Python – Adjacent elements in List

Given a List extract both next and previous element for each element.

Input : test_list = [3, 7, 9, 3] 
Output : [(None, 7), (3, 9), (7, 3), (9, None)] 
Explanation : for 7 left element is 3 and right, 9.

Input : test_list = [3, 7, 3] 
Output : [(None, 7), (3, 3), (7, None)] 
Explanation : for 7 left element is 3 and right, 3. 

Method 1: Using loop

In this, we iterate for each element and get next and previous element using element access.

Python3

# Python3 code to demonstrate working of
# Adjacent elements in List
# Using loop


# Function to find adjacent
# elements in List
def findAdjacentElements(test_list):
    res = []
    for idx, ele in enumerate(test_list):

        # Checking for all cases to append
        if idx == 0:
            res.append((None, test_list[idx + 1]))
        elif idx == len(test_list) - 1:
            res.append((test_list[idx - 1], None))
        else:
            res.append((test_list[idx - 1], test_list[idx + 1]))
    return res


# Initializing list
input_list = [3, 7, 8, 2, 1, 5, 8, 9, 3]

# Printing original list
print("The original list is:", input_list)


# printing result
print("The Adjacent elements list:", findAdjacentElements(input_list))

Output:

The original list is: [3, 7, 8, 2, 1, 5, 8, 9, 3] The Adjacent elements list: [(None, 7), (3, 8), (7, 2), (8, 1), (2, 5), (1, 8), (5, 9), (8, 3), (9, None)]

Method 2: Using List comprehension

In this approach, we use list comprehension to generate a list of tuples that contain the adjacent elements. The list comprehension iterates over the indices of the test_list, and for each index i, we get the left adjacent element with test_list[i-1] if i > 0, and None otherwise. Similarly, we get the right adjacent element with test_list[i+1] if i < len(test_list)-1, and None otherwise. We then create a tuple of the left and right adjacent elements and append it to the result list for each iteration.

Finally, we print the result list to the console, which contains the desired output.

Python3

# Initializing the list
test_list = [3, 7, 9, 3]

# Printing the input
print("The original input is: ",test_list)
# List comprehension
result = [(test_list[i-1] if i > 0 else None, test_list[i+1] if i < len(test_list)-1 else None) for i in range(len(test_list))]

# Printing the result
print("The result is: ",result)

Output

The original input is:  [3, 7, 9, 3]
The result is:  [(None, 7), (3, 9), (7, 3), (9, None)]

Time Complexity: O(n)

Auxiliary Space: O(logn)

Method 3: Use the zip() function along with list slicing. 

Python3

# Initializing the list
test_list = [3, 7, 9, 3]

# Printing the input
print("The original input is:", test_list)

# Using zip() function and list slicing
result = list(zip(test_list[:-2], test_list[2:]))

# Handling the first and last elements
result.insert(0, (None, test_list[1]))
result.append((test_list[-2], None))

# Printing the result
print("The result is:", result)

Output

The original input is: [3, 7, 9, 3]
The result is: [(None, 7), (3, 9), (7, 3), (9, None)]

Time Complexity: O(n), where n is the length of the input list.
Auxiliary Space: O(n), where n is the length of the input list.