Largest palindrome product: Difference between revisions
julia example
(→{{header|XPL0}}: faster version (226 vs. 99 ms on Pi4)) |
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Largest palindromic product of two 8-digit integers: 99999999 x 99990001 = 9999000000009999
Largest palindromic product of two 9-digit integers: 999980347 x 999920317 = 999900665566009999
</pre>
=={{header|Julia}}==
<lang julia>using Primes
function twoprodpal(factorlength)
maxpal = Int128(10)^(2 * factorlength) - 1
dig = digits(maxpal)
halfnum = dig[1:length(dig)÷2]
while any(halfnum .!= 0)
prodnum = evalpoly(Int128(10), [reverse(halfnum); halfnum])
facs = twofac(factorlength, prodnum)
if !isempty(facs)
println("For factor length $factorlength, $(facs[1]) * $(facs[2]) = $prodnum")
break
end
halfnum = digits(evalpoly(Int128(10), halfnum) - 1)
end
end
function twofac(faclength, prodnum)
f = [one(prodnum)]
for (p, e) in factor(prodnum)
f = reduce(vcat, [f * p^j for j in 1:e], init=f)
end
possiblefacs = filter(x -> length(string(x)) == faclength, f)
for i in possiblefacs
j = prodnum ÷ i
j ∈ possiblefacs && return sort([i, j])
end
return typeof(prodnum)[]
end
@Threads.threads for i in 2:12
twoprodpal(i)
end
</lang>{{out}}
<pre>
For factor length 2, 91 * 99 = 9009
For factor length 3, 913 * 993 = 906609
For factor length 4, 9901 * 9999 = 99000099
For factor length 5, 99681 * 99979 = 9966006699
For factor length 6, 999001 * 999999 = 999000000999
For factor length 7, 9997647 * 9998017 = 99956644665999
For factor length 8, 99990001 * 99999999 = 9999000000009999
For factor length 9, 999920317 * 999980347 = 999900665566009999
For factor length 10, 9999986701 * 9999996699 = 99999834000043899999
For factor length 11, 99999943851 * 99999996349 = 9999994020000204999999
For factor length 12, 999999000001 * 999999999999 = 999999000000000000999999
</pre>
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