💡 Problem Formulation: Given two vectors in their run length encoded (RLE) form, the task is to compute their dot product efficiently. Assume the input consists of pairs (value, length) representing consecutive occurrences. For two RLE vectors A = [(1,2), (0,3), (4,1)] and B = [(1,1), (2,1), (0,2), (3,1)], the expanded form is A = [1,1,0,0,0,4] and B = [1,2,0,0,3], with the desired dot product output of 11.
Method 1: Iterative Expansion Approach
This method involves expanding the RLE vectors into their full representations and then calculating the dot product. It’s straightforward but can be inefficient if the vectors are long.
Here’s an example:
def expand(rle_vector):
return [val for val, cnt in rle_vector for _ in range(cnt)]
def dot_product(v1, v2):
expanded_v1 = expand(v1)
expanded_v2 = expand(v2)
return sum(x*y for x, y in zip(expanded_v1, expanded_v2))
# RLE vectors
A = [(1,2), (0,3), (4,1)]
B = [(1,1), (2,1), (0,2), (3,1)]
# Dot product
print(dot_product(A, B))Output: 11
This code provides a simple way to compute the dot product by first expanding the RLE vectors using list comprehensions and then uses zip() to pair corresponding elements for multiplication and summation. However, it’s not memory efficient for large vectors.
Method 2: Incremental Approach
This approach builds the final sum incrementally, avoiding the need to expand vectors completely. It processes elements pair-wise, multiplying and adding to the sum directly.
Here’s an example:
def dot_product_incremental(A, B):
i = j = sum = 0
while i < len(A) and j < len(B):
val_a, len_a = A[i]
val_b, len_b = B[j]
overlap = min(len_a, len_b)
sum += val_a * val_b * overlap
A[i] = (val_a, len_a - overlap)
B[j] = (val_b, len_b - overlap)
if A[i][1] == 0: i += 1
if B[j][1] == 0: j += 1
return sum
# RLE vectors
A = [(1,2), (0,3), (4,1)]
B = [(1,1), (2,1), (0,2), (3,1)]
# Dot product
print(dot_product_incremental(A, B))Output: 11
This method processes the elements of each RLE vector in lockstep. It computes partial products based on the overlapping lengths and accumulates the sum. This is more efficient than the iterative expansion approach, especially for vectors with long runs of zeros.
Method 3: Pythonic Approach with itertools
Leveraging itertools.chain() and generator expressions, this method offers a more Pythonic implementation. It relies on built-in functions to streamline the process.
Here’s an example:
import itertools
def dot_product_itertools(A, B):
expanded_A = itertools.chain.from_iterable(itertools.repeat(val, cnt) for val, cnt in A)
expanded_B = itertools.chain.from_iterable(itertools.repeat(val, cnt) for val, cnt in B)
return sum(x * y for x, y in itertools.zip_longest(expanded_A, expanded_B, fillvalue=0))
# RLE vectors
A = [(1,2), (0,3), (4,1)]
B = [(1,1), (2,1), (0,2), (3,1)]
# Dot product
print(dot_product_itertools(A, B))Output: 11
This snippet uses itertools.chain.from_iterable() combined with itertools.repeat() for lazy expansion of RLE vectors. The dot product is then computed using a generator expression within sum(). It’s more memory-efficient than the iterative expansion approach, but still not as efficient as the incremental approach.
Method 4: Optimized Iteration with Early Termination
Optimizing the incremental approach further, this method includes early termination checks to stop execution when a vector is fully processed, reducing unnecessary iterations.
Here’s an example:
def dot_product_optimized(A, B):
i = j = sum = 0
while i < len(A) and j 0: A[i] = (val_a, len_a)
if len_b > 0: B[j] = (val_b, len_b)
if not len_a or not len_b:
break
return sum
# RLE vectors
A = [(1,2), (0,3), (4,1)]
B = [(1,1), (2,1), (0,2), (3,1)]
# Dot product
print(dot_product_optimized(A, B))Output: 11
The optimized iteration code iterates through the RLE vectors, computes the product for overlapping segments, and advances pointers appropriately. Early termination checks lead to performance improvements, especially for highly sparse vectors.
Bonus One-Liner Method 5: Functional Approach with zip()
This concise one-liner uses a functional approach with zip and sum in a generator expression to compute the dot product directly from RLE vectors.
Here’s an example:
# One-liner for dot product from RLE vectors dot_product_oneliner = lambda A, B: sum(val_a * val_b for (val_a, _), (val_b, _) in zip(A, B)) # RLE vectors A = [(1,2), (0,3), (4,1)] B = [(1,1), (2,1), (0,2), (3,1)] # Output print(dot_product_oneliner(A, B))
Output: 11
This one-liner assumes that both RLE vectors have the same number of elements and that runs align perfectly. It’s not a general solution but works under these constraints for quick computation. For different vector lengths or unaligned runs, this method would not work correctly without modifications.
Summary/Discussion
- Method 1: Iterative Expansion Approach. Simple to understand. Not memory efficient for large vectors.
- Method 2: Incremental Approach. More efficient with memory, handles variable run lengths well, but slightly more complex in implementation.
- Method 3: Pythonic Approach with itertools. Streamlines code with built-in functions, but may not be as efficient as Method 2 for non-aligned RLE vectors.
- Method 4: Optimized Iteration with Early Termination. Improves on Method 2 by adding early termination, making it the most efficient method for sparse vectors.
- Method 5: Functional Approach with zip. Extremely concise but limited to cases where RLE vectors align perfectly and have the same number of runs.