Programming Techniques to Verify the Formation of an Array from Pieces in Python

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πŸ’‘ Problem Formulation: In Python programming, we are sometimes presented with a challenge where we need to determine if a target array can be constructed by concatenating an arbitrary number of subarrays (pieces) from a given list of arrays. The task is to verify, given an array of distinct integers and a list of subarrays, whether it is possible to form the original array by combining all the pieces sequentially without any alterations. For example, if the array is [1,2,3,4] and the pieces are [[1,2],[3,4]], the program should return True as the array can be formed from these pieces.

Method 1: Using Iteration and Array Splicing

This method involves iterating through the target array and checking if the current segment in the target array matches any piece. If a match is found, the piece is spliced from the list to prevent reuse. This process continues until we can either form the entire target array or no matching piece is found, leading to a return of False.

Here’s an example:

def can_form_array(arr, pieces):
    i = 0
    while i < len(arr):
        match = False
        for piece in pieces:
            if arr[i:i+len(piece)] == piece:
                i += len(piece)
                match = True
                break
        if not match:
            return False
    return True

# Example usage
result = can_form_array([1,2,3,4], [[1,2],[3,4]])
print(result)

The output of this code snippet:

True

This code snippet defines a function can_form_array() that takes the target array arr and a list of pieces. It iterates through arr and uses a sliding window to find matching pieces, incrementing the index as pieces are found. If there is no match, it returns False. If it successfully traverses the entire array it returns True.

Method 2: Using a Dictionary for Index Mapping

Here, we create a dictionary to map the first element of each piece to its index in the list of pieces. As we iterate through the target array, we use this dictionary to quickly find the respective piece and verify if the subsequent elements match. This method potentially reduces the overall search time, especially with large numbers of pieces.

Here’s an example:

def can_form_array(arr, pieces):
    piece_map = {piece[0]: piece for piece in pieces}
    i = 0
    while i < len(arr):
        if arr[i] in piece_map and arr[i:i+len(piece_map[arr[i]])] == piece_map[arr[i]]:
            i += len(piece_map[arr[i]])
        else:
            return False
    return True

# Example usage
result = can_form_array([1,2,3,4], [[1,2],[3,4]])
print(result)

The output of this code snippet:

True

The can_form_array() function uses a dictionary, piece_map, to map the first element of each piece to the piece itself. As we iterate over arr, this map helps quickly access the corresponding piece and check for validity, enhancing performance by avoiding unnecessary nested loops.

Method 3: Greedy Approach with Queue

Utilizing a queue, we can employ a greedy approach to verify the formation of the target array. The function dequeues elements from the target array and attempts to match them with the head of the remaining pieces. Matching pieces are removed, and the process continues until either the target array is empty (successful formation) or no pieces can form the next expected element (returning False).

Here’s an example:

from collections import deque

def can_form_array(arr, pieces):
    queue = deque(arr)
    while queue:
        first = queue.popleft()
        matched = False
        for piece in pieces:
            if piece[0] == first:
                if list(queue)[:len(piece)-1] == piece[1:]:
                    for _ in range(len(piece)-1):
                        queue.popleft()
                    pieces.remove(piece)
                    matched = True
                    break
        if not matched:
            return False
    return True

# Example usage
result = can_form_array([1,2,3,4], [[1,2],[3,4]])
print(result)

The output of this code snippet:

True

This code snippet defines a function can_form_array() that uses deque from the collections module to process elements in arr. For each dequeued element, it checks for a corresponding piece and removes it from the queue and the list of pieces upon a successful match. If no match is found, the function returns False.

Method 4: Set-Based Verification

By converting both the target array and pieces into sets of tuples, we can exploit set operations to determine if the target array can be formed by the pieces. This method is based on the premise that for successful formation, the union of all piece sets should equal the set of the target array without any excess elements.

Here’s an example:

def can_form_array(arr, pieces):
    piece_sets = set(tuple(piece) for piece in pieces)
    combined_set = set()
    for piece in piece_sets:
        combined_set = combined_set.union(piece)
    return combined_set == set(arr)

# Example usage
result = can_form_array([1,2,3,4], [[1,2],[3,4]])
print(result)

The output of this code snippet:

True

The function can_form_array() creates sets of tuples for the pieces and the target array, performing a union of all piece tuples. It then compares this union with the set of the target array. If they match, it indicates that the array can be formed from the pieces.

Bonus One-Liner Method 5: Using itertools.chain and All

A concise one-liner using itertools.chain to flatten the list of pieces, the method iterates over the flattened list and checks if all corresponding elements in the target array match, using the built-in all() function.

Here’s an example:

from itertools import chain

def can_form_array(arr, pieces):
    return arr == list(chain.from_iterable(sorted(pieces, key=lambda x: arr.index(x[0]))))

# Example usage
result = can_form_array([1,2,3,4], [[1,2],[3,4]])
print(result)

The output of this code snippet:

True

The code snippet defines a function can_form_array() that relies on chain.from_iterable from the itertools module to flatten the pieces into one list, which is then compared with arr. Sorting of pieces is required based on their first element’s index in arr to maintain the right order.

Summary/Discussion

  • Method 1: Iteration and Array Splicing. Simple and straightforward, but could be inefficient for large arrays due to repeated splicing.
  • Method 2: Dictionary for Index Mapping. Offers a faster search by mapping elements to pieces, and reduces the number of comparisons required. Not as memory-efficient when dealing with small arrays or pieces.
  • Method 3: Greedy Approach with Queue. Systematic dequeuing ensures a greedy step-by-step verification, suitable for streaming data. However, this can be slower because of list slicing operations.
  • Method 4: Set-Based Verification. Highly efficient for small to medium-sized problems due to the use of set operations. May not be effective if the pieces have repeating elements or subarray ordering is important.
  • Method 5: One-Liner with itertools.chain. Extremely concise and utilizes Python’s powerful standard library, though may be less readable for beginners. Correctness depends on the unique ordering of the first elements in pieces.