Descending primes: Difference between revisions

=={{header|Delphi}}==

{{works with|Delphi|6.0}}

{{libheader|SysUtils,StdCtrls}}

<syntaxhighlight lang="Delphi">

type TProgress = procedure(Percent: integer);

function IsPrime(N: integer): boolean;

{Optimised prime test - about 40% faster than the naive approach}

var I,Stop: integer;

begin

if (N = 2) or (N=3) then Result:=true

else if (n <= 1) or ((n mod 2) = 0) or ((n mod 3) = 0) then Result:= false

else

begin

I:=5;

Stop:=Trunc(sqrt(N));

Result:=False;

while I<=Stop do

begin

if ((N mod I) = 0) or ((N mod (i + 2)) = 0) then exit;

Inc(I,6);

end;

Result:=True;

end;

end;

function IsDescending(N: integer): boolean;

{Determine if each digit is less than previous, left to right}

var S: string;

var I: integer;

begin

Result:=False;

S:=IntToStr(N);

for I:=1 to Length(S)-1 do

if S[I]<=S[I+1] then exit;

Result:=True;

end;

procedure ShowDescendingPrimes(Memo: TMemo; Prog: TProgress);

{Write Descending primes up to 123,456,789 }

{The Optional progress }

var I,Cnt: integer;

var S: string;

const Max = 123456789;

begin

if Assigned(Prog) then Prog(0);

S:='';

Cnt:=0;

for I:=2 to Max do

begin

if ((I mod 1000000)=0) and Assigned(Prog) then Prog(Trunc(100*(I/Max)));

if IsDescending(I) and IsPrime(I) then

begin

S:=S+Format('%12.0n', [I*1.0]);

Inc(Cnt);

if (Cnt mod 10)=0 then

begin

Memo.Lines.Add(S);

S:='';

end;

end;

end;

if S<>'' then Memo.Lines.Add(S);

Memo.Lines.Add('Descending Primes Found: '+IntToStr(Cnt));

end;

</syntaxhighlight>

{{out}}

<pre>

2 3 5 7 31 41 43 53 61 71

73 83 97 421 431 521 541 631 641 643

653 743 751 761 821 853 863 941 953 971

983 5,431 6,421 6,521 7,321 7,541 7,621 7,643 8,431 8,521

8,543 8,641 8,731 8,741 8,753 8,761 9,421 9,431 9,521 9,631

9,643 9,721 9,743 9,851 9,871 75,431 76,421 76,541 76,543 86,531

87,421 87,541 87,631 87,641 87,643 94,321 96,431 97,651 98,321 98,543

98,621 98,641 98,731 764,321 865,321 876,431 975,421 986,543 987,541 987,631

8,764,321 8,765,321 9,754,321 9,875,321 97,654,321 98,764,321 98,765,431

Descending Primes Found: 87

</pre>