Python/project_euler/problem_082/sol1.py

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"""
Project Euler Problem 82: https://projecteuler.net/problem=82
The minimal path sum in the 5 by 5 matrix below, by starting in any cell
in the left column and finishing in any cell in the right column,
and only moving up, down, and right, is indicated in red and bold;
the sum is equal to 994.
131 673 [234] [103] [18]
[201] [96] [342] 965 150
630 803 746 422 111
537 699 497 121 956
805 732 524 37 331
Find the minimal path sum from the left column to the right column in matrix.txt
(https://projecteuler.net/project/resources/p082_matrix.txt)
(right click and "Save Link/Target As..."),
a 31K text file containing an 80 by 80 matrix.
"""
import os
def solution(filename: str = "input.txt") -> int:
"""
Returns the minimal path sum in the matrix from the file, by starting in any cell
in the left column and finishing in any cell in the right column,
and only moving up, down, and right
>>> solution("test_matrix.txt")
994
"""
with open(os.path.join(os.path.dirname(__file__), filename)) as input_file:
matrix = [
[int(element) for element in line.split(",")]
for line in input_file.readlines()
]
rows = len(matrix)
cols = len(matrix[0])
minimal_path_sums = [[-1 for _ in range(cols)] for _ in range(rows)]
for i in range(rows):
minimal_path_sums[i][0] = matrix[i][0]
for j in range(1, cols):
for i in range(rows):
minimal_path_sums[i][j] = minimal_path_sums[i][j - 1] + matrix[i][j]
for i in range(1, rows):
minimal_path_sums[i][j] = min(
minimal_path_sums[i][j], minimal_path_sums[i - 1][j] + matrix[i][j]
)
for i in range(rows - 2, -1, -1):
minimal_path_sums[i][j] = min(
minimal_path_sums[i][j], minimal_path_sums[i + 1][j] + matrix[i][j]
)
return min(minimal_path_sums_row[-1] for minimal_path_sums_row in minimal_path_sums)
if __name__ == "__main__":
print(f"{solution() = }")