5 Effective Ways to Generate a Connell Sequence in Python

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πŸ’‘ Problem Formulation: A Connell sequence is a sequence of integer numbers that starts with 1, increases by 2, then, from the second term onward, each new term increases by an odd number which is two units greater than the previous increment. We aim to generate the Connell sequence up to the nth term. For instance, with n = 5, the desired output would be [1, 3, 6, 10, 15].

Method 1: Iterative Approach

This method iteratively builds the Connell sequence by initializing a list with the first element as 1, then adding subsequent terms based on the current odd number increment, which starts at 2 and increases by 2 after each iteration. It’s simple, straightforward, and easy to comprehend for those new to Python.

Here’s an example:

def generate_connell_sequence(n):
    sequence = [1]
    odd_increment = 2
    for _ in range(1, n):
        sequence.append(sequence[-1] + odd_increment)
        odd_increment += 2
    return sequence


Output: [1, 3, 6, 10, 15]

This code snippet defines a function generate_connell_sequence() that takes an integer n and returns the first n terms of the Connell sequence. The sequence list is generated by starting with 1, and then incrementally adding increasingly larger odd numbers.

Method 2: Using List Comprehension

List comprehension in Python can create the Connell sequence in a more Pythonic and concise way, by performing operations inline within a single line. This method is efficient but may be less readable for those unfamiliar with the list comprehension syntax.

Here’s an example:

def connell_sequence_lc(n):
    return [1 + sum(range(2, 2*i, 2)) for i in range(1, n+1)]


Output: [1, 3, 6, 10, 15]

This snippet demonstrates a function connell_sequence_lc() that employs list comprehension to generate the Connell sequence. It compactly computes the sum of the initial element and the required odd numbers to get each subsequent term.

Method 3: Recursive Approach

A recursive approach generates the Connell sequence by calling the function within itself, building the sequence backwards. This method is elegant and demonstrates the power of recursion but may suffer from limited stack depth for large n.

Here’s an example:

def connell_sequence_recursive(n, increment=2):
    if n == 1: return [1]
    prev_sequence = connell_sequence_recursive(n-1, increment+2)
    return prev_sequence + [prev_sequence[-1] + increment]


Output: [1, 3, 6, 10, 15]

In this code snippet, the connell_sequence_recursive() function calls itself to generate each term of the sequence but requires a previous term, thus building the sequence recursively. The base case is the first element of the sequence.

Method 4: Using Generator Function

A Python generator function can be used to generate each term of the Connell sequence on-the-fly. This method conserves memory by yielding values one at a time instead of building the entire list, which is favorable for very long sequences.

Here’s an example:

def connell_sequence_generator(n):
    current, step = 1, 2
    for _ in range(n):
        yield current
        current, step = current + step, step + 2


Output: [1, 3, 6, 10, 15]

The connell_sequence_generator() function is a generator that yields the Connell sequence terms one by one. This snippet prints a list converted from the generator’s output, containing the first five terms.

Bonus One-Liner Method 5: Functional Approach

A functional programming style takes advantage of Python’s higher-order functions such as reduce(). It can be used to generate the Connell sequence in a concise, albeit less intuitive manner.

Here’s an example:

from functools import reduce
print(reduce(lambda acc, x: acc + [acc[-1] + (x*2)], range(1, 5), [1]))

Output: [1, 3, 6, 10, 15]

This code uses reduce() from the functools module, which accumulates a list by starting with [1] and extending it with each new term of the Connell sequence calculated using a lambda function.


  • Method 1: Iterative Approach. Strengths: Easy to understand. Suitable for beginners. Weaknesses: Can be inefficient for large sequences.
  • Method 2: Using List Comprehension. Strengths: Pythonic and succinct. Weaknesses: Might be confusing for those not familiar with list comprehension.
  • Method 3: Recursive Approach. Strengths: Elegant and demonstrates recursion. Weaknesses: Limited by Python’s stack depth, not suitable for large n.
  • Method 4: Using Generator Function. Strengths: Memory efficient as it yields terms one at a time. Weaknesses: Not the most intuitive for beginners.
  • Method 5: Functional Approach. Strengths: Very concise one-liner. Weaknesses: Less readable and might be hard to debug for complex cases.