np
arr_2d = np.array([[1, 2, 3],[4, 5, 6], [7, 8, 9]])
# print initial array
print("Original array: ", arr_2d)
# get the flattened array
flattened_arr = arr_2d.flatten()

print("Flattened array: ", flattened_arr)

Output

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

These are the few ways to get the flattened 1D array in python programming.