Truncatable primes: Difference between revisions
Added Forth solution
Drkameleon (talk | contribs) (obsolete) |
(Added Forth solution) |
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Line 1,181:
Left: 998443
Right: 739399
</pre>
=={{header|Forth}}==
The prime sieve code is borrowed from [[Sieve of Eratosthenes#Forth]].
<lang forth>: prime? ( n -- ? ) here + c@ 0= ;
: notprime! ( n -- ) here + 1 swap c! ;
: sieve ( n -- )
here over erase
0 notprime!
1 notprime!
2
begin
2dup dup * >
while
dup prime? if
2dup dup * do
i notprime!
dup +loop
then
1+
repeat
2drop ;
: left_truncatable_prime? ( n -- flag )
dup prime? invert if
drop false exit
then
dup 10
begin
dup 3 pick <
while
2 pick over mod
dup 3 pick = if
2drop 2drop false exit
then
dup prime? invert if
2drop 2drop false exit
then
rot drop swap
10 *
repeat
2drop drop true ;
: right_truncatable_prime? ( n -- flag )
dup prime? invert if
drop false exit
then
10 /
begin
dup 0 >
while
dup prime? invert if
drop false exit
then
10 /
repeat
drop true ;
: max_left_truncatable_prime ( n -- )
begin
dup 0 >
while
dup left_truncatable_prime? if . cr exit then
1-
repeat drop ;
: max_right_truncatable_prime ( n -- )
begin
dup 0 >
while
dup right_truncatable_prime? if . cr exit then
1-
repeat drop ;
1000000 constant limit
limit 1+ sieve
." Largest left truncatable prime: "
limit max_left_truncatable_prime
." Largest right truncatable prime: "
limit max_right_truncatable_prime</lang>
{{out}}
<pre>
Largest left truncatable prime: 998443
Largest right truncatable prime: 739399
</pre>
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