Snippet

project_euler/32.rb

Code:

# Project Euler Problem 32: Pandigital Products

# https://projecteuler.net/problem=32

# We shall say that an n-digit number is pandigital if it makes use of all
# the digits 1 to n exactly once; for example, the 5-digit number, 
# 15234, is 1 through 5 pandigital.

# The product 7254 is unusual, as the identity, 39 × 186 = 7254,
# containing multiplicand, multiplier, and product is 1 through 9 pandigital.

# Find the sum of all products whose multiplicand/multiplier/product identity
# can be written as a 1 through 9 pandigital.

def divisor_pairs(n)
    (1..Math.sqrt(n)).map { |a| n % a == 0 ? [a, n / a] : nil }.compact
end

def pandigital?(a, b, c)
    d = [a, b, c].map(&:to_s).join
    return false unless d.size == 9
    return d.chars.sort == '123456789'.chars
end

def solve()
    results = []
    (1_000...10_000).each do |c|
        for a, b in divisor_pairs(c) do
            if pandigital?(a, b, c)
                puts "#{a} × #{b} = #{c}"
                results << c
            end
        end
    end
    return results.uniq.sum
end

puts solve()

Output:

28 × 157 = 4396
18 × 297 = 5346
27 × 198 = 5346
12 × 483 = 5796
42 × 138 = 5796
4 × 1738 = 6952
39 × 186 = 7254
48 × 159 = 7632
4 × 1963 = 7852
45228