Find prime n such that reversed n is also prime: Difference between revisions
Find prime n such that reversed n is also prime (view source)
Revision as of 11:10, 20 March 2021
, 3 years agoAdded Algol W
m (clarified task (decimal digits)) |
(Added Algol W) |
||
Line 3:
;Task
Find prime '''n''' for '''0 < n < 500''' which are also primes when the (decimal) digits are reversed.
=={{header|ALGOL W}}==
<lang algolw>begin % find some primes whose digits reversed is also prime %
% sets p( 1 :: n ) to a sieve of primes up to n %
procedure Eratosthenes ( logical array p( * ) ; integer value n ) ;
begin
p( 1 ) := false; p( 2 ) := true;
for i := 3 step 2 until n do p( i ) := true;
for i := 4 step 2 until n do p( i ) := false;
for i := 3 step 2 until truncate( sqrt( n ) ) do begin
integer ii; ii := i + i;
if p( i ) then for pr := i * i step ii until n do p( pr ) := false
end for_i ;
end Eratosthenes ;
integer MAX_NUMBER, maxPrime;
MAX_NUMBER := 500;
% approximate the largest prime we need to consider ( 10 ^ number of digits in MAX_NUMBER ) %
begin
integer v;
v := MAX_NUMBER;
maxPrime := 1;
while v > 0 do begin
v := v div 10;
maxPrime := maxPrime * 10
end while_v_gt_0
end;
begin
logical array prime( 1 :: maxPrime);
integer pCount;
% sieve the primes to maxPrime %
Eratosthenes( prime, maxPrime );
% find the primes that are reversable %
pCount := 0;
for i := 1 until MAX_NUMBER - 1 do begin
if prime( i ) then begin
integer pReversed, v;
v := i;
pReversed := 0;
while v > 0 do begin
pReversed := ( pReversed * 10 ) + v rem 10;
v := v div 10
end while_v_gt_0 ;
if prime( pReversed ) then begin
writeon( i_w := 4, s_w := 0, " ", i );
pCount := pCount + 1;
if pCount rem 20 = 0 then write()
end if_prime_pReversed
end if_prime_i
end for_i ;
write( i_w := 1, s_w := 0, "Found ", pCount, " reversable primes below ", MAX_NUMBER )
end
end.</lang>
{{out}}
<pre>
2 3 5 7 11 13 17 31 37 71 73 79 97 101 107 113 131 149 151 157
167 179 181 191 199 311 313 337 347 353 359 373 383 389
Found 34 reversable primes below 500
</pre>
=={{header|Phix}}==
| |||