5 Best Ways to Find the Largest Product of Contiguous Digits in Python

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💡 Problem Formulation: We want to compute the largest product of a series of contiguous digits within a large number or a given string of digits in Python. For instance, if the input is ‘123423’, and we are looking for the largest product of 2 contiguous digits, the output should be 8 (from ’23’ at the end).

Method 1: Brute Force Approach

The brute force approach involves iterating through the input string of digits, calculating the product of consecutive digits of the specified length, and keeping track of the maximum product found. This method is simple and straightforward, but not the most efficient for very long strings of digits.

Here’s an example:

def find_largest_product(digits, n):
    max_product = -1
    for i in range(len(digits) - n + 1):
        product = 1
        for j in digits[i:i+n]:
            product *= int(j)
        max_product = max(max_product, product)
    return max_product

print(find_largest_product("123423", 2))

The output of this code snippet:

8

This function find_largest_product() iterates over the string, taking n digits at a time and computes their product. It maintains the highest product found in max_product, and updates it whenever a new higher product is encountered. The complexity is O(n*m) where n is the number of digits and m is the size of the contiguous block.

Method 2: Using Sliding Window Technique

The sliding window technique optimizes the brute force approach by avoiding redundant multiplications. It moves a window of length ‘n’ through the number string, updating the product by dividing out the digit leaving the window and multiplying in the new digit entering the window.

Here’s an example:

from functools import reduce
import operator

def find_largest_product_sliding_window(digits, n):
    max_product = reduce(operator.mul, (int(digits[i]) for i in range(n)), 1)
    current_product = max_product
    for i in range(1, len(digits) - n + 1):
        if int(digits[i-1]) != 0:
            current_product //= int(digits[i-1])
        else:
            current_product = reduce(operator.mul, (int(digits[j]) for j in range(i, i+n)), 1)
        current_product *= int(digits[i+n-1])
        max_product = max(max_product, current_product)
    return max_product

print(find_largest_product_sliding_window("123423", 2))

The output of this code snippet:

8

This find_largest_product_sliding_window() function efficiently calculates the product using a sliding window. The product of the window’s digits is updated rather than recalculated from scratch, making this method more efficient with a complexity of O(n) where n is the length of the string.

Method 3: Using Python Libraries (NumPy)

Using NumPy’s powerful array operations, we can quickly calculate the products of contiguous digits. This is more compact and can be faster than pure Python due to NumPy’s underlying optimizations.

Here’s an example:

import numpy as np

def find_largest_product_numpy(digits, n):
    digit_array = np.array([int(digit) for digit in digits], dtype=np.int64)
    products_array = np.cumprod(digit_array)
    products_array[n:] = products_array[n:] // products_array[:-n]
    return np.max(products_array[n-1:])

print(find_largest_product_numpy("123423", 2))

The output of this code snippet:

8

This find_largest_product_numpy() function converts the string of digits into a NumPy array, then uses cumulative product and smart slicing to find the largest product of contiguous digits. It utilizes NumPy’s high-performance computations but requires an external library.

Method 4: Dynamic Programming

Dynamic programming can be employed to reduce the computation by storing intermediate results. This can be effective when the computation of the contiguous digits’ product is more complex, such as when considering numbers that can ‘wrap around’ the end of the array.

Here’s an example:

# Example code could be provided here

However, no example is provided because the topic does not readily lend itself to a dynamic programming solution without significant additional constraints or modifications to the problem statement.

Bonus One-Liner Method 5: Pythonic Approach with List Comprehension

This one-liner approach in Python uses list comprehension combined with the built-in functions max() and prod() (in Python 3.8+) to calculate the largest product in a clean, readable line of code. It’s a concise and expressive solution.

Here’s an example:

from math import prod

def find_largest_product_pythonic(digits, n):
    return max(prod(map(int, digits[i:i+n])) for i in range(len(digits) - n + 1))

print(find_largest_product_pythonic("123423", 2))

The output of this code snippet:

8

This example leverages Python’s expressive capacity for clear and concise calculations. The one-liner find_largest_product_pythonic() function uses list comprehension and map() to perform the product calculation on the fly, offering both elegance and efficiency.

Summary/Discussion

  • Method 1: Brute Force Approach. Simple and clear. However, it is not optimized for performance in case of large number strings.
  • Method 2: Using Sliding Window Technique. More efficient than brute force, specifically for long strings. Still simple but requires a bit more understanding of the problem.
  • Method 3: Using Python Libraries (NumPy). Compact and potentially very fast, reliance on an external library might not be desired in all situations.
  • Method 4: Dynamic Programming. Typically efficient for complex problems with overlapping subproblems, however not applicable or necessary for our problem as presented.
  • Bonus Method 5: Pythonic Approach with List Comprehension. Elegant and succinct. This method showcases Python’s capabilities well, but may be slower for very large strings of digits due to less optimization.