Python/project_euler/problem_04/sol1.py

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"""
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Problem:
A palindromic number reads the same both ways. The largest palindrome made from
the product of two 2-digit numbers is 9009 = 91 x 99.
Find the largest palindrome made from the product of two 3-digit numbers which
is less than N.
"""
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def solution(n: int = 998001) -> int:
"""Returns the largest palindrome made from the product of two 3-digit
numbers which is less than n.
>>> solution(20000)
19591
>>> solution(30000)
29992
>>> solution(40000)
39893
>>> solution(10000)
Traceback (most recent call last):
...
ValueError: That number is larger than our acceptable range.
"""
# fetches the next number
for number in range(n - 1, 9999, -1):
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# converts number into string.
str_number = str(number)
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# checks whether 'str_number' is a palindrome.
if str_number == str_number[::-1]:
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divisor = 999
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# if 'number' is a product of two 3-digit numbers
# then number is the answer otherwise fetch next number.
while divisor != 99:
if (number % divisor == 0) and (len(str(number // divisor)) == 3.0):
return number
divisor -= 1
raise ValueError("That number is larger than our acceptable range.")
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if __name__ == "__main__":
import doctest
doctest.testmod()
print(solution(int(input().strip())))