Python - Reverse Arrays

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In this chapter, we shall explore the different ways to rearrange the given array in the reverse order of the index. In Python, array is not one of the built-in data types. However, Python's standard library has array module. The array class helps us to create a homogenous collection of string, integer or float types.

The syntax used for creating array is −

import array
obj = array.array(typecode[, initializer])

Let us first create an array consisting of a few objects of int type −

import array as arr
a = arr.array('i', [10,5,15,4,6,20,9])

The array class doesn't have any built-in method to reverse array. Hence, we have to use another array. An empty array "b" is declared as follows −

b = arr.array('i')

Reverse an Array by Traversing Elements in Reverse Order

Next, we traverse the numbers in array "a" in reverse order, and append each element to the "b" array −

for i in range(len(a)-1, -1, -1):
b.append(a[i])

The array "b" now holds numbers from original array in reverse order.

Example

Here is the complete code to reverse an array in Python −

import array as arr
a = arr.array('i', [10,5,15,4,6,20,9])
b = arr.array('i')
for i in range(len(a)-1, -1, -1):
   b.append(a[i])
print (a, b)

It will produce the following output −

array('i', [10, 5, 15, 4, 6, 20, 9]) array('i', [9, 20, 6, 4, 15, 5, 10])

Reverse an Array Using reverse() Method

We can also reverse the sequence of numbers in an array using the reverse() method in list class. List is a built-in type in Python.

We have to first transfer the contents of an array to a list with tolist() method of array class −

a = arr.array('i', [10,5,15,4,6,20,9])
b = a.tolist()

We can call the reverse() method now −

b.reverse()

If we now convert the list back to an array, we get the array with reversed order,

a = arr.array('i')
a.fromlist(b)

Example

Here is the complete code −

from array import array as arr
a = arr.array('i', [10,5,15,4,6,20,9])
b = a.tolist()
b.reverse()
a = arr.array('i')
a.fromlist(b)
print (a)

It will produce the following output −

array('i', [10, 5, 15, 4, 6, 20, 9])