Python | Triplet iteration in List (original) (raw)

Last Updated : 22 Apr, 2023

List iteration is common in programming, but sometimes one requires to print the elements in consecutive triplets. This particular problem is quite common and having a solution to it always turns out to be handy. Lets discuss certain way in which this problem can be solved.

Method #1 : Using list comprehension List comprehension can be used to print the triplets by accessing current, next and next to next element in the list and then printing the same. Care has to be taken while pairing the last elements with the first ones to form a cyclic triplet pairs.

Python3 `

Python3 code to demonstrate

Triplet iteration in List

using list comprehension

from itertools import compress

initializing list

test_list = [0, 1, 2, 3, 4, 5]

printing original list

print ("The original list is : " + str(test_list))

using list comprehension

Triplet iteration in List

res = [((i), (i + 1) % len(test_list), (i + 2) % len(test_list)) for i in range(len(test_list))]

printing result

print ("The triplet list is : " + str(res))

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Output

The original list is : [0, 1, 2, 3, 4, 5] The triplet list is : [(0, 1, 2), (1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 0), (5, 0, 1)]

Time Complexity: O(n)

Auxiliary space: O(n)

Method#2 : Using zip() function zip function can be used to pair the element of the list provided. We generate the list whose starting element is next element and next to next element. Take care while generating list while merging the last element to first element to form the cyclic triplet.

Python3 `

Python3 code to demonstrate

Triplet iteration in List

using list comprehension

import itertools

initializing list

l = [0, 1, 2, 3, 4, 5]

Printing original list

print("The original list is : " + str(l))

Generating list for pairing

def gen(l, n): k1 = itertools.cycle(l); k2 = itertools.dropwhile(lambda x: x!=n, k1) k3 = itertools.islice(k2, None, len(l)) return list(k3)

using zip function

Triplet iteration in List

ans = [] for i in zip(l, gen(l, 1), gen(l, 2)): ans.append(tuple(i))

printing result

print ("The triplet list is : " + str(ans))

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Output

The original list is : [0, 1, 2, 3, 4, 5] The triplet list is : [(0, 1, 2), (1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 0), (5, 0, 1)]

Time Complexity: O(n)

Auxiliary space: O(n)

Method#3: Using For loop For loop can be used to print the triplets by accessing current, next and next to next in the list then print the same. Care has to been taken while pairing the last element with first element when forming cyclic triplet.

Python3 `

Python3 code to demonstrate

Triplet iteration in List

using list comprehension

initializing list

l = [0, 1, 2, 3, 4, 5] k = len(l)

Printing original list

print("The original list is : " + str(l))

using zip function

Triplet iteration in List

ans = [] for i in range(k): x = (l[i], l[(i+1)%k], l[(i+2)%k]); ans.append(x)

printing result

print ("The triplet list is : " + str(ans))

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Output

The original list is : [0, 1, 2, 3, 4, 5] The triplet list is : [(0, 1, 2), (1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 0), (5, 0, 1)]

Time Complexity: O(n)

Auxiliary space: O(n)

Method#4: Using zip() and slicing:

Python3 `

test_list = [0, 1, 2, 3, 4, 5]

Using zip() and slicing

result = list(zip(test_list, test_list[1:] + test_list[:1], test_list[2:] + test_list[:2])) print(result) #This code is contributed by Jyothi pinjala.

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Output

[(0, 1, 2), (1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 0), (5, 0, 1)]

Time Complexity: O(n)

Auxiliary space: O(n)

Method #5: Using recursion

Here's a solution using recursion in Python:

Python3 `

def get_triplets_recursive(lst, triplet_lst, index): """ A recursive function that takes the original list, a list to store the triplets and an index and appends the triplets to the triplet list. """ # Base case: when the index has reached the end of the list, return if index == len(lst): return

# Append the current triplet to the triplet list
triplet_lst.append((lst[index], lst[(index + 1) % len(lst)], lst[(index + 2) % len(lst)]))

# Recursively call the function with the next index
get_triplets_recursive(lst, triplet_lst, index + 1)

def get_triplets(lst): """ A function that takes the original list and returns the list of triplets. """ # Initialize an empty list to store the triplets triplet_lst = []

# Call the recursive function with the original list, the empty list of triplets and the starting index 0
get_triplets_recursive(lst, triplet_lst, 0)

# Return the list of triplets
return triplet_lst

Example usage

original_list = [0, 1, 2, 3, 4, 5] triplet_list = get_triplets(original_list) print(triplet_list)

#This code is contributed by Edula Vinay Kumar Reddy

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Output

[(0, 1, 2), (1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 0), (5, 0, 1)]

Time complexity: O(n)

Auxiliary Space: O(n)

Method 6 : using the itertools module

step by step:

• Import the itertools module.
• Initialize the list.
• Use the itertools.cycle() function to cycle through the list.
• Use the itertools.islice() function to slice the list into triplets.
• Convert the sliced triplets into a list.
• Print the resulting list of triplets. Python3 `

Python code to demonstrate

Triplet iteration in List

using itertools module

import itertools module

import itertools

initializing list

l = [0, 1, 2, 3, 4, 5]

Printing original list

print("The original list is : " + str(l))

using itertools.cycle() and itertools.islice() functions

Triplet iteration in List

ans = [list(itertools.islice(itertools.cycle(l), i, i+3)) for i in range(len(l))]

printing result

print ("The triplet list is : " + str(ans))

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Output

The original list is : [0, 1, 2, 3, 4, 5] The triplet list is : [[0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5], [4, 5, 0], [5, 0, 1]]

Time complexity: O(n), where n is the length of the list.

Auxiliary space: O(n), where n is the length of the list.