55 lines
1.4 KiB
Ruby
55 lines
1.4 KiB
Ruby
descr=%{
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The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.
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Find the sum of the only eleven primes that are both truncatable from left to right and right to left.
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NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.
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}
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puts "Read primes"
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primes=File.read("firsts_1MM_primes.txt").split().collect! { |p| p.to_i }
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is_prime={}
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puts "Init datastructure"
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primes.each do |p|
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is_prime[p]=true
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end
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puts "search truncatable left to right and right to left"
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truncatable=[]
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primes.each do |p|
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# verify if it is truncatable
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next if p<10
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# puts p
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not_truncatable=false
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str=p.to_s.split('')
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while str.length > 1
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str.pop
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num=str.join('').to_i
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# puts "> #{num}"
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if not is_prime[num]
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not_truncatable=true
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break
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end
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end
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next if not_truncatable
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str=p.to_s.split('')
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while str.length > 1
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str.shift()
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num=str.join('').to_i
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# puts "> #{num}"
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if not is_prime[num]
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not_truncatable=true
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break
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end
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end
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next if not_truncatable
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puts "Trunkatable: #{p}"
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truncatable<<=p
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if truncatable.length == 11
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break
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end
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end
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puts "NB: #{truncatable.inject(0){|sum,n| sum+n}}"
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