Compare Adjacent Elements in an Array

Introduction

This article will discuss how to compare adjacent elements in a List using Python. Suppose you want to create a new list from a given list by calculating the difference between adjacent elements of in the list, for example:

Input: test_list = [26, 19, 12, 6]

Output: new_list = [7, 7, 6]

Explanation:

26 – 19 = 7

19 – 12 = 7

12 – 6 = 6

There are several methods to achieve this. Each method demonstrated will use the same input list of [26, 19, 12, 6] and produce the same output of [7, 7, 6]

Method 1: Using the range() function:

def adjacent_items(seq):
    new_list = []
    for i in range(1, len(seq)):
        previous_item = seq[i - 1]
        current_item = seq[i]
        new_list.append(previous_item - current_item)

    return new_list   # Output [7, 7, 6]

Method 2: Using the zip() function and slices:

def adjacent_items(seq):
    new_list = []
    list1 = seq[0::]  # Copy of list starting at index 0
    list2 = seq[1::]  # copy of list starting at index 1       

    for previous_item, current_item in zip(list1, list2):
        new_list.append(previous_item - current_item)
    return new_list  # Output [7, 7, 6]

Method 3: Using the zip() function, slices, and a list comprehension:

new_list = [previous_item - current_item 
            for (previous_item, current_item) 
            in zip(test_list[0::], test_list[1::])]
print(new_list)  # Output [7, 7, 6]

Method 4: Using a generator and list comprehension:

First lets create the generator.

def adjacent_items(seq):
    iterable_list = iter(seq)
    prev = next(iterable_list)
    for item in iterable_list:
        yield (prev - item)
        prev = item

• Note: Since this is a generator, you must iterate over it to use it. We’ll use a list comprehension to produce the results:

new_list = [item for item in adjacent_items(test_list)]
print(new_list)  # Output [7, 7, 6]

Method 5: Using the more_itertool and pairwise() function:

• Note: You must use pip to install more_itertools

pip install more_itertools

from more_itertools import pairwise

def adjacent_items(seq):
    new_list = []
    for previous_item, current_item in pairwise(lst):
        new_list.append(previous_item - current_item)
    return new_list  # Output [7, 7, 6]

Of course the above example can be achieved using a list comprehension as well.

from more_itertools import pairwise

new_list = [previous_item - current_item
            for (previous_item , current_item)
            in pairwise(test_list)]
print(new_list)  # Output [7, 7, 6]

For more information on more_itertools visit Python docs.

Conclusion

The purpose of this article was to demonstrate how to compare adjacent elements in a List with Python. The code examples used in this article all create a new list from a given list by calculating the difference between adjacent elements of in the list. However, these functions can modify to suit your purpose. Hopefully you have a better understanding on how to compare adjacent elements in a List using Python. There are other solutions; these are just a few. If you found this article helpful or have any feedback, please leave a comment.