5 Best Ways to Find the Nth Term in the Look and Say Sequence in Python

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πŸ’‘ Problem Formulation: We aim to determine the nth term in the ‘Look and Say’ sequence, which is a series of integers where each term is derived from the previous one by describing its digits. Starting with the number 1, the second term is “11” since the first term has “one 1”. If the input is n=5, for instance, the expected output is "111221", the fifth term in the sequence.

Method 1: Iterative Approach

This method utilizes a straightforward iterative approach to build up the Look and Say sequence starting from the number “1”. It generates each subsequent term by analyzing the previous term, counting the number of continuous digits (say x counts of digit y), and concatenating x and y to form the next term in the sequence.

Here’s an example:

def look_and_say(n):
    term = "1"
    for _ in range(n - 1):
        next_term, i = "", 0
        while i < len(term):
            count = 1
            while i + 1 < len(term) and term[i] == term[i + 1]:
                i += 1
                count += 1
            next_term += str(count) + term[i]
            i += 1
        term = next_term
    return term

print(look_and_say(5))

Output:

"111221"

This code defines a function called look_and_say which computes the nth term of the Look and Say sequence iteratively. It initializes the first term to “1” and runs a loop ‘n-1’ times, each time constructing the next term based on the current one.

Method 2: Recursive Approach

The recursive approach solves the problem by breaking it down into simpler sub-problems. It calculates the nth term by calling itself to find the (n-1)th term, which it then processes to generate the next term in the sequence, continuing this way until it reaches the base case.

Here’s an example:

def look_and_say_recursive(n, term="1"):
    if n == 1:
        return term
    next_term, i = "", 0
    while i < len(term):
        count = 1
        while i + 1 < len(term) and term[i] == term[i + 1]:
            i += 1
            count += 1
        next_term += str(count) + term[i]
        i += 1
    return look_and_say_recursive(n - 1, next_term)

print(look_and_say_recursive(5))

Output:

"111221"

This code snippet demonstrates the use of a recursive function look_and_say_recursive to generate the nth term. If n reaches 1, it returns the current term, otherwise, it constructs the next term and makes a recursive call to process the next iteration.

Method 3: Using Regular Expressions

The regular expression approach leverages the power of Python’s regex capabilities to match sequences of identical digits and thereby construct each term. It’s a compact and efficient way to express the process of counting and grouping the digits.

Here’s an example:

import re

def look_and_say_regex(n):
    term = "1"
    for _ in range(n - 1):
        term = "".join([str(len(match.group(0))) + match.group(1)
                        for match in re.finditer(r"(\d)\1*", term)])
    return term

print(look_and_say_regex(5))

Output:

"111221"

In this function, look_and_say_regex, the regex pattern r"(\d)\1*" is used to find all consecutive identical digits. The join function combines the counts and the digits to create the next term, which replaces the current term in each iteration of the loop.

Method 4: Using Groupby from the Itertools Library

The Groupby method provided by Python’s itertools library is another sophisticated tool that can be employed for creating the Look and Say sequence. It groups the sequence by its elements, which can then be easily counted and combined to form the next term.

Here’s an example:

from itertools import groupby

def look_and_say_groupby(n):
    term = "1"
    for _ in range(n - 1):
        term = ''.join(str(len(list(group))) + digit for digit, group in groupby(term))
    return term

print(look_and_say_groupby(5))

Output:

"111221"

This example uses the groupby function to iterate over grouped values of the sequence. The output from groupby is a series of tuples containing the digit and the group; the size of the group and digit are concatenated to form the next sequence.

Bonus One-Liner Method 5: Using Functional Python

This quick one-liner utilizes Python’s functional programming capabilities to condense the problem into a single line of code, using higher-order functions like reduce(). It is a compact but less readable solution.

Here’s an example:

from functools import reduce
import re

look_and_say_oneliner = lambda n: reduce(lambda term, _: re.sub(r'(\d)(\1*)', lambda m: str(len(m.group(0))) + m.group(1), term), range(n-1), '1')

print(look_and_say_oneliner(5))

Output:

"111221"

A lambda function combined with reduce() is used here, employing the re.sub() regex substitution along with a pattern to match identical digits. Running this for ‘n-1’ iterations derives the nth term in the sequence.

Summary/Discussion

  • Method 1: Iterative Approach. Straightforward and easy to understand. Can be memory-intensive for large n due to string concatenations.
  • Method 2: Recursive Approach. Elegant and inherent stack trace visualization of the sequence’s construction. Not as memory-efficient and possible stack overflow for very large n.
  • Method 3: Using Regular Expressions. More compact and potentially faster for large sequences due to optimized regex engine. However, regex can be difficult to read and maintain.
  • Method 4: Using Groupby from the Itertools Library. Utilizes a powerful built-in library, making the code concise. It might be less intuitive for those not familiar with itertools.
  • Bonus One-Liner Method 5: Using Functional Python. Offers a very concise solution. Sacrifices readability for brevity, which can make debugging challenging.