Descending primes
editable example
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ghul
use IO.Std.write_line
use Collections
use Ghul.Pipes
is_prime(n: int) -> bool =>
if n < 2 then false
elif n % 2 == 0 then n == 2
else
let divisor mut = 3
while divisor * divisor <= n /\ n % divisor != 0 do
divisor = divisor + 2
od
divisor * divisor > n
fi
descending(so_far: int, next_digit: int, found: LIST[int]) is
if so_far > 0 /\ is_prime(so_far) then
found.add(so_far)
fi
for digit in 0::next_digit do
descending(so_far * 10 + digit, digit - 1, found)
od
si
let found = LIST()
descending(0, 9, found)
let primes = found |> collect_list()
primes.sort()
for row in 0::(primes.count - 1) / 6 do
write_line(
(0::5)
|> filter(column => row * 6 + column < primes.count)
|> map(column => "{primes[row * 6 + column],12}")
|> join("")
)
od
write_line("{primes.count} primes with strictly descending digits")